Conservation of mass, relative formula mass & measurement uncertainty – GCSE Chemistry exam questions with mark scheme

1Grade 1–33 marks

Relative atomic masses ($A_r$): H = 1, C = 12, O = 16, S = 32, Ca = 40

(a) Calculate the relative formula mass ($M_r$) of calcium carbonate, $\text{CaCO}_3$. [1]

(b) Calculate the $M_r$ of sulfuric acid, $\text{H}_2\text{SO}_4$. [1]

$\text{Mg} + \ldots\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$

(a) 100(1)
(b) 98(1)
(c) 2 (in front of HCl)(1)
Answer: (a) 100 (b) 98 (c) Mg + 2HCl → MgCl₂ + H₂
Examiner tip: Multiply every atom inside a formula by its subscript: O₃ is 3 × 16.

2Grade 4–53 marks

A student heated magnesium ribbon in a crucible. The magnesium reacted with oxygen. The mass of the crucible and contents increased.

(a) Explain why the mass increased. [2]

(b) The student then heated calcium carbonate in an open tube. The mass decreased. Explain why. [1]

(a) oxygen (from the air) reacts / combines with the magnesium allow magnesium oxide is formed from magnesium and oxygen; do not accept 'oxygen is made/created'(1)
(a) so the mass of oxygen that has combined is added to the mass of magnesium / magnesium oxide has a greater mass than the magnesium used allow the oxygen came from outside the system (air), so mass is still conserved; ignore 'heat adds mass'(1)
(b) carbon dioxide (gas) is produced and escapes (into the air) allow 'a gas is given off and lost'; both 'gas' and 'escapes/lost' ideas needed; do not accept 'mass is destroyed'(1)
Answer: (a) oxygen from the air combines with the magnesium, so the mass of the oxygen is added (b) carbon dioxide gas is produced and escapes
Examiner tip: Mass is never created or destroyed — always name the gas that entered or left the system.

3Grade 1–3Foundation3 marks

A student put a conical flask containing copper sulfate solution and a small tube of sodium hydroxide solution on a balance. The flask was sealed with a stopper.

The total mass was 96.45 g.

The student tipped the flask so the two solutions mixed. A blue solid formed.

The equation for the reaction is:

$$\text{CuSO}_4 + 2\text{NaOH} \rightarrow \text{Cu(OH)}_2 + \text{Na}_2\text{SO}_4$$

(a) Give the total mass of the sealed flask and contents after the reaction. [1]

(b) Give a reason for your answer to part (a). [1]

(a) 96.45 (g) allow the same / no change (in mass)(1)
(b) no atoms are lost or made (during the reaction) allow mass of products equals mass of reactants / atoms are rearranged / nothing can escape (because the flask is sealed); allow mass is conserved; ignore 'it is a closed system' on its own(1)
(c) 6(1)
Answer: (a) 96.45 g (b) no atoms are lost or made / nothing escapes the sealed flask (c) 6
Examiner tip: In (c) count the oxygen in both reactants and multiply the 2NaOH by 2: students often write 5 by forgetting the big number.

4Grade 1–33 marks

Relative atomic masses ($A_r$): H = 1, N = 14, O = 16, Mg = 24, S = 32

(a) Calculate the relative formula mass ($M_r$) of magnesium hydroxide, $\text{Mg(OH)}_2$. [1]

(b) Calculate the relative formula mass ($M_r$) of ammonium sulfate, $\text{(NH}_4\text{)}_2\text{SO}_4$. [2]

(a) 58(1)
(b) correct use of the bracket: (2 × 14) + (8 × 1) + 32 + (4 × 16) allow 2 × 18 + 96 / 36 + 96; allow any correct working that shows 2 N and 8 H(1)
(b) 132 correct answer with no working scores 2 marks; 114 (bracket ignored) scores 0(1)
Answer: (a) 58 (b) 132
Examiner tip: Everything inside a bracket is multiplied by the number after it: (NH₄)₂ means 2 N and 8 H, not 1 N and 4 H.

5Grade 4–54 marks

Ammonium nitrate, $\text{NH}_4\text{NO}_3$, is used as a fertiliser.

Relative atomic masses ($A_r$): H = 1, N = 14, O = 16

(a) Calculate the relative formula mass ($M_r$) of ammonium nitrate. [1]

(b) Calculate the percentage by mass of nitrogen in ammonium nitrate. [2]

A farmer wants to add as much nitrogen as possible to a field using 50 kg of fertiliser.

Which fertiliser should the farmer use? Give a reason for your answer. [1]

(a) 80(1)
(b) $\dfrac{2 \times 14}{80} \times 100$ allow 28 ÷ 80 × 100; allow error carried forward from (a)(1)
(b) 35 (%) correct answer with no working scores 2 marks; 17.5% (only one N counted) scores 1 mark only if working shown(1)
(c) urea and it has a higher percentage (by mass) of nitrogen (46.7% compared with 35%) allow ecf from (b) if the comparison is correct for their value; 'urea' with no reason scores 0(1)
Answer: (a) 80 (b) 35% (c) urea, because it contains a higher percentage of nitrogen by mass
Examiner tip: Ammonium nitrate has two nitrogen atoms (one in NH₄ and one in NO₃): count every one before dividing by $M_r$.

6Grade 4–54 marks

Iron reacts with chlorine to produce iron(III) chloride.

(a) Balance the equation for the reaction. [1]

$$\ldots\text{Fe} + \ldots\text{Cl}_2 \rightarrow \ldots\text{FeCl}_3$$

(b) Relative atomic masses ($A_r$): Cl = 35.5, Fe = 56

Use your balanced equation to show that the total relative formula mass of the reactants is equal to the total relative formula mass of the products. [2]

(c) Explain why the total mass of reactants is always equal to the total mass of products in a chemical reaction. [1]

(a) 2Fe + 3Cl₂ → 2FeCl₃ allow multiples, e.g. 4, 6, 4(1)
(b) reactants: (2 × 56) + (3 × 71) = 325 allow 112 + 213 = 325(1)
(b) products: 2 × 162.5 = 325 (so they are equal) both totals must be shown for this mark; allow ecf for a correctly balanced multiple of the equation (e.g. 650 = 650)(1)
(c) (because) atoms are not created or destroyed / atoms are only rearranged allow no atoms are lost or made; ignore 'mass is conserved' on its own (restates the question)(1)
Answer: (a) 2Fe + 3Cl₂ → 2FeCl₃ (b) 2 × 56 + 3 × 71 = 325 and 2 × 162.5 = 325 (c) no atoms are lost or made, they are only rearranged
Examiner tip: In (b) multiply each $M_r$ by the balancing number in front of it, and remember Cl₂ has $M_r$ 71, not 35.5.

7Grade 4–56 marks

Five groups of students each heated 2.50 g of copper carbonate in an open crucible until there was no further change. They measured the mass of solid left at the end.

Group	A	B	C	D	E
Mass of solid left in g	1.62	1.66	1.58	1.83	1.66

(a) Which group's result is anomalous? [1]

(b) Suggest one reason for this anomalous result. [1]

(d) The uncertainty of a mean can be estimated from the range of the results.

Give the mean mass in the form: mean ± uncertainty. [2]

(a) D allow 1.83 (g)(1)
(b) not heated for long enough / not all of the copper carbonate decomposed / heated less strongly allow some carbon dioxide was still in the solid / a weighing error; ignore 'human error' or 'mistake' unqualified(1)
(c) $\dfrac{1.62 + 1.66 + 1.58 + 1.66}{4}$ allow 6.52 ÷ 4(1)
(c) 1.63 (g) correct answer with no working scores 2 marks; 1.67 (anomaly included) scores 0(1)
(d) range = 1.66 − 1.58 = 0.08 (g) allow ecf for a range using their values excluding the anomaly(1)
(d) 1.63 ± 0.04 (g) allow ecf from (c) for the mean, e.g. their mean ± 0.04; ± 0.08 scores the first mark only(1)
Answer: (a) D (b) not heated long enough, so not all the copper carbonate decomposed (c) 1.63 g (d) 1.63 ± 0.04 g
Examiner tip: The uncertainty is half the range of the repeat results: students who write ± 0.08 lose the final mark.

8Grade 6–76 marks

A student added marble chips (calcium carbonate) to dilute hydrochloric acid in a conical flask on a balance. The top of the flask was loosely plugged with cotton wool.

The equation for the reaction is:

$$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$

The graph shows how the mass of the flask and contents changed during the reaction.

(a) Use the graph to determine the total decrease in mass. [1]

(b) Explain why the mass of the flask and contents decreased. Use the equation. [2]

(d) The student repeated the experiment with a stopper sealing the flask. Assume the stopper stays in place.

Predict how the mass of the flask and contents would change. Explain your answer. [2]

(a) 0.90 (g) allow 0.88–0.92; allow 182.40 − 181.50(1)
(b) carbon dioxide (gas) is produced(1)
(b) (the gas) escapes from the flask / leaves the system allow 'is lost to the air' or 'goes through the cotton wool'; do not accept 'mass/atoms are destroyed'(1)
(c) lets the gas escape but stops acid (droplets / spray) leaving the flask allow stops liquid splashing out; ignore 'for safety' unqualified(1)
(d) (mass would) stay the same / not change allow 'stays at 182.40 g'(1)
(d) the carbon dioxide cannot escape / no atoms leave the flask (so total mass of products = total mass of reactants) a reason without the prediction can score this mark(1)
Answer: (a) 0.90 g (b) carbon dioxide gas forms and escapes from the flask (c) lets CO₂ out but stops acid spray escaping (d) no change, because the CO₂ cannot leave the sealed flask
Examiner tip: Mass is never 'destroyed': link the decrease to a named gas that leaves the flask, using the (g) state symbol in the equation as your clue.

9Grade 8–9Higher6 marks

Element X is a metal. When it is heated in oxygen it forms an oxide with the formula $\text{X}_2\text{O}_3$.

$$4\text{X} + 3\text{O}_2 \rightarrow 2\text{X}_2\text{O}_3$$

In an experiment, 10.8 g of X reacted completely to form 20.4 g of $\text{X}_2\text{O}_3$.

(a) Calculate the mass of oxygen that reacted. [1]

(b) The total relative formula mass of the products in the equation is 204.

Relative atomic mass ($A_r$): O = 16

Calculate the relative atomic mass of X.

Use the periodic table to identify element X. [3]

Give your answer to 3 significant figures. [2]

(a) 9.6 (g) allow 20.4 − 10.8(1)
(b) $M_r$ of $\text{X}_2\text{O}_3$ = 204 ÷ 2 = 102(1)
(b) $2A_r(\text{X}) = 102 − (3 \times 16) = 54$ so $A_r$ = 27 allow 204 − 96 = 108, 108 ÷ 4 = 27; correct answer with no working scores 2 marks(1)
(b) aluminium / Al allow ecf: the element matching their $A_r$ if it is a metal(1)
(c) $\dfrac{54}{102} \times 100$ allow ecf from (b): $\dfrac{2 \times \text{their } A_r}{\text{their } M_r} \times 100$; allow 10.8 ÷ 20.4 × 100(1)
(c) 52.9 (%) must be 3 s.f. for this mark; 52.94... or 53 scores the first mark only(1)
Answer: (a) 9.6 g (b) $A_r$ = 27, X is aluminium (c) 52.9%
Examiner tip: Divide the total of 204 by 2 first: the '2' in front of X₂O₃ means 204 is the mass of two formula units.

10Grade 6–76 marks

A class did three experiments.

Experiment	What was done	Mass at start in g	Mass at end in g
A	Copper powder heated strongly in an open crucible	1.27	1.59
B	Zinc carbonate heated strongly in an open crucible	2.50	1.62
C	Barium chloride solution mixed with sodium sulfate solution in a sealed flask	124.30	124.30

The equations for the reactions are:

A: $2\text{Cu(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CuO(s)}$

B: $\text{ZnCO}_3\text{(s)} \rightarrow \text{ZnO(s)} + \text{CO}_2\text{(g)}$

C: $\text{BaCl}_2\text{(aq)} + \text{Na}_2\text{SO}_4\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}$

A student said: “Experiments A and B show that mass is not always conserved in chemical reactions.”

Evaluate the student’s statement.

Use the results, the equations and your knowledge of particles in your answer. [6]

A: copper atoms combine with oxygen (atoms / molecules) from the air(1)
A: the oxygen was not weighed at the start because it came from outside the crucible (a non-enclosed system)(1)
A: the increase of 0.32 g is the mass of oxygen that reacted(1)
B: carbon dioxide gas is produced and escapes into the air(1)
B: the decrease of 0.88 g is the mass of carbon dioxide that left the crucible(1)
C: no gas is involved / the flask is sealed, so no atoms can enter or leave and the mass stays the same(1)
in every reaction atoms are not created or destroyed, only rearranged (shown by the balanced equations having the same number of each type of atom on both sides)(1)
if the oxygen used (A) or carbon dioxide lost (B) were included / measured, total mass of reactants = total mass of products(1)
conclusion: the student is wrong; mass is conserved in all three reactions, the changes in A and B are only apparent because A and B are not enclosed systems(1)

Level	Marks	What it looks like
Level 3	5–6	Explains all three results using the particle model, linking each mass change to a gas joining or leaving the system, supported by correct balanced equations or mass data, and concludes clearly that mass is conserved so the student's claim is wrong.
Level 2	3–4	Explains at least two of the results by linking the change in mass to oxygen being gained or carbon dioxide being lost, with some reference to atoms or equations; the conclusion may be implied or only partly justified.
Level 1	1–2	Makes simple relevant statements, e.g. that a gas is given off or that oxygen is added, but with little or no explanation of why mass appears to change.
Level 0	0	No relevant content.

Answer: The student is wrong. In A, oxygen from the air (not weighed at the start) combined with the copper, adding 0.32 g. In B, carbon dioxide gas (0.88 g) escaped from the open crucible. In C nothing could enter or leave, so the mass stayed the same. Atoms are only rearranged, so mass is conserved in every reaction; A and B just weren't enclosed systems.
Examiner tip: 'Evaluate' needs a judgement: top answers explain each experiment and finish by saying clearly whether the student is right, with the reason.

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