1

### NEET 2013 (Karnataka)

Consider the half-cell reduction reaction
Mn2+ + 2e$-$ $\to$ Mn, Eo = $-$1.18 V
Mn2+ $\to$ Mn3+ + e$-$, Eo = $-$ 1.51 V
The $E$o for the reaction 3 Mn2+ $\to$ Mno + 2Mn3+, and possibility of the forward reaction are respectively
A
$-$ 4.18 V and yes
B
+ 0.33 V and yes
C
+ 2.69 V and no
D
$-$ 2.69 V and no

## Explanation

Mn2+ + 2e$-$ $\to$ Mn, Eo = $-$1.18 V

Mn2+ $\to$ Mn3+ + e$-$, Eo = $-$ 1.51 V

3Mn2+ $\to$ Mno + 2Mn3+, $\Delta$Eo = - 1.81 - 1.51 = $-$ 2.69 V

Since $\Delta$E° is negative,

$\therefore$ $\Delta$G = –nFE°, $\Delta$G will have positive value so, forward reaction is not possible.
2

### NEET 2013

At 25oC molar conductance of 0.1 molar aqueous solution of ammonium hydroxide is 9.54 ohm$-$1 cm2 mol$-$1 and at infinite dilution its molar conductance is 238 ohm$-$1 cm2 mol$-$1. The degree of ionisation of ammonium hydroxide at the same concentration and temperature is
A
4.008%
B
40.800%
C
2.080%
D
20.800%

## Explanation

Degree of dissociation($\alpha$) = ${{9.54} \over {238}}$ = 0.04008 = 4.008%
3

### NEET 2013

A button cell used in watches function as following.
Zn(s) + Ag2O(s) + H2O(l) $\rightleftharpoons$ 2Ag(s) + Zn2+(aq) + 2OH$-$(aq)

If half cell potentials are
Zn2+(aq) + 2e$-$ $\to$ Zn(s);  Eo = $-$0.76 V
Ag2O(s) + H2O(l) + 2e$-$ $\to$ 2Ag(s) + 2OH$-$(aq), Eo = 0.34 V

The cell potential will be
A
0.84 V
B
1.34 V
C
1.10 V
D
0.42 V

## Explanation

At anode :Zn2+(aq) + 2e$-$ $\to$ Zn(s);  Eo = $-$0.76 V

At cathode : Ag2O(s) + H2O(l) + 2e$-$ $\to$ 2Ag(s) + 2OH$-$(aq), Eo = 0.34 V

cell = E°cathode – E°anode

= 0.34 – (–0.76) = 1.10 V
4

### AIPMT 2012 Mains

Molar conductivities $\left( {\Lambda _m^o} \right)$ at infinite dilution of NaCl, Hcl and CH3COONa are 126.4, 425.9 and 91.0 S cm2 mol$-$1 respectively. $\left( {\Lambda _m^o} \right)$ for CH3COOH will be
A
425.5 S cm2 mol$-$1
B
180.5 S cm2 mol$-$1
C
290.8 S cm2 mol$-$1
D
390.5 S cm2 mol$-$1

## Explanation

$\Lambda$oCH3COOH = $\Lambda$oCH3COONa + $\Lambda$oHCl - $\Lambda$oNaCl

= = 91 + 425.9 – 126.4 = 390.5