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141. Three electrolytic cells A, B and C containing solutions of ZnSO₄(zinc sulphate), AgNO₃(silver nitrate) and CuSO₄ (copper sulphate), respectively are connected in series. A steady current of 1.5 ampere was passed through them until 1.45 g of silver is deposited at the cathode of cell B. How long did the current flow? What mass of copper and zinc were deposited? (Atomic masses: Ag = 108, Zn = 65.4, Cu = 63.5, all in amu).

{Ag}^{+} + e^{-} \to Ag 1 mol 1 mol (= 108 g) (Cathode reaction in cell B)

108 g of silver is deposited at cathode when 1 mol of electrons are passed or 108 g of silver deposit needs = 1 Faraday = 96,500 coulombs. Therefore, 1.45 g of silver needs

= \frac{96, 500 \times 1.45}{108} = 1295.6 columbs

But quantity of electricity passed
= current x time
= 1295.6 C = 1.5 A x time (in sec.)
or time for which current is passed

= \frac{1295.6}{1.5} seconds = 863.7 s = 14.40 \min .

The cathode reaction in copper sulphate cell is

{Cu}^{2 +} + 2 e^{-} \to Cu 2 mol 1 mol (63.5 g) (2 \times 96500 c)

2 x 96500 coulombs gives a deposit of 63.5 g of Cu.
Therefore, 1295.6 coulombs will deposit of

= \frac{63.5 \times 1295.6}{2 \times 96, 500} = 0.4263 g

Similarly,

{Zn}^{2 +} + 2 e^{-} \to Zn 2 mol 1 mol (65.4 g) (2 \times 96500)

(cathode reaction in zinc sulphate cell)
Mass of zinc deposited

= \frac{65.4 x 1295.6}{2 \times 96, 500} g = 0.44 g .

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Short Answer Type

142. Calculate the standard cell potentials of galvanic cell in which the following reactions take place:
2Cr(s) + 3Cd²⁺ (aq) → 2Cr³⁺ (aq) + 3Cd
(Calculate the Δ_rG° and equilibrium constant of the reaction.)

257 Views

143. Calculate the standard cell potentials of galvanic cell in which the following reactions take place:
Fe²⁺ (aq) + Ag⁺ (aq) → Fe³⁺(aq) + Ag(s)
Calculate the Δ_rG° and equilibrium constant of the reaction.

146 Views

Long Answer Type

144. In the button cell widely used in watches and other devices the following reaction takes place:
Zn(s) + Ag₂O(s) + H₂O(l) → Zn²⁺(aq) + 2Ag(s) + 2OH^– (aq)
Determine Δ_rG° and E° for the reaction.

550 Views

Short Answer Type

145. The resistance of a conductivity cell containing 0.001 M KCl solution at 298 K is 1500Ω. What is the cell constant if conductivity of 0.001 M KCl solution at 298 K is 0.146 x 10^–3 s cm^–1.

219 Views

Long Answer Type

146. Conductivity of 0.00241 M acetic acid solution is 7.896 x 10^–5 S cm^-1. Calculate its molar conductivity in this solution. If Λ°_m for acetic acid be 390.5 S cm² mol^–1, what would be its dissociation constant?

183 Views

Short Answer Type

147. Calculate the emf of the cell Zn/Zn²⁺(0.1 M) || Cd²⁺ (0.01 M) | Cd at 298 k. (given)
E°_Zn²⁺/Zn = – 0.76 V and E°_Cd²⁺/Cd = – 0.40 V).

1204 Views

Long Answer Type

148.

Cu²⁺ + 2e^– → Cu E° = + 0.34 V
Ag⁺ + 1e^– → Ag E° = + 0.80 V
(i) Construct a galvanic cell using the above data.
(ii) For what concentration of Ag⁺ ions will the emf of the cell be zero at 25°C, if the concentration of Cu²⁺ is 0.01 M ? [log 3.919 = 0.593]

357 Views

149. Silver is electrodeposited on a metallic vessel of total surface area 900 cm² by passing a current of 0.5 amp for two hours. Calculate the thickness of silver deposited. [Given : Density of silver = 10.5 g cm^–3, Atomic mass of silver = 108 amu, F = 96,500 C mol^–1]

603 Views

Short Answer Type

150. Calculate the number of coulombs required for the oxidation of 1 mole of water to oxygen as per equation:
2H₂O → 4H⁺ + O₂ + 4e^–
[Given: 1 F = 96,500 C mol^–1]

190 Views