Grade 10 Physical Sciences · Chemical Change

Calculate concentration of a solution

Calculate the concentration of a solution with c = n/V, volume in dm³, or work out a dilution (2 mol in 4 dm³ is 0,5 mol/dm³).

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Question 5.3 · November 2023 · Paper 2 · National (DBE) · 3 marks

The reaction between EXCESS dilute hydrochloric acid and sodium thiosulphate is used to investigate factors that influence reaction rate.

Na_{2} S_{2} O_{3} (aq) + 2 HCl(aq) \to 2 NaCl(aq) + S(s) + H_{2} O (ℓ) + SO_{2} (g)

The concentration of HCl(aq) used is

1

mol·dm

^{- 3}

. The same volume of HCl(aq) is used in each run.

The time taken for the cross on the paper under the flask to become invisible is measured.

The table below summarises the reaction conditions and results of the experiment. Diagram for this question

5.3 Calculate the value of P in the table.(3)

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

More past paper questions: calculate concentration of a solution

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3. Question 7.1.1 · May/June 2024 · Paper 2 · National (DBE) · 3 marks
7.1 A standard solution is prepared by dissolving 10 g of sodium carbonate, $Na_{2} CO_{3}$ (s), in $0, 7$ dm $^{3}$ of water.

Answer the questions that follow.
7.1.1 Calculate the concentration of the solution.(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $c = \frac{m}{M V}$
  1 mark · Calculate concentration of a solution
2. ✓
  Substitution $c = \frac{10}{( 106 ) ( 0 , 7 )}$
  1 mark · Calculate concentration of a solution
3. ✓
  Answer with unit $c = 0, 13$ mol·dm $^{- 3}$
  1 mark · Calculate concentration of a solution
4. Question 7.2.1 · November 2023 · Paper 2 · National (DBE) · 3 marks
To identify metal M in an unknown metal carbonate, $MCO_{3}$ , the following procedure is carried out:

Step 1: $0, 198$ g of IMPURE $MCO_{3}$ is reacted with $25$ cm $^{3}$ of $0, 4$ mol·dm $^{- 3}$ nitric acid, $HNO_{3}$ (aq).

Step 2: The EXCESS $HNO_{3}$ (aq) is then neutralised with $20$ cm $^{3}$ of $0, 15$ mol·dm $^{- 3}$ barium hydroxide, $Ba(OH)_{2}$ (aq).

Assume that the volumes are additive.

The following reactions take place:

$2 HNO_{3} (aq) + MCO_{3} (s) \to M(NO_{3})_{2} (aq) + CO_{2} (g) + H_{2} O (ℓ)$

$2 HNO_{3} (aq) + Ba(OH)_{2} (aq) \to Ba(NO_{3})_{2} (aq) + 2 H_{2} O (ℓ)$
7.2 Calculate the:
7.2.1 Number of moles of $Ba(OH)_{2}$ (aq) that reacted with the excess $HNO_{3}$ (aq)(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $n (Ba(OH)_{2}) = c V$
  1 mark · Calculate concentration of a solution
2. ✓
  Substitution $= (0, 15) (0, 02)$
  1 mark · Calculate concentration of a solution
3. ✓
  Answer with unit $= 0, 003$ mol
  1 mark · Calculate concentration of a solution
5. Question 5.3.1 · May/June 2023 · Paper 2 · National (DBE) · 4 marks
Consider the following decomposition reaction that takes place in a sealed $2$ dm $^{3}$ 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.
5.3 Calculate the:
5.3.1 Mass of $NO_{2}$ (g) present in the container at $400$ s(4)
Show the memo (4 ticks)Memo
1. ✓
  Formula $c (NO_{2}) = \frac{m}{M V}$
  1 mark · Calculate concentration of a solution
2. ✓
  Substitute change in concentration $200 \times 1 0^{- 4} = \frac{m}{...}$
  1 mark · Calculate concentration of a solution
3. ✓
  Substitute M (46) and V (2) $200 \times 1 0^{- 4} = \frac{m}{( 46 ) ( 2 )}$
  1 mark · Calculate concentration of a solution
4. ✓
  Final answer $m = 1, 84$ g
  1 mark · Calculate concentration of a solution
6. Question 7.2.1 · November 2022 · Paper 2 · National (DBE) · 3 marks
7.2 A flask contains $300$ cm $^{3}$ of dilute sodium hydroxide, NaOH(aq), of concentration $0, 167$ mol·dm $^{- 3}$ .

Answer the questions that follow.
7.2.1 Calculate the number of moles of sodium hydroxide in the flask.(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $n (NaOH) = c V$
  1 mark · Calculate concentration of a solution
2. ✓
  Substitution $n = (0, 167) (0, 300)$
  1 mark · Calculate concentration of a solution
3. ✓
  Answer with unit $n (NaOH) = 0, 05$ mol $(5 \times 1 0^{- 2}$ mol $)$
  1 mark · Calculate concentration of a solution

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