The electrolysis of sodium chloride solution

3 marks · Normal

During the electrolysis of sodium chloride solution, 0.1 mol of hydrogen gas is collected at the cathode.

Calculate the number of moles of hydroxide ions formed around the cathode by the time this hydrogen gas is collected.

Assume the half-equation at the cathode is:

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0.2 mol (±0.01)

Also accept: 0.200 mol

Common mistakes: Dividing instead of multiplying by 2.

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Marking points

M1 · 1 mark · Identifies correct mole ratio from half-equation (1:2, H2:OH-)

M2 · 1 mark · Calculates number of moles of OH- formed as twice the amount of H2 collected. (follow-through)

A1 · 1 mark · Gives final answer: 0.2 mol (follow-through)

Worked solution

From the half-equation:

1 mol H2 is formed with 2 mol OH-.

So, 0.1 mol H2 gives:

Final answer: 0.2 mol OH- formed.