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

The pH is also affected by the concentration of the acid. A higher concentration of acid means a greater number of H⁺ ions present in the solution. Consequently, as the concentration increases, the pH decreases. Thus, a more concentrated acid will have a lower pH.

The student calculated the mean volume from titration numbers 2 to 5, which were more accurate than the first titration. This was done to reduce anomalies, ensuring a reliable mean value was used for the titration calculation.

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

$\text{moles } Ba(OH)_2 = \frac{23.50}{1000} \times 0.100 = 0.00235 \text{ mol}$

Since the mole ratio from the equation is 2:1 (2 moles of HCl for each mole of Ba(OH)₂), it follows that:

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

Next, the concentration is calculated as:

$\text{concentration } = \frac{0.00470 \text{ mol}}{25.00 \text{ cm}^3} \times 1000 = 0.188 \text{ mol/dm}³$

After neutralisation, the mixture contains solid barium sulfate, which is not soluble and does not dissociate into ions. Therefore, there are no free ions to conduct electricity in the solution, resulting in a conductivity of zero.

Adding more barium hydroxide solution introduces additional hydroxide ions, OH⁻, into the solution. The presence of these free ions allows the solution to conduct electricity better, increasing the electrical conductivity.