A-Level Chemistry May/June 2025 Q4(a)(iv): The reaction between Na2S2O3(aq) and HCl(aq) is repeated in a second experiment. In thi…

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  Step 1: Determine the limiting reactant

  First, calculate the moles of Na₂S₂O₃:

  $Moles\ of\ Na_2S_2O_3 = concentration \times volume$

  $Moles\ of\ Na_2S_2O_3 = 0.050\ mol\ dm^{-3} \times \frac{25.0}{1000}\ dm^3 = 0.00125\ mol$

  The moles of HCl are given as $0.0020\ mol$.

  The balanced equation for the relevant species is:

  $Na_2S_2O_3(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + SO_2(g) + S(s)$

  The stoichiometric ratio of $Na_2S_2O_3 : HCl$ is $1:2$.

  To react completely with $0.00125\ mol$ of $Na_2S_2O_3$, the moles of HCl required would be:

  $Moles\ of\ HCl\ needed = 2 \times 0.00125 = 0.00250\ mol$

  Since only $0.0020\ mol$ of HCl is available, HCl is the limiting reactant.

  Step 2: Calculate moles of sulfur produced

  The amount of sulfur produced is determined by the limiting reactant, HCl.

  The stoichiometric ratio of $HCl : S$ is $2:1$.

  $Moles\ of\ S = \frac{Moles\ of\ HCl}{2}$

  $Moles\ of\ S = \frac{0.0020}{2} = 0.0010\ mol$ (M1)

  Step 3: Calculate the number of sulfur atoms

  Use the Avogadro constant, $L = 6.02 \times 10^{23}\ mol^{-1}$.

  $Number\ of\ atoms = moles \times L$

  $Number\ of\ S\ atoms = 0.0010 \times (6.02 \times 10^{23})$

  $Number\ of\ S\ atoms = 6.02 \times 10^{20}$ (M2)

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  How the marks are awarded

  • M1 — The first mark is for correctly calculating the moles of sulfur produced as 0.0010 mol. This requires identifying HCl as the limiting reactant and then using the 2:1 stoichiometric ratio from the balanced equation between HCl and S.
  • M2 — The second mark is for correctly converting the moles of sulfur calculated in the first step into the number of individual atoms by multiplying by the Avogadro constant (6.02 x 10²³), giving the final answer of 6.02 x 10²⁰.

  Common mistakes

  • Incorrectly identifying Na₂S₂O₃ as the limiting reactant and using its moles (0.00125 mol) to calculate the moles of sulfur, leading to an incorrect final answer.
  • Using the wrong stoichiometric ratio, for example, a 1:1 ratio between HCl and S, which would result in calculating 0.0020 mol of sulfur.
  • Stopping after calculating the moles of sulfur (0.0010 mol) and not proceeding to calculate the number of atoms using the Avogadro constant.
  • Making a calculation error when using standard form, such as incorrectly entering 6.02 x 10²³ into the calculator or misplacing the decimal point in the final answer.

  Examiner tip: Always determine the limiting reactant first in any calculation where the quantities of more than one reactant are provided.

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