AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2020: Question 10

10 marks · Standard Demand difficulty · Short Answer

Explain conservation of mass using experimental data, describe purification steps for an insoluble salt, and calculate the percentage atom economy to 3 significant figures.

Practise this question

Question

Question 10 is about silver iodide and conservation of mass. Part 10.1 gives a 5-step method mixing silver nitrate and sodium iodide solutions in beakers A and B, accompanied by Table 7 showing masses before and after mixing (Beaker A: 78.26 g before, 108.22 g after; Beaker B: 78.50 g before, 48.54 g after), asking to explain how results demonstrate conservation of mass. Parts 10.2 to 10.4 ask how to separate the insoluble silver iodide, suggest an impurity removed by rinsing with water, and explain why the silver iodide is warmed. Part 10.5 asks to calculate percentage atom economy to 3 significant figures using given relative formula masses, and 10.6 asks for one industrial reason for using reactions with high atom economy.
Question text

10 This question is about silver iodide.

Silver iodide is produced in the reaction between silver nitrate solution and

sodium iodide solution.

The equation for the reaction is:

AgNO3(aq) + NaI(aq) → AgI(s) + NaNO3(aq)

10.1 A student investigated the law of conservation of mass.

This is the method used.

1. Pour silver nitrate solution into a beaker labelled A.

2. Pour sodium iodide solution into a beaker labelled B.

3. Measure the masses of both beakers and their contents.

4. Pour the solution from beaker B into beaker A.

5. Measure the masses of both beakers and their contents again.

Table 7 shows the student’s results.

Table 7

Mass before mixing in g Mass after mixing in g

Beaker A and contents 78.26 108.22

Beaker B and contents 78.50 48.54

Explain how the results demonstrate the law of conservation of mass.

You should use data from Table 7 in your answer.

[2 marks]

10.2 Suggest how the student could separate the insoluble silver iodide from the mixture at

the end of the reaction.

[1 mark]

The student purified the separated silver iodide.

This is the method used.

1. Rinse the silver iodide with distilled water.

2. Warm the silver iodide.

10.3 Suggest one impurity that was removed by rinsing with water.

[1 mark]

10.4 Suggest why the student warmed the silver iodide.

[1 mark]

10.5 Calculate the percentage atom economy for the production of silver iodide in

this reaction.

The equation for the reaction is:

AgNO3(aq) + NaI(aq) → AgI(s) + NaNO3(aq)

Give your answer to 3 significant figures.

Relative formula masses (Mr): AgNO3 = 170 NaI = 150 AgI = 235 NaNO3 = 85

[4 marks]

Percentage atom economy (3 significant figures) = %

10.6 Give one reason why reactions with a high atom economy are used in industry.

[1 mark]

Mark scheme

Show the mark scheme Mark scheme for Question 10 shows: 10.1 awards 1 mark for showing total mass before equals total mass after (156.76 g) or mass change equal and opposite (+/- 29.96 g), and 1 mark for stating mass of products equals mass of reactants / no change in mass; 10.2 awards 1 mark for filtration; 10.3 awards 1 mark for sodium nitrate solution, silver nitrate, or sodium iodide; 10.4 awards 1 mark for removing/evaporating water or drying; 10.5 awards 4 marks across total Mr (320), division (235/320 * 100), unrounded percentage (73.4375%), and rounding to 3 significant figures (73.4%); 10.6 awards 1 mark for sustainable development, economic reasons, or higher proportion of useful product/reducing waste.

Question 10

AO /

Question Answers Extra information Mark

Spec. Ref.

10.1 (total) mass before = 156.76 (g) allow 78.26 + 78.50 = 156.76 1 AO2

and and

(total) mass after = 156.76 (g) 108.22 + 48.54 = 156.76

or

increase in mass of beaker A allow 108.22 – 78.26 = 29.96

and contents = 29.96 (g) and

and 48.54 – 78.50 = – 29.96

decrease in mass of beaker B

and contents = 29.96 (g)

(so) the mass of products allow (so) no atoms were lost or 1 AO1

equals the mass of the reactants made during the reaction

or

4.3.1.1

(so) there is no change in mass

during the reaction

10.2 filter / filtration allow a description of filtration 1 AO2

4.1.1.2

10.3 allow correct formulae AO2

4.1.1.2

sodium nitrate (solution) allow sodium / nitrate / silver / 1

or iodide ions

silver nitrate (solution)

or

sodium iodide (solution)

10.4 to remove / evaporate the water allow to dry (the solid) 1 AO3

4.1.1.2

Question 10 continued

AO /

Spec. Ref.

10.5 AO2

(total Mr = 170 + 150) = 320 allow (235 + 85) = 320 1 4.3.3.2

(% atom economy =)

24 235

allow correct use of incorrectly 1

×100 calculated total M

320 r

= 73.4375 (%)

= 73.4 (%) 1

allow an answer correctly

calculated to 3 significant figures

from an incorrect percentage

calculation which uses the

values in the question

10.6 any one from: 1 AO1

4.3.3.2

• for sustainable development allow to reduce waste

• for economic reasons

• to produce a high(er)

percentage of useful product

Total 10

How to answer it

Precipitation of Silver Iodide: Quantitative Analysis & Separation

📌 What this question tests

This question assesses practical and quantitative chemistry skills in AQA GCSE Chemistry Paper 1 (Topics 4.1 & 4.3):

  • Conservation of Mass: Calculating total mass before and after a reaction to prove mass is conserved.
  • Separation & Purification: Identifying filtration, washing precipitates to remove soluble salts, and drying to remove water.
  • Atom Economy Calculation: Using relative formula masses (Mr) to calculate percentage atom economy to a required number of significant figures (3 s.f.).
  • Industrial Chemistry: Understanding the economic and environmental benefits of high atom economy.
Question 10.1

Conservation of Mass from Experimental Data

Explain how the results demonstrate the law of conservation of mass. [2 marks]

✅ Correct Answer

  • Mark 1 (Data): Total mass before = 78.26 + 78.50 = 156.76 g
    AND total mass after = 108.22 + 48.54 = 156.76 g
    (OR: Increase in Beaker A = 29.96 g AND Decrease in Beaker B = 29.96 g)
  • Mark 2 (Explanation): Total mass of reactants equals the total mass of products (or: no mass was gained or lost / no atoms were made or destroyed).

🧠 Exam Technique

The prompt explicitly says: "You should use data from Table 7". Whenever a question says to use data, you must show numerical totals. Stating the law without proving it with numbers gets 0 of the first mark.

❌ Common Errors

  • Only adding one side (e.g. calculating 156.76 g before, but failing to state the sum after).
  • Quoting individual beaker masses without adding them together to find the totals.
Mark breakdown: [1 mark] for both totals calculated correctly (156.76 g). [1 mark] for clearly stating that mass of products = mass of reactants / no change in mass.
Question 10.2

Separating an Insoluble Salt

Suggest how the student could separate the insoluble silver iodide from the mixture. [1 mark]

✅ Correct Answer

Filter or filtration (or a description of filtering using filter funnel and paper).

💡 Key Knowledge

Check state symbols in the equation: AgI(s) is a solid precipitate, while the other substances are aqueous solutions (aq) . An insoluble solid is always separated from a solution using filtration.

❌ Common Errors

Confusing filtration with crystallisation, distillation, or chromatography. Crystallisation is used to separate a soluble solid from a solvent, not an insoluble precipitate!

Mark breakdown: [1 mark] for identifying filtration.
Questions 10.3 & 10.4

Purification: Washing and Drying the Precipitate

✅ 10.3: Impurity Removed by Rinsing [1 mark]

  • Sodium nitrate / NaNO₃ (solution)
  • Also accepted: Unreacted silver nitrate ( AgNO₃ ) or unreacted sodium iodide ( NaI ); or named ions (sodium / nitrate / silver / iodide ions).

✅ 10.4: Purpose of Warming [1 mark]

  • To remove / evaporate the water
  • Also accepted: To dry the solid / precipitate.

💡 Key Knowledge: Precipitate Preparation

  1. Filter: Collect the solid residue.
  2. Wash with distilled water: Removes soluble by-products (here, NaNO₃ ) clinging to the wet solid.
  3. Warm / Dry: In an oven or warm desiccator to evaporate residual wash water, giving a pure, dry sample.

❌ Common Errors

  • 10.3: Answering "AgI" (that is the desired product, not an impurity!).
  • 10.4: Saying "to speed up the reaction" — the reaction is already finished!
Mark breakdown: 10.3 [1 mark] for naming a soluble chemical species present in the mixture. 10.4 [1 mark] for removing water / drying.
Question 10.5

Calculating Percentage Atom Economy

Calculate the percentage atom economy for the production of silver iodide. Give your answer to 3 significant figures. [4 marks]

📐 Step-by-Step Calculation

Equation: AgNO₃(aq) + NaI(aq) → AgI(s) + NaNO₃(aq)

Given values: Mr(AgNO₃) = 170, Mr(NaI) = 150, Mr(AgI) = 235, Mr(NaNO₃) = 85

  1. Step 1: Calculate total Mr of all reactants (or all products)
    Total Mr = 170 + 150 = 320
    (Check: 235 + 85 = 320 — total mass of reactants equals total mass of products) [1 mark]
  2. Step 2: Set up the Atom Economy formula
    $\text{Atom Economy} = \frac{M_{\text{r}}\text{ of desired product}}{\text{Total } M_{\text{r}} \text{ of all reactants}} \times 100$ Desired product is AgI (Mr = 235):
    $\text{Atom Economy} = \frac{235}{320} \times 100$ [1 mark]
  3. Step 3: Calculate the unrounded percentage
    $\frac{235}{320} \times 100 = 73.4375\%$ [1 mark]
  4. Step 4: Round to 3 significant figures
    73.4375 rounded to 3 s.f. = 73.4% [1 mark]

❌ Common Errors & Traps

  • Significant figures penalty: Leaving the answer as 73% (2 s.f.) or 73.44% (4 s.f.) loses the 4th mark. Always check instructions for rounding!
  • Wrong denominator: Dividing 235 by only one of the reactants instead of the sum of all reactants.
  • Inverting the fraction: Dividing total Mr by desired Mr gives an answer > 100%, which is impossible!

🧠 Top Examiner Tip

Full error-carried-forward (ECF) applies here! If you make an arithmetic error in Step 1, you can still gain Marks 2, 3, and 4 if you carry your incorrect denominator through correctly and round your final answer to 3 significant figures.

Question 10.6

Industrial Importance of High Atom Economy

Give one reason why reactions with a high atom economy are used in industry. [1 mark]

✅ Any One Acceptable Reason

  • For sustainable development / conserve natural resources
  • For economic reasons (cheaper / saves money / higher profit)
  • To reduce waste (less useless by-product to dispose of)
  • To produce a higher proportion of useful product

💡 Atom Economy vs Percentage Yield

Atom economy is about reaction design: how many reactant atoms end up in the desired product rather than waste.

High atom economy means less raw material is wasted, saving costs on raw materials and disposal of unwanted by-products.

❌ Common Errors

Vague answers like "it's quicker", "it gives a higher yield", or "it's more efficient" without qualification will not score. Focus on reducing waste or reducing costs.

Mark breakdown: [1 mark] for any clear economic, environmental, or waste-reduction reason.

Topics

Chemistry · C1: Atomic Structure and the Periodic Table · C3: Quantitative Chemistry

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.