Figure 3 shows the apparatus that can be used to electrolyse sodium sulfate solution using inert electrodes - Edexcel - GCSE Chemistry - Question 2 - 2020 - Paper 1

The gas that forms at the positive electrode is ammonia.

To calculate the concentration of the sodium sulfate solution, we use the formula:

$\text{Concentration} = \frac{\text{mass of solute (g)}}{\text{volume of solution (dm}^3)}$

The mass of sodium sulfate is 28.4 g, and the volume is 250 cm³, which is equivalent to 0.250 dm³. Therefore:

$\text{Concentration} = \frac{28.4 \, ext{g}}{0.250 \, \text{dm}^3} = 113.6 \, \text{g dm}^{-3}$

To express the answer to three significant figures, the final concentration is 114 g dm⁻³.

The ions present in sodium sulfate are sodium ions (Na⁺) and sulfate ions (SO₄²⁻). The formula for sodium sulfate is Na₂SO₄.