Question 1

Which of the following expressions indicates the correct relationship between molar conductivity of strong electrolyte and its concentration?

Accepted Answer

The relationship between molar conductivity ( ) and concentration (c) for a strong electrolyte is given by the Debye-Huckel-Onsager equation. The equation is = _0 - a c , where _0 is the limiting molar conductivity at infinite dilution and a is a constant that depends on the nature of the electrolyte, solvent, and temperature. Answer: = _0 - a c

Question 2

If molar conductance at infinite dilution for CH _3 COO ^- and H ^+ ions are 50\ S cm ^2\ mol ^ -1 and 350\ S cm ^2\ mol ^ -1 respectively, molar conductivity of 5 10^ -2 M CH _3 COOH is 20\ S cm ^2 m

Accepted Answer

Molar conductivity at infinite dilution for CH _3 COOH is given by Kohlrausch's law: _m^ = ^ ( CH _3 COO ^-) + ^ ( H ^+) _m^ = 50 + 350 = 400\ S cm ^2\ mol ^ -1 Degree of dissociation is: = _m _m^ = 20 400 = 0.05 Hydrogen ion concentration is: [ H ^+] = C = (5 10^ -2 ) 0.05 = 2.5 10^ -3 \ mol/dm ^3 Answer: 2.5 10^ -3

Question 3

Determine the electrode potential of Sn ^ 2+ (0.01 M )\ |\ Sn(s) at 25^ C if E^ _ Sn is -0.136 V.

Accepted Answer

The reduction half-reaction is Sn ^ 2+ + 2e^ - Sn(s) . Using the Nernst equation: E = E^ - 0.0591 n 1 [ Sn ^ 2+ ] Substituting the given values (n = 2, [ Sn ^ 2+ ] = 0.01 M , E^ = -0.136 V ): E = -0.136 - 0.0591 2 1 0.01 E = -0.136 - 0.02955 (100) E = -0.136 - 0.02955 2 E = -0.136 - 0.0591 E = -0.1951 V -0.195 V Answer: -0.195 V

Question 4

What is the molar conductivity of 0.02 M KI solution if its conductivity is 4.37 10^ -4 \, ^ -1 \, cm ^ -1 ?

Accepted Answer

Given: Conductivity, = 4.37 10^ -4 \, ^ -1 \, cm ^ -1 Molarity, M = 0.02 \, M The formula for molar conductivity is: _m = 1000 M Substituting the given values: _m = 4.37 10^ -4 1000 0.02 _m = 0.437 0.02 _m = 21.85 \, ^ -1 \, cm ^2 \, mol ^ -1 Answer: 21.85\ ^ -1 \, cm ^2 mol ^ -1

Question 5

What is the quantity of electricity required to produce 8 g of Mg (molar mass = 24\ g mol ^ -1 ) from MgCl _2 solution ?

Accepted Answer

The reduction reaction of magnesium is given by: Mg ^ 2+ + 2e^ - Mg From the stoichiometry of the reaction, 1 mole of Mg requires 2 moles of electrons, which corresponds to 2 F of electricity. Number of moles of Mg to be produced = Mass Molar mass = 8 24 = 1 3 mol Quantity of electricity required = 1 3 2 F = 2 3 F = 0.666 F Answer: 0.666 F

Question 6

Which of the following reactions occurs at cathode during recharging of lead accumulator ?

Accepted Answer

During the discharging of a lead accumulator, the cell acts as a galvanic cell with the following reactions: At anode (oxidation): Pb _ (s) + SO _4^ 2- _ (aq.) PbSO _ 4(s) + 2e^- At cathode (reduction): PbO _ 2(s) + SO _4^ 2- _ (aq.) + 4 H ^+_ (aq.) + 2e^- PbSO _ 4(s) + 2 H _2 O _ ( ) During recharging, an external voltage is applied and the cell acts as an electrolytic cell. The reactions are reversed. The electrode that acted as the anode during discharge becomes the cathode (where reduction o

Question 7

What must be the molarity of BaCl _2 solution to have molar conductivity 240\, ^ -1 cm ^2 mol ^ -1 and conductivity 0.012\, ^ -1 cm ^ -1 at 25^ C ?

Accepted Answer

The relationship between molar conductivity _m, conductivity , and molarity M is given by the formula: _m = 1000 M Given values are: _m = 240 \, ^ -1 cm ^2 mol ^ -1 = 0.012 \, ^ -1 cm ^ -1 Substituting the given values into the formula: 240 = 0.012 1000 M 240 = 12 M M = 12 240 M = 0.05 \, M Answer: 0.05 M

Question 8

What is molar conductivity of CH _3 COOH at zero concentration if the molar conductivities of H _2 SO _4, K _2 SO _4 and CH _3 COOK at zero concentrations are respectively x, y and z ^ -1 cm ^2 mol ^

Accepted Answer

By Kohlrausch's law of independent migration of ions: ^ _m(H_2SO_4) = 2 ^ (H^+) + ^ (SO_4^ 2- ) = x ^ _m(K_2SO_4) = 2 ^ (K^+) + ^ (SO_4^ 2- ) = y ^ _m(CH_3COOK) = ^ (CH_3COO^-) + ^ (K^+) = z The molar conductivity of CH_3COOH at zero concentration is: ^ _m(CH_3COOH) = ^ (CH_3COO^-) + ^ (H^+) Subtracting the second equation from the first: x - y = 2 ^ (H^+) - 2 ^ (K^+) x - y 2 = ^ (H^+) - ^ (K^+) Adding this to the third equation: x - y 2 + z = [ ^ (H^+) - ^ (K^+)] + [ ^ (CH_3COO^-) + ^ (K^+)] x

Question 9

During the electrolysis of aqueous NaCl the product formed at cathode is?

Accepted Answer

In the electrolysis of aqueous NaCl , the ions present in the solution are Na ^ + , Cl ^ - , H ^ + , and OH ^ - . At the cathode, reduction occurs. The competing reactions are: Na ^ + + e^ - Na _ (s) (E^ = -2.71 V ) 2 H _ 2 O _ (l) + 2e^ - H _ 2(g) + 2 OH ^ - _ (aq) (E^ = -0.83 V ) Since the standard reduction potential of water is greater than that of sodium ions, water is preferentially reduced at the cathode. Thus, hydrogen gas ( H _ 2(g) ) is liberated at the cathode. Answer: H _ 2(g)

Question 10

What is the conductivity of 0.02\ M \ AgNO _3 solution having cell constant 1.2\ cm ^ -1 and resistance 95.0\ ohms ?

Accepted Answer

Given: Resistance, R = 95.0\ Cell constant, G^ * = 1.2\ cm ^ -1 The conductivity is given by the relation: = 1 R G^ * Substituting the given values: = 1 95.0 1.2 = 0.01263\ ^ -1 \ cm ^ -1 = 1.263 10^ -2 \ ^ -1 \ cm ^ -1 Answer: 1.263 10^ -2 \ ^ -1 \ cm ^ -1

Question 11

What is number of moles of electron passed when a current of 2 Amp flows through solution of electrolyte for 10 minutes?

Accepted Answer

Given, current I = 2 A and time t = 10 minutes = 10 60 = 600 s. Total charge passed is given by Q = I t Q = 2 600 = 1200 C The charge of one mole of electrons is 1 Faraday (F 96500 C). Number of moles of electrons = Q F Number of moles of electrons = 1200 96500 = 0.012435 mol Number of moles of electrons 1.243 10^ -2 mol Answer: 1.243 10^ -2

Question 12

If standard reduction potential of four electrodes A, B, C, D are +2.5\, V , +3.0\, V , -2.0\, V and -1.5\, V respectively. When is the standard emf of cell maximum?

Accepted Answer

The standard emf of a cell is given by E^ _ cell = E^ _ cathode - E^ _ anode . To obtain the maximum standard emf, the cathode must have the highest standard reduction potential and the anode must have the lowest standard reduction potential. Given standard reduction potentials are: E^ _ A = +2.5 V E^ _ B = +3.0 V E^ _ C = -2.0 V E^ _ D = -1.5 V The highest reduction potential is for electrode B (+3.0 V ), so B should act as the cathode. The lowest reduction potential is for electrode C (-2.0 V

Question 13

If standard reduction potential of Zn, Ni and Fe are -0.76\, V , -0.23\, V and -0.44\, V respectively. For the reaction (Stated below) to be spontaneous find out electrodes X and Y considering above e

Accepted Answer

For the given reaction X _ (s) + Y ^ +2 _ (aq) X ^ +2 _ (aq) + Y _ (s) , X undergoes oxidation and acts as the anode, while Y undergoes reduction and acts as the cathode. For the reaction to be spontaneous, the standard cell potential E^ _ cell must be positive. E^ _ cell = E^ _ cathode - E^ _ anode = E^ _ Y - E^ _ X > 0 This implies E^ _ Y > E^ _ X . Given standard reduction potentials: E^ _ Zn = -0.76\, V E^ _ Ni = -0.23\, V E^ _ Fe = -0.44\, V Checking the given options: If X = Ni, Y = Fe: E^

Question 14

A galvanic cell consist copper electrode and standard hydrogen electrode. If E^ ( Cu ^ +2 _ (aq) | Cu _ (s) ) = +0.34\, V , then identify a reaction taking place at positive electrode during working o

Accepted Answer

Given E^ ( Cu ^ +2 / Cu ) = +0.34\, V and E^ ( H ^+/ H _2) = 0.00\, V . Since the standard reduction potential of the copper electrode is greater than that of the standard hydrogen electrode, copper acts as the cathode and the hydrogen electrode acts as the anode. In a galvanic cell, the cathode is the positive electrode where reduction takes place. The reduction reaction at the positive electrode (copper cathode) is: Cu ^ +2 _ (aq) + 2e^- Cu _ (s)

Question 15

Which of the following net cell reactions occurs in a galvanic cell containing cadmium electrode and standard hydrogen electrode? E ^ _ ( Cd ^ 2+ _ (aq) | Cd _ (s) ) = -0.403\ V .

Accepted Answer

Given E^ _ Cd ^ 2+ | Cd = -0.403\ V and E^ _ H ^+| H _2 = 0.00\ V . Since the standard reduction potential of cadmium is lower than that of hydrogen, cadmium has a higher tendency to get oxidized. Therefore, cadmium will act as the anode (oxidation) and the standard hydrogen electrode will act as the cathode (reduction). At anode: Cd _ (s) Cd ^ 2+ _ (aq) + 2 e ^- At cathode: 2 H ^+_ (aq) + 2 e ^- H _ 2(g) Adding the two half-cell reactions, the net cell reaction is: Cd _ (s) + 2 H ^+_ (aq) Cd ^

Question 16

For a certain redox reaction in galvanic cell X _ (s) + Y ^ 2+ _ (aq) X ^ 2+ _ (aq) + Y _ (s) , E ^ cell is 0.0296\ V at 298\ K . What is equilibrium constant of reaction?

Accepted Answer

The relation between standard cell potential and equilibrium constant at 298\ K is given by: E^ _ cell = 0.0591 n K_c For the given reaction, the number of electrons transferred is n = 2. Substituting the given values: 0.0296 = 0.0591 2 K_c 0.0296 0.02955 K_c K_c = 0.0296 0.02955 1 K_c = 10^1 = 10 Answer: 10

Question 17

If standard reduction potential (E^0) of ( Al ^ +3 _ (aq) | Al _ (s) ), ( Fe ^ +2 _ (aq) | Fe _ (s) ), ( Cu ^ +2 _ (aq) | Cu _ (s) ) and ( Ag ^ +1 _ (aq) | Ag _ (s) ) are -1 66 V, -0 44 V, +0 34 V and

Accepted Answer

For a reaction to be non-spontaneous, the standard cell potential E^ 0 _ cell must be negative, where E^ 0 _ cell = E^ 0 _ cathode - E^ 0 _ anode . For the reaction 2 Ag _ (s) + Fe ^ +2 _ (aq) 2 Ag ^ +1 _ (aq) + Fe _ (s) : Ag undergoes oxidation (anode) and Fe ^ +2 undergoes reduction (cathode). E^ 0 _ cell = E^ 0 _ Fe ^ +2 / Fe - E^ 0 _ Ag ^ + / Ag E^ 0 _ cell = -0.44 - 0.79 = -1.23 V Since E^ 0 _ cell For the other reactions, E^ 0 _ cell is positive, making them spontaneous: 2 Al _ (s) + 3 Cu

Question 18

The graphical variation of molar conductivity ( ) against square root of molar concentration ( c ) of certain electrolyte 'X' is linear with intercept on y axis. Identify X from following.

Accepted Answer

According to the Debye-Hückel-Onsager equation, the variation of molar conductivity _m with the square root of concentration c is given by _m = _m^ - A c . This linear relationship with a definite y-intercept ( _m^ ) is observed only for strong electrolytes. Weak electrolytes show a steep curve at low concentrations and do not have a linear plot or a well-defined intercept. Among the given options, CH _3 COOH , NH _4 OH , and HCOOH are weak electrolytes. CH _3 COONa is a salt of a weak acid and

Question 19

What is the change in oxidation number of Pb at positive electrode of lead accumulator acting as galvanic cell ?

Accepted Answer

In a lead accumulator acting as a galvanic cell (discharging), the positive electrode acts as the cathode where reduction takes place. The reaction at the positive electrode is: PbO_2(s) + SO_4^ 2- (aq) + 4H^+(aq) + 2e^- PbSO_4(s) + 2H_2O(l) The oxidation state of Pb in PbO_2 is +4. The oxidation state of Pb in PbSO_4 is +2. The oxidation number of Pb changes from +4 to +2, which means it decreases by 2. Answer: decreases by 2

Question 20

The conductivity of 0.02 M of KCl solution at 298 K is 0.0123 ohm ^ -1 \ cm ^ -1 . If the resistance of the cell containing this solution is 120 ohms, what is the value of the cell constant ?

Accepted Answer

The relation between conductivity , resistance R, and cell constant G^* is given by: G^* = R Given: = 0.0123 ohm ^ -1 \ cm ^ -1 R = 120 ohms Substituting the values: G^* = 0.0123 120 G^* = 1.476 cm ^ -1 Answer: 1.476\ cm ^ -1

Electrochemistry — Previous Year Questions & Solutions