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2018 O Level Chem P1 Q13 — Electrolysis / Quantitative Electrolysis Solution

Electrolysis / Quantitative Electrolysis 2018 O Level Pure Chemistry · Paper 1 · Q13

Question

In the diagram, each cell contains an aqueous solution of a single salt. All four electrodes are platinum. Electrodes Q and S increase in mass during the electrolysis but no gas is given off at Q or S. The increase in mass of Q is greater than the increase in mass of S. Which statement must be correct? Question diagram

Question diagram

AThe cation in cell 1 is the same as in cell 2 but the cation of the solution in cell 1 is more concentrated than in cell 2.
BThe cation of the solution in cell 1 is different from the cation of the solution in cell 2.
CThe current flowing in cell 1 is greater than the current flowing in cell 2.
DThe loss of mass of electrode P is less than the loss of mass of electrode R.

Correct answer: B

Video Solution

Explanation

Answer: B. The two cells are in series, so the same current (and the same number of moles of electrons) passes through both in the same time. This makes statement C impossible.

Cathodes Q and S gain mass with no gas, so a metal is deposited at each. Because the same charge passes through both cells but the masses deposited differ, the metals must be different, so the two solutions contain different cations. That is statement B.

Statement A is wrong: for a given charge the mass deposited depends on the metal, not on concentration. Statement D is wrong: all four electrodes are inert platinum, so the anodes P and R lose no mass at all (oxygen is released there). Answer B.

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