Grade 10 Physical Sciences · Chemical Change

Use the mole ratio

Use the numbers in a balanced equation to change moles of one substance into moles of another (2 mol H₂ makes 2 mol H₂O).

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Stoichiometry: Mole Ratios | Grade 10, 11 and 12 ChemistryMiss Martins Maths and Science

How to Find the Mole Ratio to Solve Stoichiometry ProblemsWayne Breslyn (Dr. B.)

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Question 5.2 · May/June 2023 · Paper 2 · National (DBE) · 2 marks

Consider the following decomposition reaction that takes place in a sealed

2

^{3}

container:

2 N_{2} O_{5} (g) \to 4 NO_{2} (g) + O_{2} (g)

The graph below shows how the concentrations of

N_{2} O_{5}

(g) and

NO_{2}

(g) change with time. Diagram for this question

5.2 Consider the statement below:

The rate of decomposition of

N_{2} O_{5}

(g) is half the rate of formation of

NO_{2}

(g).

Is this statement TRUE or FALSE? Give a reason for the answer.(2)

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

More past paper questions: use the mole ratio

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3. Question 5.2.2 · November 2025 · Paper 2 · National (DBE) · 5 marks
5.2 Carbon monoxide, CO(g), reacts with oxygen, $O_{2}$ (g), to form carbon dioxide, $CO_{2}$ (g), in a sealed container according to the balanced equation:

$2 CO(g) + O_{2} (g) \to 2 CO_{2} (g)$

The graph below shows the concentration of $O_{2}$ (g) versus time.

Answer the questions that follow.
5.2.2 The reaction takes place in a $3$ dm $^{3}$ container. Calculate the average rate (in mol·s $^{- 1}$ ) at which $CO_{2}$ (g) is formed in the first 10 s.(5)
Show the memo (5 ticks)Memo
1. ✓
  (a) Calculate the change in concentrationRate $= - \frac{Δ c ( O _{2} )}{Δ t}$
  $= - \frac{( 0 , 15 - 0 , 27 )}{\dots}$
  1 mark · Calculate reaction rate
2. ✓
  (b) Substitute 10 s $= - \frac{( 0 , 15 - 0 , 27 )}{10 - 0}$
  $= 0, 012$ mol·dm $^{- 3}$ ·s $^{- 1}$
  1 mark · Calculate reaction rate
3. ✓
  (c) Multiply by 3 dm3Rate ( $O_{2}$ ) in mol·s $^{- 1}$ $= c V = (0, 012) (3) = 0, 036$ mol·s $^{- 1}$
  1 mark · Calculate concentration of a solution
4. ✓
  (d) Use the mol ratioRate ( $CO_{2}$ ) $= 2 \times$ rate ( $O_{2}$ ) $= 2 \times 0, 036$
  1 mark · Use the mole ratio
5. ✓
  (e) Final answer $= 0, 072$ (mol·s $^{- 1}$ )
  1 mark · Use the mole ratio
4. Question 2.3.2 · November 2025 · Paper 2 · National (DBE) · 5 marks
The letters A to G in the table below represent organic compounds.
2.3 Compound E, $C_{3} H_{8}$ (g), reacts with oxygen, $O_{2}$ (g), according to the balanced equation:

$C_{3} H_{8} (g) + 5 O_{2} (g) \to 3 CO_{2} (g) + 4 H_{2} O(g)$

Initially $8$ cm $^{3}$ of compound E and $50$ cm $^{3}$ of oxygen were injected into a container of adjustable volume and allowed to react, as shown below.

Answer the questions that follow.
2.3.2 Calculate the TOTAL volume of the GASES present in the container at the end of the reaction.(5)
Show the memo (5 ticks)Memo
1. ✓
  (a) Volume of O2 used (with volume of C3H8) $V (O_{2})_{used} = 5 V (C_{3} H_{8}) = (5) (8) = 40$ cm $^{3}$
  1 mark · Use the mole ratio
2. ✓
  (b) Volumes of CO2 and H2O produced $V (CO_{2})_{formed} = 3 V (C_{3} H_{8}) = (3) (8) = 24$ cm $^{3}$
  $V (H_{2} O)_{formed} = 4 V (C_{3} H_{8}) = (4) (8) = 32$ cm $^{3}$
  1 mark · Use the mole ratio
3. ✓
  (c) Volume of O2 remaining $V (O_{2})_{unused} = 50 - 40 = 10$ cm $^{3}$
  1 mark · Identify the limiting reagent
4. ✓
  (d) Addition of the three volumesVolume of gas in the container $= 10 + 24 + 32$
  1 mark · Identify the limiting reagent
5. ✓
  (e) Final answerTotal volume of gas in the container $= 66$ cm $^{3}$ (or $0, 066$ dm $^{3}$ )
  1 mark · Identify the limiting reagent
5. Question 7.2.1 · May/June 2023 · Paper 2 · National (DBE) · 3 marks
7.2 A hydrochloric acid solution, HCℓ(aq), is standardised by titrating it against $25$ cm $^{3}$ of a $0, 04$ mol·dm $^{- 3}$ sodium carbonate solution $Na_{2} CO_{3}$ (aq). At the endpoint, it is found that $19, 5$ cm $^{3}$ of HCℓ(aq) has reacted.

The balanced equation for the reaction is:

$Na_{2} CO_{3} (aq) + 2 HCl(aq) \to 2 NaCl(aq) + CO_{2} (g) + H_{2} O (ℓ)$

Answer the questions that follow.
7.2.1 Calculate the concentration of the HCℓ(aq).(3)
Show the memo (3 ticks)Memo
1. ✓
  Substitute 0,04 mol·dm⁻³, 25 cm³ and 19,5 cm³ $\frac{c _{a} V _{a}}{c _{b} V _{b}} = \frac{n _{a}}{n _{b}}$
  $\frac{c _{a} ( 19 , 5 )}{( 0 , 04 ) ( 25 )} = ...$
  1 mark · Do titration calculations
2. ✓
  Use mol ratio n(Na2CO3) : n(HCℓ) = 1 : 2 $\frac{c _{a} ( 19 , 5 )}{( 0 , 04 ) ( 25 )} = \frac{2}{1}$
  1 mark · Use the mole ratio
3. ✓
  Final answer $c_{a} = 0, 10$ mol·dm $^{- 3}$ ( $0, 103$ )
  1 mark · Do titration calculations
6. Question 9.4 · May/June 2024 · Paper 2 · National (DBE) · 5 marks
The simplified diagram below represents the cell used for electroplating ornaments with silver, Ag. P and Q are the two terminals of the battery.
9.4 Calculate the current needed to electroplate the ornament with $3, 25$ g of silver in $30$ minutes.(5)
Show the memo (5 ticks)Memo
1. ✓
  Moles of silver $n (Ag) = \frac{m}{M} = \frac{3 , 25}{108} = 0, 03$ mol
  1 mark · Calculate number of moles
2. ✓
  Moles of electrons $n (e^{-}) = \frac{N}{N _{A}}$ : $0, 03 = \frac{N}{\dots}$
  1 mark · Use the mole ratio
3. ✓
  Substitute Avogadro's number $0, 03 = \frac{N}{6 , 02 \times 1 0 ^{23}}$
  $N_{e^{-}} = 1, 81 \times 1 0^{22}$
  $Q = N_{e^{-}} q_{e} = (1, 81 \times 1 0^{22}) (1, 6 \times 1 0^{- 19}) = 2889, 6$ C
  1 mark · Calculate number of moles
4. ✓
  Substitute the time $I = \frac{Q}{Δ t} = \frac{2 889 , 6}{30 ( 60 )}$
  1 mark · Calculate current or voltage from charge
5. ✓
  Final answer $I = 1, 61$ A
  1 mark · Calculate current or voltage from charge

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