This question is about acids and alkalis - AQA - GCSE Chemistry - Question 8 - 2022 - Paper 1

The concentration of the acid also affects its pH. A higher concentration of acid means that more acid molecules are present in the solution, which leads to a greater number of H⁺ ions being released. Thus, as the concentration increases, so does the concentration of H⁺ ions, causing a further decrease in pH.

The student used a volume of 23.50 cm³ of barium hydroxide solution because this value is the mean of the titrations from the second to the fifth measurements, excluding the first titration which is often an anomalous result. Averaging the consistent values provides a more reliable figure for the titration calculation.

First, we calculate the moles of barium hydroxide used:

$\text{Moles of Ba(OH)₂} = \frac{23.50}{1000} \times 0.100 = 0.00235$

Since the balanced equation shows a ratio of 2:1 for HCl to Ba(OH)₂, the moles of HCl will be:

$\text{Moles of HCl} = 0.00235 \times 2 = 0.00470$

To find the concentration of hydrochloric acid:

$\text{Concentration of HCl} = \frac{0.00470}{0.02500} \times 1000 = 0.188 \text{ mol/dm}^3$

When sulfuric acid is neutralised, the resulting barium sulfate precipitates as a solid and does not dissociate into ions in solution. Since there are no free-moving ions to carry the electric charge, the electrical conductivity is zero. The reaction can be represented as:

$\text{H}_2 ext{SO}_4(aq) + ext{Ba(OH)}_2(aq) → ext{BaSO}_4(s) + 2 ext{H}_2 ext{O}(l)$

The electrical conductivity of the mixture increased as the addition of barium hydroxide provides more hydroxide ions (OH⁻) in solution, which are free to move and carry charge, thereby enhancing the overall conductivity.