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Question 51

When an electric current is passes through acidified water, 112 mL of hydrogen gas at N.T.P. was collected at the cathode in 965 seconds. The current passed, in ampere, is:

At normal temperature and pressure (N.T.P.), one mole of any ideal gas occupies a molar volume of $$22.4\ \text{L}=22400\ \text{mL}.$$

We have collected only $$112\ \text{mL}$$ of hydrogen. The number of moles of hydrogen obtained is therefore

$$n(\text{H}_2)=\frac{\text{volume of H}_2}{\text{molar volume}}=\frac{112\ \text{mL}}{22400\ \text{mL mol}^{-1}}=0.005\ \text{mol}.$$

During electrolysis of acidified water, hydrogen forms at the cathode according to the half-reaction

$$2\text{H}^+(aq)+2e^- \longrightarrow \text{H}_2(g).$$

This equation shows that $$2$$ moles of electrons are needed to produce $$1$$ mole of $$\text{H}_2.$$ Hence, to produce $$0.005\ \text{mol}$$ of $$\text{H}_2,$$ the moles of electrons required are

$$n(e^-)=2\times n(\text{H}_2)=2\times0.005=0.010\ \text{mol}.$$

The charge carried by one mole of electrons is the Faraday constant $$F=96500\ \text{C mol}^{-1}.$$ Therefore, the total charge that has passed through the solution is

$$Q=n(e^-)\,F=0.010\ \text{mol}\times96500\ \text{C mol}^{-1}=965\ \text{C}.$$

The problem states that this charge was delivered in $$t=965\ \text{s}.$$ Electric current is defined by the relation $$I=\dfrac{Q}{t}.$$ Substituting the values gives

$$I=\frac{965\ \text{C}}{965\ \text{s}}=1.0\ \text{A}.$$

Hence, the correct answer is Option D.

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