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JEE Main 2019 · Chemistry · Electrochemistry · medium

The same current is passed for 1930 s through two electrolytic cells in series, one containing aqueous AgNO3 and the other aqueous CuSO4, with inert…

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The same current is passed for 1930 s1930\ \mathrm{s} through two electrolytic cells in series, one containing aqueous AgNO3\mathrm{AgNO_3} and the other aqueous CuSO4\mathrm{CuSO_4}, with inert platinum electrodes in both. The current is 0.50 A0.50\ \mathrm{A}. Taking the molar volume of gas at STP as 22.4 L22.4\ \mathrm{L}, the total volume of O2\mathrm{O_2} liberated at the two anodes, measured at STP, is (F=96500 C mol−1F = 96500\ \mathrm{C\,mol^{-1}})
  1. A. 56 mL56\ \mathrm{mL}
  2. B. 224 mL224\ \mathrm{mL}
  3. C. 28 mL28\ \mathrm{mL}
  4. D. 112 mL112\ \mathrm{mL}
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Answer: (D) 112 mL112\ \mathrm{mL}

Charge Q=0.50×1930=965 CQ = 0.50\times1930 = 965\ \mathrm{C}, i.e. 965/96500=0.0100965/96500 = 0.0100 mol of electrons. At each platinum anode water is oxidised: 2H2O→O2+4H++4e−2\mathrm{H_2O} \to \mathrm{O_2} + 4\mathrm{H^+} + 4e^-, so each cell gives 0.0100/4=2.5×10−30.0100/4 = 2.5\times10^{-3} mol O2=56 mL\mathrm{O_2} = 56\ \mathrm{mL} at STP. Because the cells are in series the same charge flows through both, giving 2×56=112 mL2\times56 = 112\ \mathrm{mL}. The metal deposited (Ag\mathrm{Ag} or Cu\mathrm{Cu}) does not matter for the oxygen volume.
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