Work Out What Fraction of a Fertiliser Bag Is Actually Nitrogen
Learn to balance equations by hit and trial, read everything a balanced equation tells you and what it leaves out, define relative atomic and molecular mass against the carbon-12 standard, and calculate percentage composition.
How much of a bag of ammonium nitrate is really nitrogen?
A farmer buys a sack labelled ammonium nitrate, , and wants the nitrogen in it. How much of the bag is nitrogen and how much is packaging in the chemical sense?
Add up the masses in the formula, taking , and :
Of that total, parts are nitrogen, so
Just over a third of the bag is nitrogen, and the rest is hydrogen and oxygen that the plant does not need from this source. That single calculation lets a farmer compare two fertilisers on the only basis that matters.
Nothing in it required a laboratory. A formula and a table of atomic masses were enough — which is what makes the chemical equation such a powerful piece of writing.
This page covers the second part of the ICSE Class 9 Chemistry chapter on the language of chemistry — balancing equations, what a balanced equation does and does not tell you, relative atomic and molecular mass, and percentage composition.
Add up the masses in the formula, taking , and :
Of that total, parts are nitrogen, so
Just over a third of the bag is nitrogen, and the rest is hydrogen and oxygen that the plant does not need from this source. That single calculation lets a farmer compare two fertilisers on the only basis that matters.
Nothing in it required a laboratory. A formula and a table of atomic masses were enough — which is what makes the chemical equation such a powerful piece of writing.
This page covers the second part of the ICSE Class 9 Chemistry chapter on the language of chemistry — balancing equations, what a balanced equation does and does not tell you, relative atomic and molecular mass, and percentage composition.
How do you balance an equation by hit and trial?
Count each element on both sides and adjust the coefficients in front of the formulae until every count matches. The formulae themselves are never changed — only the numbers in front of them.
Why balancing is necessary. Matter is neither created nor destroyed in a chemical change, so every atom that goes in must come out. An unbalanced equation claims otherwise.
A useful order of work. Balance the element that appears in the fewest formulae first, leave hydrogen and oxygen until last, and treat a polyatomic group that survives the reaction as a single unit.
Worked example 1 — two products. Iron and steam:
The right-hand side needs iron and oxygen, so put before and before . That brings hydrogen atoms in, so before :
Check: Fe ; O ; H . Balanced.
Worked example 2. Zinc and hydrochloric acid:
Check: Zn ; H ; Cl .
Worked example 3. Slaked lime and hydrochloric acid:
Check: Ca ; O ; H against ; Cl .
Worked example 4 — three products. Marble chips and hydrochloric acid:
Check: Ca ; C ; O against ; H ; Cl .
Worked example 5 — three products, harder. Copper and dilute nitric acid:
Check: Cu ; H ; N against ; O against . Balanced.
Worked example 6 — three products with concentrated acid. Zinc and concentrated nitric acid:
Check: Zn ; H ; N against ; O against .
Worked example 7 — decomposition into three products. Heating lead nitrate:
Check: Pb ; N ; O against .
Never change a subscript to balance an equation. Writing instead of to supply an extra oxygen does balance the count — and it changes the substance from water to hydrogen peroxide, which is a different compound with different reactions. Subscripts are fixed by the valencies of the previous part of this chapter, and only the coefficients are free.
Always finish with the check written out. Counting each element on both sides costs one line and catches every error, and an equation offered without that check is an equation nobody has verified.
Why balancing is necessary. Matter is neither created nor destroyed in a chemical change, so every atom that goes in must come out. An unbalanced equation claims otherwise.
A useful order of work. Balance the element that appears in the fewest formulae first, leave hydrogen and oxygen until last, and treat a polyatomic group that survives the reaction as a single unit.
Worked example 1 — two products. Iron and steam:
The right-hand side needs iron and oxygen, so put before and before . That brings hydrogen atoms in, so before :
Check: Fe ; O ; H . Balanced.
Worked example 2. Zinc and hydrochloric acid:
Check: Zn ; H ; Cl .
Worked example 3. Slaked lime and hydrochloric acid:
Check: Ca ; O ; H against ; Cl .
Worked example 4 — three products. Marble chips and hydrochloric acid:
Check: Ca ; C ; O against ; H ; Cl .
Worked example 5 — three products, harder. Copper and dilute nitric acid:
Check: Cu ; H ; N against ; O against . Balanced.
Worked example 6 — three products with concentrated acid. Zinc and concentrated nitric acid:
Check: Zn ; H ; N against ; O against .
Worked example 7 — decomposition into three products. Heating lead nitrate:
Check: Pb ; N ; O against .
Never change a subscript to balance an equation. Writing instead of to supply an extra oxygen does balance the count — and it changes the substance from water to hydrogen peroxide, which is a different compound with different reactions. Subscripts are fixed by the valencies of the previous part of this chapter, and only the coefficients are free.
Always finish with the check written out. Counting each element on both sides costs one line and catches every error, and an equation offered without that check is an equation nobody has verified.
What does a balanced equation tell you, and what does it leave out?
A balanced equation carries a surprising amount of information — and leaves out almost everything about how the reaction actually behaves.
What it conveys.
- The names and formulae of every reactant and product
- The relative number of molecules, from the coefficients. In , two molecules of hydrogen need one of oxygen
- The relative masses, once the formula masses are worked out. Those same coefficients say that parts by mass of hydrogen need parts of oxygen to give parts of water
- The physical states, through state symbols: solid, liquid, gas, dissolved in water
- The conditions, written above or below the arrow — heat, a catalyst, a pressure, a temperature
- Whether a gas is evolved, shown by an upward arrow, or a precipitate forms, shown by a downward one
- Whether the reaction is reversible, shown by a double arrow
Worked reading. Consider
This says: solid zinc reacts with sulphuric acid in solution; one formula unit of each is needed; the zinc sulphate produced stays dissolved; and hydrogen is given off as a gas. With , , and , it also says that parts by mass of zinc need parts of sulphuric acid.
What it does NOT tell you — the limitations.
- The concentration of the reactants, unless dilute or concentrated is written in
- Whether the reaction is exothermic or endothermic, unless the heat change is stated
- The rate of the reaction — whether it takes an instant or a week
- The time taken to complete
- Whether the reaction goes to completion or stops part way
- The colour changes and physical appearance of what happens
- The conditions needed, unless they are written above the arrow
- The mechanism — how the atoms actually rearrange
Worked contrast. The equation
is identical whether the carbon is burning fiercely in a furnace or oxidising imperceptibly over years. The equation cannot tell the two apart, because it records only what goes in and what comes out.
A balanced equation is a statement of bookkeeping, not of behaviour. It guarantees that the atoms add up and says nothing about whether the reaction is worth attempting. So an equation being balanced is no evidence that the reaction happens at all — one can write a perfectly balanced equation for a change that never occurs, and that is the sharpest of the limitations.
What it conveys.
- The names and formulae of every reactant and product
- The relative number of molecules, from the coefficients. In , two molecules of hydrogen need one of oxygen
- The relative masses, once the formula masses are worked out. Those same coefficients say that parts by mass of hydrogen need parts of oxygen to give parts of water
- The physical states, through state symbols: solid, liquid, gas, dissolved in water
- The conditions, written above or below the arrow — heat, a catalyst, a pressure, a temperature
- Whether a gas is evolved, shown by an upward arrow, or a precipitate forms, shown by a downward one
- Whether the reaction is reversible, shown by a double arrow
Worked reading. Consider
This says: solid zinc reacts with sulphuric acid in solution; one formula unit of each is needed; the zinc sulphate produced stays dissolved; and hydrogen is given off as a gas. With , , and , it also says that parts by mass of zinc need parts of sulphuric acid.
What it does NOT tell you — the limitations.
- The concentration of the reactants, unless dilute or concentrated is written in
- Whether the reaction is exothermic or endothermic, unless the heat change is stated
- The rate of the reaction — whether it takes an instant or a week
- The time taken to complete
- Whether the reaction goes to completion or stops part way
- The colour changes and physical appearance of what happens
- The conditions needed, unless they are written above the arrow
- The mechanism — how the atoms actually rearrange
Worked contrast. The equation
is identical whether the carbon is burning fiercely in a furnace or oxidising imperceptibly over years. The equation cannot tell the two apart, because it records only what goes in and what comes out.
A balanced equation is a statement of bookkeeping, not of behaviour. It guarantees that the atoms add up and says nothing about whether the reaction is worth attempting. So an equation being balanced is no evidence that the reaction happens at all — one can write a perfectly balanced equation for a change that never occurs, and that is the sharpest of the limitations.
Formula
What is relative atomic mass measured against?
Against one twelfth of the mass of a single carbon-12 atom, which is the agreed standard.
Relative atomic mass (RAM) of an element is the number of times one atom of that element is heavier than th of an atom of carbon-12:
Relative molecular mass (RMM) is the same comparison for a molecule:
and in practice it is found by adding the relative atomic masses of every atom in the formula.
Both quantities have NO unit. They are ratios of one mass to another, so the units cancel — and writing *the relative atomic mass of oxygen is grams* is a marked error. It is simply .
Why carbon-12 is the standard. Its mass is defined as exactly units, so that th of it is a convenient reference of almost exactly the mass of a hydrogen atom. That choice makes the relative atomic masses of the light elements come out close to whole numbers, which is why hydrogen is about and oxygen about .
Worked calculations of relative molecular mass, taking , , , , , , , , , .
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Worked example with water of crystallisation. :
Multiply through the brackets before adding. In the subscript applies to the whole hydroxide group, so both the oxygen and the hydrogen are doubled. **Taking it to apply only to the hydrogen gives instead of , and that is the most frequent arithmetic error in these calculations.
Relative atomic mass need not be a whole number.** Chlorine's is and copper's is , because the natural element is a mixture of isotopes and the value is a weighted average. So a fractional relative atomic mass is not a rounding error — it is a statement about the mixture, and the atomic-structure chapter explains where it comes from.
Relative atomic mass (RAM) of an element is the number of times one atom of that element is heavier than th of an atom of carbon-12:
Relative molecular mass (RMM) is the same comparison for a molecule:
and in practice it is found by adding the relative atomic masses of every atom in the formula.
Both quantities have NO unit. They are ratios of one mass to another, so the units cancel — and writing *the relative atomic mass of oxygen is grams* is a marked error. It is simply .
Why carbon-12 is the standard. Its mass is defined as exactly units, so that th of it is a convenient reference of almost exactly the mass of a hydrogen atom. That choice makes the relative atomic masses of the light elements come out close to whole numbers, which is why hydrogen is about and oxygen about .
Worked calculations of relative molecular mass, taking , , , , , , , , , .
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Worked example with water of crystallisation. :
Multiply through the brackets before adding. In the subscript applies to the whole hydroxide group, so both the oxygen and the hydrogen are doubled. **Taking it to apply only to the hydrogen gives instead of , and that is the most frequent arithmetic error in these calculations.
Relative atomic mass need not be a whole number.** Chlorine's is and copper's is , because the natural element is a mixture of isotopes and the value is a weighted average. So a fractional relative atomic mass is not a rounding error — it is a statement about the mixture, and the atomic-structure chapter explains where it comes from.
How do you calculate percentage composition by mass?
Find the total mass of the element in one formula unit, divide by the relative molecular mass, and multiply by a hundred.
**Worked example 1 — calcium carbonate, .** The relative molecular mass is , which makes the arithmetic easy:
Check: . The percentages must add to a hundred, and that is a free verification on every such calculation.
**Worked example 2 — water, .** The relative molecular mass is :
Check: .
**Worked example 3 — carbon dioxide, .** Relative molecular mass :
**Worked example 4 — sulphuric acid, .** Relative molecular mass :
Check: .
Worked example 5 — the water of crystallisation in a hydrate. In , relative molecular mass , the water contributes :
So over a third of the mass of blue copper sulphate crystals is water, which is why heating them causes such a large loss in mass — the effect the next chapter on water investigates.
Worked example 6 — comparing two fertilisers. Which supplies more nitrogen per kilogram, ammonium nitrate or ammonium sulphate ?
- , relative molecular mass , nitrogen :
- , relative molecular mass , nitrogen :
Both contain two nitrogen atoms, and the percentages differ sharply because the rest of each formula weighs a different amount. That is exactly the comparison a farmer needs, and neither formula reveals it by inspection.
Count every atom of the element, including ones in different parts of the formula. In the nitrogen appears twice — once in the ammonium group and once in the nitrate group — so the mass of nitrogen is and not . Using only the first occurrence halves the answer, and a formula with an element in two places is precisely where that error is set.
The percentage is by MASS, not by number of atoms. Water contains twice as many hydrogen atoms as oxygen atoms and is nearly nine tenths oxygen by mass, because each oxygen atom is sixteen times heavier. So "most of water is hydrogen" is true by atom count and false by mass, and a question always means mass unless it says otherwise.
**Worked example 1 — calcium carbonate, .** The relative molecular mass is , which makes the arithmetic easy:
Check: . The percentages must add to a hundred, and that is a free verification on every such calculation.
**Worked example 2 — water, .** The relative molecular mass is :
Check: .
**Worked example 3 — carbon dioxide, .** Relative molecular mass :
**Worked example 4 — sulphuric acid, .** Relative molecular mass :
Check: .
Worked example 5 — the water of crystallisation in a hydrate. In , relative molecular mass , the water contributes :
So over a third of the mass of blue copper sulphate crystals is water, which is why heating them causes such a large loss in mass — the effect the next chapter on water investigates.
Worked example 6 — comparing two fertilisers. Which supplies more nitrogen per kilogram, ammonium nitrate or ammonium sulphate ?
- , relative molecular mass , nitrogen :
- , relative molecular mass , nitrogen :
Both contain two nitrogen atoms, and the percentages differ sharply because the rest of each formula weighs a different amount. That is exactly the comparison a farmer needs, and neither formula reveals it by inspection.
Count every atom of the element, including ones in different parts of the formula. In the nitrogen appears twice — once in the ammonium group and once in the nitrate group — so the mass of nitrogen is and not . Using only the first occurrence halves the answer, and a formula with an element in two places is precisely where that error is set.
The percentage is by MASS, not by number of atoms. Water contains twice as many hydrogen atoms as oxygen atoms and is nearly nine tenths oxygen by mass, because each oxygen atom is sixteen times heavier. So "most of water is hydrogen" is true by atom count and false by mass, and a question always means mass unless it says otherwise.
Exam tip
Exam tip: write the check line and never touch a subscript
Balance only with coefficients. Changing to changes the substance — subscripts come from valency and are fixed.
Write the element-by-element check after balancing: *Fe , O , H *. One line, and it catches every slip.
Leave hydrogen and oxygen until last, and balance the element in the fewest formulae first.
Relative atomic mass and relative molecular mass have NO unit — they are ratios against th of a carbon-12 atom. Never write grams.
Multiply through brackets before adding: is , not .
For a hydrate, add the water separately: .
**Percentage , and the percentages must total — check it.
Count the element everywhere it appears.** Nitrogen in is , not .
Percentage is by MASS, not by number of atoms.
List what an equation conveys and what it omits as two separate lists — the omissions (concentration, rate, time, heat change, colour, completeness) are marked separately.
And use state symbols , , , and the arrows for a gas or a precipitate when the question asks what an equation tells you.
Write the element-by-element check after balancing: *Fe , O , H *. One line, and it catches every slip.
Leave hydrogen and oxygen until last, and balance the element in the fewest formulae first.
Relative atomic mass and relative molecular mass have NO unit — they are ratios against th of a carbon-12 atom. Never write grams.
Multiply through brackets before adding: is , not .
For a hydrate, add the water separately: .
**Percentage , and the percentages must total — check it.
Count the element everywhere it appears.** Nitrogen in is , not .
Percentage is by MASS, not by number of atoms.
List what an equation conveys and what it omits as two separate lists — the omissions (concentration, rate, time, heat change, colour, completeness) are marked separately.
And use state symbols , , , and the arrows for a gas or a precipitate when the question asks what an equation tells you.
Did you know
Why two fertilisers with the same nitrogen atoms are not equally good
Ammonium nitrate and ammonium sulphate both contain exactly two nitrogen atoms per formula unit. By atom count they look identical as nitrogen sources.
By mass they are not close.
A kilogram of the first delivers about a third of a kilogram of nitrogen; the second delivers barely a fifth. The same two nitrogen atoms are carried by very different amounts of other material — a sulphate group weighing against a formula that carries only hydrogen and oxygen.
That gap has consequences a farmer can measure. Transporting nitrogen as ammonium sulphate means hauling considerably more sacks for the same effect, and spreading more material over the same field.
The same reasoning runs through industry. Iron is extracted from ores, and two ores with the same iron atoms per formula unit can differ in iron percentage because of what else is attached. Comparing ores, fertilisers, fuels or food supplements always comes down to a percentage-by-mass calculation, never to an atom count.
And it explains a curiosity about water. has twice as many hydrogen atoms as oxygen atoms, and it is about oxygen by mass. Two light atoms lose comfortably to one heavy one, because each oxygen atom weighs sixteen times as much as each hydrogen atom.
So "how many atoms" and "how much mass" are two genuinely different questions about the same formula, and almost every practical decision in chemistry turns on the second one.
By mass they are not close.
A kilogram of the first delivers about a third of a kilogram of nitrogen; the second delivers barely a fifth. The same two nitrogen atoms are carried by very different amounts of other material — a sulphate group weighing against a formula that carries only hydrogen and oxygen.
That gap has consequences a farmer can measure. Transporting nitrogen as ammonium sulphate means hauling considerably more sacks for the same effect, and spreading more material over the same field.
The same reasoning runs through industry. Iron is extracted from ores, and two ores with the same iron atoms per formula unit can differ in iron percentage because of what else is attached. Comparing ores, fertilisers, fuels or food supplements always comes down to a percentage-by-mass calculation, never to an atom count.
And it explains a curiosity about water. has twice as many hydrogen atoms as oxygen atoms, and it is about oxygen by mass. Two light atoms lose comfortably to one heavy one, because each oxygen atom weighs sixteen times as much as each hydrogen atom.
So "how many atoms" and "how much mass" are two genuinely different questions about the same formula, and almost every practical decision in chemistry turns on the second one.
Exam relevance
How do equations and mole calculations feed into JEE Main and NEET?
Because the balanced equation is the starting line of every stoichiometry question, and relative molecular mass becomes the molar mass.
This is the foundation for Class 11 Chemistry Some Basic Concepts of Chemistry, examined in JEE Main and NEET. The relative molecular mass calculated here becomes the molar mass in grams per mole, and the coefficients of a balanced equation become a mole ratio:
**So having a relative molecular mass of means that g of it is one mole, and the whole of stoichiometry follows from that single bridge.
Percentage composition becomes empirical-formula determination, run backwards. Class 11 gives the percentages by mass and asks for the formula: divide each percentage by the atomic mass, take the simplest ratio, and the empirical formula appears. The calculation on this page is that procedure reversed, and questions of both kinds appear in JEE Main and NEET. The molecular formula then follows from the empirical formula and the molar mass.
Limiting reagent is the main new idea. When two reactants are supplied in amounts that do not match the equation's ratio, one runs out first and fixes the yield — and identifying it requires exactly the mass-to-mole conversion above. Limiting-reagent numericals are a recurring JEE Main type, and they are impossible without a correctly balanced equation.
Balancing itself gets harder and gains a method.** Class 11 Redox Reactions introduces the oxidation-number and ion-electron methods for equations that hit and trial cannot handle, and Class 12 Electrochemistry uses them constantly. Hit and trial remains the right tool for ordinary equations, and the discipline of writing the check line is what carries over.
The carbon-12 standard is examined directly. Class 11 defines the atomic mass unit as exactly th of the mass of a carbon-12 atom, and connects it to Avogadro's number — the number of atoms in g of carbon-12. The definition given on this page is the one used there, and the point that relative masses carry no unit while molar masses do is a standard discrimination question.
The limitations list reappears as chemical kinetics and thermodynamics. Everything an equation cannot tell you — the rate, the heat change, whether it goes to completion — becomes a whole chapter apiece: Class 11 Chemical Kinetics, Thermodynamics and Equilibrium. So the limitations are not a footnote but a syllabus map.
What the questions look like. For board work, expect balance given equations, state what an equation conveys and its limitations as two lists, define relative atomic and molecular mass with the carbon-12 reference, calculate a relative molecular mass, and find the percentage composition or the percentage of water of crystallisation. For JEE Main and NEET, expect mole calculations, empirical formulae from percentages, limiting reagent and redox balancing.
How board and competitive emphasis differ. A board paper rewards the check line after balancing and the stated standard in a definition. A competitive paper assumes both and tests the mole arithmetic built on them, usually with a limiting reagent buried in it.
The single trap that costs the most marks. Counting an element only where it first appears. Nitrogen in totals , not , because it sits in both the ammonium and the nitrate group — and the same happens with hydrogen in and with oxygen in almost every hydrate. The defence is to write each element's total on its own line before dividing: *N: *, which makes a missed occurrence visible immediately.
This is the foundation for Class 11 Chemistry Some Basic Concepts of Chemistry, examined in JEE Main and NEET. The relative molecular mass calculated here becomes the molar mass in grams per mole, and the coefficients of a balanced equation become a mole ratio:
**So having a relative molecular mass of means that g of it is one mole, and the whole of stoichiometry follows from that single bridge.
Percentage composition becomes empirical-formula determination, run backwards. Class 11 gives the percentages by mass and asks for the formula: divide each percentage by the atomic mass, take the simplest ratio, and the empirical formula appears. The calculation on this page is that procedure reversed, and questions of both kinds appear in JEE Main and NEET. The molecular formula then follows from the empirical formula and the molar mass.
Limiting reagent is the main new idea. When two reactants are supplied in amounts that do not match the equation's ratio, one runs out first and fixes the yield — and identifying it requires exactly the mass-to-mole conversion above. Limiting-reagent numericals are a recurring JEE Main type, and they are impossible without a correctly balanced equation.
Balancing itself gets harder and gains a method.** Class 11 Redox Reactions introduces the oxidation-number and ion-electron methods for equations that hit and trial cannot handle, and Class 12 Electrochemistry uses them constantly. Hit and trial remains the right tool for ordinary equations, and the discipline of writing the check line is what carries over.
The carbon-12 standard is examined directly. Class 11 defines the atomic mass unit as exactly th of the mass of a carbon-12 atom, and connects it to Avogadro's number — the number of atoms in g of carbon-12. The definition given on this page is the one used there, and the point that relative masses carry no unit while molar masses do is a standard discrimination question.
The limitations list reappears as chemical kinetics and thermodynamics. Everything an equation cannot tell you — the rate, the heat change, whether it goes to completion — becomes a whole chapter apiece: Class 11 Chemical Kinetics, Thermodynamics and Equilibrium. So the limitations are not a footnote but a syllabus map.
What the questions look like. For board work, expect balance given equations, state what an equation conveys and its limitations as two lists, define relative atomic and molecular mass with the carbon-12 reference, calculate a relative molecular mass, and find the percentage composition or the percentage of water of crystallisation. For JEE Main and NEET, expect mole calculations, empirical formulae from percentages, limiting reagent and redox balancing.
How board and competitive emphasis differ. A board paper rewards the check line after balancing and the stated standard in a definition. A competitive paper assumes both and tests the mole arithmetic built on them, usually with a limiting reagent buried in it.
The single trap that costs the most marks. Counting an element only where it first appears. Nitrogen in totals , not , because it sits in both the ammonium and the nitrate group — and the same happens with hydrogen in and with oxygen in almost every hydrate. The defence is to write each element's total on its own line before dividing: *N: *, which makes a missed occurrence visible immediately.
Key takeaways
Balancing, relative mass and percentage composition: quick revision
- Balance with coefficients only — never change a subscript, since and are different substances.
- Order of work: the element in the fewest formulae first, hydrogen and oxygen last, and treat a surviving group as one unit.
- (Fe , O , H ).
- ; .
- — two reactants, three products.
- (O ).
- ; .
- Always write the element-by-element check.
- An equation CONVEYS: names and formulae; relative numbers of molecules; relative masses; state symbols , , , ; conditions above the arrow; a gas evolved or a precipitate formed; reversibility.
- An equation OMITS: concentration, whether it is exothermic or endothermic, the rate, the time, whether it goes to completion, the colour changes, and the mechanism.
- A balanced equation is no evidence that the reaction happens — balance is bookkeeping, not behaviour.
- Relative atomic mass compares one atom with th of a carbon-12 atom; relative molecular mass does the same for a molecule and is found by adding the atomic masses.
- Both have NO unit — never write grams.
- Relative molecular masses: ; ; ; ; ; ; ; ; .
- Multiply through brackets: is , not .
- A fractional relative atomic mass such as for chlorine reflects a mixture of isotopes, not a rounding error.
- **Percentage **, and the percentages must total .
- : Ca , C , O . : H , O . : C , O .
- : H , S , O .
- **Water in ** is — over a third of the crystal's mass.
- Fertiliser comparison: is nitrogen and is , despite both having two nitrogen atoms.
- Count the element everywhere — nitrogen in is , not .
- Percentage is by MASS, not atom count — water is oxygen by mass with twice as many hydrogen atoms.
Take the formula printed on any fertiliser or supplement packet and work out the percentage of the element it is sold for — then compare it with what the label claims.
- Order of work: the element in the fewest formulae first, hydrogen and oxygen last, and treat a surviving group as one unit.
- (Fe , O , H ).
- ; .
- — two reactants, three products.
- (O ).
- ; .
- Always write the element-by-element check.
- An equation CONVEYS: names and formulae; relative numbers of molecules; relative masses; state symbols , , , ; conditions above the arrow; a gas evolved or a precipitate formed; reversibility.
- An equation OMITS: concentration, whether it is exothermic or endothermic, the rate, the time, whether it goes to completion, the colour changes, and the mechanism.
- A balanced equation is no evidence that the reaction happens — balance is bookkeeping, not behaviour.
- Relative atomic mass compares one atom with th of a carbon-12 atom; relative molecular mass does the same for a molecule and is found by adding the atomic masses.
- Both have NO unit — never write grams.
- Relative molecular masses: ; ; ; ; ; ; ; ; .
- Multiply through brackets: is , not .
- A fractional relative atomic mass such as for chlorine reflects a mixture of isotopes, not a rounding error.
- **Percentage **, and the percentages must total .
- : Ca , C , O . : H , O . : C , O .
- : H , S , O .
- **Water in ** is — over a third of the crystal's mass.
- Fertiliser comparison: is nitrogen and is , despite both having two nitrogen atoms.
- Count the element everywhere — nitrogen in is , not .
- Percentage is by MASS, not atom count — water is oxygen by mass with twice as many hydrogen atoms.
Take the formula printed on any fertiliser or supplement packet and work out the percentage of the element it is sold for — then compare it with what the label claims.