This question is about electrolysis - AQA - GCSE Chemistry Combined Science - Question 2 - 2022 - Paper 1

From the experiment described in Figure 2, the volumes of gases collected are:

• Volume of hydrogen = 30 cm³
• Volume of oxygen = 15 cm³

The student’s hypothesis states that the ratio of volumes collected should mirror the ratio of atoms in water (H₂O). Here, the volume of hydrogen collected is 30 cm³, and that of oxygen is 15 cm³. The ratio of hydrogen to oxygen can be calculated as:

$\text{Ratio} = \frac{\text{Volume of Hydrogen}}{\text{Volume of Oxygen}} = \frac{30}{15} = 2:1$

This ratio corresponds exactly to the 2 hydrogen atoms to 1 oxygen atom in a water molecule, thus supporting the student's hypothesis.

To calculate the measure of uncertainty, we will determine the range of the volumes:

• Minimum volume = 6 cm³
• Maximum volume = 11 cm³

The range is calculated as:

$\text{Range} = \text{Max} - \text{Min} = 11 \text{ cm}³ - 6 \text{ cm}³ = 5 \text{ cm}³$

Since the range is usually expressed as half of this value, the measure of uncertainty would be: $\frac{5}{2} = 2.5 \text{ cm}³$ This value can be approximated, and the closest option available is 9 ± 2 cm³.

To find out how much potassium sulfate is needed for 1.0 dm³ (1000 cm³) of solution:

1. The concentration of the solution can be calculated as: $\text{Concentration} = \frac{0.86 \text{ g}}{25 \text{ cm}³} = 0.0344 \text{ g/cm}³$

2. For 1.0 dm³: $\text{Mass needed} = 0.0344 \text{ g/cm}³ \times 1000 \text{ cm}³ = 34.4 \text{ g}$

Thus, approximately 34 g of potassium sulfate is needed to make 1.0 dm³ of solution.