Q4. How many coulombs are needed to produce 1 mol of calcium from molten CaCl2?
Ca2+ + 2e− → Ca needs 2F = 2 × 96 487 = 1.93 × 105 C (NCERT page 52 for F).
Q5. How long must a current of 2.0 A flow to deposit 1.27 g of copper from CuSO4 solution? (Cu 63.5 g mol−1.)
1.27/63.5 = 0.0200 mol of Cu needs 0.0400 mol of electrons: Q = 0.0400 × 96 487 = 3859 C, so t = 3859/2.0 = 1930 s, about 32 minutes (NCERT page 52).
Q6. 9650 C is passed through acidified water with platinum electrodes. How many moles of H2 and of O2 form? (Take F = 96 500 C mol−1.)
9650/96 500 = 0.100 mol of electrons. 2H+ + 2e− → H2 gives 0.0500 mol of H2; 2H2O → O2 + 4H+ + 4e− gives 0.0250 mol of O2 (NCERT Eqs. (2.33) and (2.38); reasoned). At 273.15 K and 1 bar, with 22.7 L mol−1 (NCERT Class 11 Chemistry, page 20), that is 1.14 L of H2 and 0.568 L of O2.
Q7. Predict the products of electrolysing aqueous AgNO3 with platinum electrodes.
Cathode: silver, since Ag+/Ag (0.80 V) is well above H+/H2 (0.00 V) (NCERT Table 2.1, page 37). Anode: under ordinary aqueous conditions nitrate is not readily oxidised, so water is oxidised, giving oxygen; the solution becomes nitric acid (reasoned from NCERT pages 52–53).
Q8. Assertion: electrolysis of concentrated aqueous NaCl gives chlorine, not oxygen, at the anode. Reason: E⊖ for O2/H2O is lower than for Cl2/Cl−. Are both true, and does the reason explain the assertion?
Both are true, but the reason does not explain the assertion. By E⊖ alone water would be oxidised first; chlorine forms because oxygen needs an overpotential (NCERT page 53).
Q9. The same charge deposits silver from Ag+ and aluminium from Al3+. What is the ratio of the masses deposited? (Ag 108, Al 27 g mol−1.)
By Faraday’s second law the masses are in the ratio of the equivalent weights, 108/1 : 27/3 = 108 : 9 = 12 : 1 (NCERT page 51).
Q10. Check NCERT’s statement that a current of 50 000 A is about 0.518 F per second.
In one second 50 000 C passes; 50 000/96 487 = 0.518 mol of electrons, that is 0.518 F (NCERT page 52).