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

The pH of an acid also depends on its concentration; an increased concentration of the acid results in a greater number of H⁺ ions in solution, which lowers the pH further.

The student used the mean of the titration results from titrations 2 to 5, which is 23.50 cm³, as it provides a more accurate measurement. This value is less influenced by any anomalous results.

To calculate the concentration of hydrochloric acid, we first find the moles of barium hydroxide:

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

Since the reaction ratio is 2:1, the moles of HCl will be:

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

Now, we can find the concentration of HCl:

$\text{concentration} = \frac{0.00470}{0.025} \times 1000 = 0.188 \text{ mol/dm³}$

When sulfuric acid is neutralised, it forms barium sulfate which is a solid and does not have ions that are free to move in solution. Consequently, there are no ions present to carry the charge, leading to zero electrical conductivity.

The increase in electrical conductivity after adding more barium hydroxide is due to the presence of excess hydroxide ions, OH⁻, which are now available in solution and can carry charge.