Grade 10 Physical Sciences · Chemical Change

Calculate number of moles

Calculate number of moles from mass, particles or gas volume using n = m/M (36 g of water is 2 mol).

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How to calculate moles using mass | Stoichiometry grade 10, 11 and 12Miss Martins Maths and Science

Quantitative Aspects of Chemical Change: Calculate molsKevinmathscience

Avogadros number | How to calculate number of moles | Grade 10 ChemistryMiss Martins Maths and Science

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Question 9.4 · May/June 2025 · Paper 2 · National (DBE) · 4 marks

The simplified diagram below represents an electrolytic cell used to demonstrate the electrolysis of a concentrated sodium chloride solution, NaCℓ(aq).

X and Y are carbon electrodes. Diagram for this question

9.4 Calculate the number of electrons transferred through the external circuit when

300

^{3}

gas is collected at electrode X.

Take the molar gas volume as

24

^{3}

·mol

^{- 1}

.(4)

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: calculate number of moles

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3. Question 5.4.2 · May/June 2022 · Paper 2 · National (DBE) · 2 marks
Learners use the reaction of $MgCO_{3}$ (s) with EXCESS dilute HC $ℓ$ (aq) to investigate the relationship between temperature and the rate of a chemical reaction.

The balanced equation for the reaction is:

$MgCO_{3} (s) + 2 HC ℓ (aq) \to MgC ℓ_{2} (aq) + CO_{2} (g) + H_{2} O (ℓ)$

The results obtained are represented in the graph below.
5.4 The learners obtained the graph above using $5$ g $MgCO_{3}$ (s) with EXCESS HC $ℓ$ at $40$ °C.

Calculate the:
5.4.2 Molar gas volume at $40$ °C if $1, 5$ dm $^{3}$ $CO_{2}$ is collected in a syringe(2)
Show the memo (2 ticks)Memo
1. ✓
  Substitution of n(CO2) and 1,5 dm³ $n = \frac{V}{V _{m}}$
  $0, 06 = \frac{1 , 5}{V _{m}}$
  1 mark · Calculate number of moles
2. ✓
  Final answer $V_{m} = 25$ dm $^{3}$ ·mol $^{- 1}$ ( $25, 21$ dm $^{3}$ ·mol $^{- 1}$ )
  1 mark · Calculate number of moles
4. Question 9.2.3 · November 2025 · Paper 2 · National (DBE) · 6 marks
The diagram below represents a cell used for the refining of copper.

The unrefined copper contains zinc as the only impurity.
9.2 When the switch is closed, it is found after T hours that the amount of $Cu^{2 +}$ (aq) ions in the electrolyte changed by $0, 05$ moles and $0, 15$ moles of Cu(s) were deposited on electrode Q.

Answer the questions that follow.
9.2.3 Calculate the change in mass of electrode R after T hours.(6)
Show the memo (6 ticks)Memo
1. ✓
  (a) Using ratio n(Cu2+) : n(Zn2+) = 1 : 1 $n (Zn^{2 +}) = n (Cu^{2 +}) = 0, 05$ mol
  1 mark · Use the mole ratio
2. ✓
  (b) Formula n = m/M $m (Zn) = n M$
  1 mark · Calculate number of moles
3. ✓
  (c) Substitute 65 and n(Zn2+) $= (0, 05) (65) = 3, 25$ g Zn from R
  1 mark · Calculate number of moles
4. ✓
  (d) Subtraction of moles $0, 15 - 0, 05 = 0, 1$ mol Cu (oxidised at R)
  1 mark · Describe an electrolytic cell
5. ✓
  (e) Substitute 63,5 and n(Cu) $m (Cu) = n M = 0, 1 (63, 5) = 6, 35$ g Cu from R
  1 mark · Calculate number of moles
6. ✓
  (f) Final answerChange in mass $= 6, 35 + 3, 25 = 9, 6$ g (decrease in mass)
  1 mark · Calculate number of moles
5. Question 5.3.2 · November 2022 · Paper 2 · National (DBE) · 4 marks
Three experiments, A, B and C, are carried out to investigate some of the factors that affect the rate of decomposition of hydrogen peroxide, $H_{2} O_{2} (ℓ)$ .

The balanced equation for the reaction is:

$2 H_{2} O_{2} (ℓ) \to 2 H_{2} O (ℓ) + O_{2} (g)$

Identical samples of hydrogen peroxide are used in each experiment.

The conditions used in each experiment are summarised in the table below.
5.3 The volume of oxygen gas, $O_{2}$ (g), produced in experiment B during the first $3, 6$ s is collected in a syringe, as shown below.
5.3.2 Calculate the mass of water, $H_{2} O (ℓ)$ , that was produced during the first $3, 6$ s. Take the molar gas volume to be $24000$ cm $^{3}$ ·mol $^{- 1}$ at $25$ °C.(4)
Show the memo (4 ticks)Memo
1. ✓
  Substitute into n = V/Vm $n (O_{2}) = \frac{V}{V _{m}} = \frac{560}{24 000} = 0, 023$ mol $(0, 0233)$
  1 mark · Calculate number of moles
2. ✓
  Mole ratio $n (H_{2} O) = 2 n (O_{2}) = 2 (0, 023) = 0, 046$ mol $(0, 0467)$
  1 mark · Use the mole ratio
3. ✓
  Substitute into m = nM $m = n M = 0, 046 \times 18$
  1 mark · Calculate number of moles
4. ✓
  Final answer $m = 0, 83$ g
  1 mark · Calculate number of moles
6. Question 5.4 · May/June 2025 · Paper 2 · National (DBE) · 6 marks
The reaction between magnesium carbonate pellets, $MgCO_{3}$ (s), and EXCESS dilute hydrochloric acid, HCℓ(aq), is used to investigate the effect of concentration on the rate of a reaction.

$MgCO_{3} (s) + 2 HC ℓ (aq) \to MgC ℓ_{2} (aq) + H_{2} O (ℓ) + CO_{2} (g)$

The apparatus used for this investigation is shown below.
5.4 For curve B, calculate the average rate at which $CO_{2}$ (g) is produced for the first $120$ s in dm $^{3}$ ·s $^{- 1}$ . The molar gas volume is $24, 5$ dm $^{3}$ ·mol $^{- 1}$ .(6)
Show the memo (6 ticks)Memo
1. ✓
  Mass subtraction $m (CO_{2}) = 144, 5 - 143, 87 = 0, 63$ g
  1 mark · Use conservation of mass
2. ✓
  Formula $n (CO_{2}) = \frac{m}{M}$
  1 mark · Calculate number of moles
3. ✓
  Substitute M = 44 $n (CO_{2}) = \frac{0 , 63}{44} = 1, 43 \times 1 0^{- 2}$ mol
  1 mark · Calculate number of moles
4. ✓
  Substitute 24,5 dm3 in V = nVm $V (CO_{2}) = n V_{m} = (1, 43 \times 1 0^{- 2}) (24, 5) = 0, 35$ dm $^{3}$
  1 mark · Calculate number of moles
5. ✓
  Substitute into rate formulaAve rate $= \frac{Δ V ( CO _{2} )}{Δ t} = \frac{0 , 35 - 0}{120 - 0}$
  1 mark · Calculate reaction rate
6. ✓
  Final answer $= 2, 92 \times 1 0^{- 3}$ dm $^{3}$ ·s $^{- 1}$
  1 mark · Calculate reaction rate

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