5.3.1.1Conservation of mass and balanced chemical equations
AQA GCSE Combined Science (8464), Higher tier · Chemistry › Quantitative chemistry › Chemical measurements, conservation of mass and the
Practise Conservation of mass and balanced chemical equations. 15 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The law of conservation of mass says that no atoms are lost or made in a chemical reaction, so the total mass of the products equals the total mass of the reactants. You use it to work out missing masses and to write and balance symbol equations. It is tested on Paper 1, and balancing equations is needed across both papers.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State the law of conservation of massNo atoms are lost or made in a reaction, so the mass of the products equals the mass of the reactants.
3
Count the atoms in a formulaMultiply the small numbers by the big number in front, e.g. 2H2O contains 4 hydrogen atoms and 2 oxygen atoms.
4
Calculate a missing mass in a reactionTotal mass of reactants = total mass of products, so subtract the known masses from the total.
5
Balance a symbol equationChange only the big numbers in front of formulae until every element has the same number of atoms on both sides.
6
Write balanced equations from word equationsWrite the correct formula for each substance (e.g. O2, H2, HCl, MgCl2) and then balance it.
7
Balance equations with brackets in formulaeCount atoms inside brackets carefully, e.g. Ca(OH)2 + 2HNO3 → Ca(NO3)2 + 2H2O.
8
Balance ionic and half equations (HT)Atoms and total charge must both be equal on each side, e.g. Cu2+ + 2e− → Cu.
Notes
The law of conservation of mass
In a chemical reaction the atoms are rearranged to make new substances. No atoms are lost or made.
So the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass.
Example: 12 g of magnesium reacts completely with 8 g of oxygen, so 20 g of magnesium oxide forms.
If the mass seems to change in an open container, a gas has escaped or has been taken in from the air.
Reading formulae
A small number after a symbol multiplies only that atom: CO2 has 1 carbon atom and 2 oxygen atoms.
A small number after a bracket multiplies everything inside it: Mg(OH)2 has 1 Mg, 2 O and 2 H.
A big number in front multiplies the whole formula: 3H2O has 6 H and 3 O.
Seven elements exist as molecules of two atoms: H2, N2, O2, F2, Cl2, Br2 and I2.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Balancing symbol equations
A balanced equation has the same number of atoms of each element on both sides.
Only change the big numbers in front of formulae. Never change the small numbers: that would make a different substance.
Balance one element at a time. Leave elements that appear on their own (such as O2, H2 or Fe) until last, then recount everything.
Worked example: Na + H2O → NaOH + H2. There are 2 H on the left but 3 on the right. Use 2H2O and 2NaOH (4 H on each side), then 2Na. Result: 2Na + 2H2O → 2NaOH + H2.
Higher tier: in ionic equations and half equations the charges must balance as well as the atoms, e.g. 2Cl− → Cl2 + 2e− (total charge 2− on each side). grade 8+
Cheatsheet
Law of conservation of mass: no atoms are lost or made, so mass of products = mass of reactants
Missing mass = total mass on one side − known masses on the other side
A small number multiplies the atom (or bracket) just before it
A big number in front multiplies the whole formula
Balance by changing the big numbers only, never the small numbers
Diatomic elements: H2, N2, O2, F2, Cl2, Br2, I2
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous
Ionic and half equations: atoms and charges both balance (HT) grade 8+
How to answer each type of question
Calculate a missing mass
1 to 2 marks4
Write down: total mass of reactants = total mass of products.
Add up the masses on the side where you know them all.
Subtract the masses you know on the other side, and give the unit.
Example. Calcium carbonate breaks down when it is heated. CaCO3 → CaO + CO2 25.0 g of calcium carbonate was heated until it had all decomposed. 14.0 g of calcium oxide formed. Calculate the mass of carbon dioxide formed. [2 marks]
Show the model answer
mass of CO2 = 25.0 − 14.0 (1) = 11.0 g (1)
Balance a symbol equation
1 mark5
List each element and count its atoms on both sides.
Change the big numbers one at a time, leaving lone elements such as O2 until last.
Recount every element on both sides before you move on.
Example. Balance the equation for the complete combustion of propane. ....C3H8 + ....O2 → ....CO2 + ....H2O [1 mark]
Show the model answer
C3H8 + 5O2 → 3CO2 + 4H2O (1) Check: C 3 = 3; H 8 = 8; O 10 = 6 + 4.
Write a balanced symbol equation from a word equation
2 marks6
Write the correct formula for every reactant and product (remember H2, O2, Cl2).
Balance it by changing the big numbers only.
Add state symbols only if the question asks for them.
Example. Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen. Write a balanced symbol equation for this reaction. [2 marks]
Say that atoms are not lost or made, only rearranged.
Say that nothing can escape: no gas is made, or the container is sealed.
Example. A student mixed silver nitrate solution with sodium chloride solution in a beaker on a balance. A white solid formed. The total mass did not change. Explain why. [2 marks]
Show the model answer
No atoms are lost or made in the reaction; they are only rearranged (1). No gas is produced, so nothing escapes from the beaker (1).
Shortcuts and memory tricks
Make a tally table: list each element under both sides and tick it off when it balances.
Leave lone elements (O2, H2, Fe) until last: changing them does not upset anything else.
Stuck with an odd number of oxygen atoms? Double every balancing number, e.g. 2C2H6 + 7O2 → 4CO2 + 6H2O.
The diatomic elements N, O, F, Cl, Br and I make a '7' shape on the periodic table. Add hydrogen and you have all seven.
Quick check: mass in = mass out. A product mass bigger than the total reactant mass is wrong.
Where marks are lost
Changing the small numbers (e.g. writing H2O2 to balance oxygen) instead of the big numbers in front.
Forgetting that a number after a bracket multiplies everything inside: Ca(NO3)2 has 6 oxygen atoms, not 3.
Writing O, H or Cl on their own for oxygen, hydrogen or chlorine gas instead of O2, H2 or Cl2.
Writing only 'mass is conserved' when asked to explain. Say that no atoms are lost or made.
Leaving a fraction such as 3½O2 in the final equation instead of whole numbers.
Exam technique
After your last change, recount every element on both sides. One miscount loses the mark.
In 'explain' questions about mass, use the word atoms: no atoms are lost or made, they are only rearranged.
Show the subtraction in a missing-mass calculation so you can still get a method mark if you slip.
If you add state symbols when they are not asked for, make sure they are correct.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Balance the equation for the reduction of iron(III) oxide by carbon monoxide. Fe2O3 + ....CO → ....Fe + ....CO2
Fe2O3 + 3CO → 2Fe + 3CO2
Lithium reacts with water to produce lithium hydroxide (LiOH) and hydrogen. Write a balanced symbol equation for this reaction.
2Li + 2H2O → 2LiOH + H2
Balance the equation. ....NH3 + ....O2 → ....NO + ....H2O
4NH3 + 5O2 → 4NO + 6H2O
Aluminium reacts with iodine to produce aluminium iodide, AlI3. A teacher sealed 2.70 g of aluminium powder and 25.40 g of iodine in a glass tube and warmed it. After the reaction:
all of the iodine had reacted
0.90 g of aluminium had not reacted.
Balance the equation. ....Al + ....I2 → ....AlI3
2Al + 3I2 → 2AlI3
A chemist heated 7.56 g of ammonium dichromate in a sealed container. The products were 4.56 g of Cr2O3 and 0.84 g of N2. Calculate the mass of X produced.
2.16 g
Symbol equations are balanced by changing the numbers written in front of formulae. The small numbers within a formula must never be changed. How many oxygen atoms are shown in the formula 2Fe(OH)3?
6
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Sodium reacts with chlorine to produce sodium chloride. 2Na + Cl2 → 2NaCl
(a) Complete the sentence. Tick (✓) one box. The total mass of the products of a chemical reaction is ...[1]
less than the total mass of the reactants.
equal to the total mass of the reactants.
greater than the total mass of the reactants.
half of the total mass of the reactants.
(b) How many chlorine atoms are shown on the left-hand side of the equation?[1]
(c) A teacher reacted 2.30 g of sodium with 3.55 g of chlorine in a fume cupboard. All of the sodium and all of the chlorine reacted. Calculate the mass of sodium chloride produced.[1]
(d) Explain why the mass of sodium chloride produced is equal to the total mass of the sodium and chlorine that reacted.[1]
Show the answer and mark scheme
(a)Answer: equal to the total mass of the reactants.
(b)Answer: 2
2
(c)Answer: 5.85 g
5.85 (g)
(d)
no atoms are lost or made during a chemical reaction
Question 2Medium5 marks
A student investigated the reaction between barium chloride solution and sodium sulfate solution. The student:
poured 25 cm3 of each solution into two separate beakers
placed both beakers on a balance and recorded the total mass
poured one solution into the other and put both beakers back on the balance
recorded the total mass again.
A white solid formed when the solutions were mixed.
(a) Complete the balanced equation for the reaction. BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + ....NaCl(aq)[1]
(b) The total mass at the start was 148.62 g. Predict the total mass at the end. Give a reason for your answer.[2]
(c) In a second experiment, 2.08 g of barium chloride reacted completely with 1.42 g of sodium sulfate to produce 2.33 g of barium sulfate. Calculate the mass of sodium chloride produced.[2]
Show the answer and mark scheme
(a)Answer: 2NaCl
2 (NaCl)
(b)Answer: 148.62 g — no gas is produced, so no products escape
148.62 (g)
no gas is produced / nothing escapes from the beakers (so no atoms are lost)
(c)Answer: 1.17 g
(2.08 + 1.42) − 2.33
1.17 (g)
Question 3Hard7 marks
In the first stage of the manufacture of nitric acid, ammonia reacts with oxygen to produce nitrogen monoxide (NO) and water.
(b) Butane (C4H10) burns completely in oxygen to produce carbon dioxide and water. Write a balanced symbol equation for this reaction.[2]
(c) A liquid fuel, X, burns completely in oxygen. The balanced equation is: X + 3O2 → 2CO2 + 3H2O Deduce the formula of X. Explain how you worked out your answer.[3]
Show the answer and mark scheme
(a)Answer: 4NH3 + 5O2 → 4NO + 6H2O
4NH3, 4NO and 6H2O (in the ratio 2 : 2 : 3)
5O2 with the rest of the equation correct
(b)Answer: 2C4H10 + 13O2 → 8CO2 + 10H2O
correct formulae for all four substances
correctly balanced: 2C4H10 + 13O2 → 8CO2 + 10H2O
(c)Answer: C2H6O (C2H5OH)
products contain 2 carbon atoms and 6 hydrogen atoms (which must come from X)
products contain 7 oxygen atoms but 3O2 provides only 6