1
MCQ (Single Correct Answer)

### AIPMT 2007

A weak acid, HA, has a Ka of 1.00 $\times$ 10$-$5. If 0.100 mol of this acid is dissolved in one litre of water, the percentage of acid dissociated at equilibrium is closest to
A
1.00%
B
99.9%
C
0.100%
D
99.0%

## Explanation

For weak acid degree of dissociation,

$\alpha$ = $\sqrt {{{{K_a}} \over C}}$

= $\sqrt {{{1 \times {{10}^{ - 5}}} \over {0.1}}} = {10^{ - 2}}$ = 1.00 %
2
MCQ (Single Correct Answer)

### AIPMT 2007

The equilibrium constants of the following are

N2 + 3H2 $\rightleftharpoons$ 2NH3;     K1

N2 + O2 $\rightleftharpoons$ 2NO;     K2

H2 + ${1 \over 2}$O2 $\rightleftharpoons$ H2O;     K3

The equilibrium constant (K) of the reaction :

2NH3 + ${5 \over 2}$ O2 $\rightleftharpoons$ 2NO + 3H2O will be
A
K2K33/K1
B
K2K3/K1
C
K23K3/K1
D
K1K33/K2

## Explanation

2NH3 $\rightleftharpoons$ N2 + 3H2;     ${1 \over {{K_1}}}$

N2 + O2 $\rightleftharpoons$ 2NO;     K2

3H2 + ${3 \over 2}$O2 $\rightleftharpoons$ 3H2O;     (K3)3

By adding all equations, we get

2NH3 + ${5 \over 2}$ O2 $\rightleftharpoons$ 2NO + 3H2O

$\therefore$ K = ${{{K_2} \times {{\left( {{K_3}} \right)}^3}} \over {{K_1}}}$
3
MCQ (Single Correct Answer)

### AIPMT 2006

For the reaction :
CH4(g) + 2O2(g) $\rightleftharpoons$ CO2(g) + 2H2O(l),
$\Delta$Hr = $-$ 170.8 kJ mol$-$1.
Which of the following statements is not true?
A
The reaction is exothermic.
B
At equilibrium, the concentrations of CO2(g) and H2O(l) are not equal.
C
The equilibrium constant for the reaction is given by Kp = ${{\left[ {C{O_2}} \right]} \over {\left[ {C{H_4}} \right]\left[ {{O_2}} \right]}}$
D
Addition of CH4(g) or O2(g) at equilibrium will cause a shift to the right.

## Explanation

Option C is incorrect as the value of KP given is wrong. It should have been

KP = ${{{P_{C{O_2}}}} \over {{P_{C{H_4}}} \times {{\left[ {{P_{{O_2}}}} \right]}^2}}}$
4
MCQ (Single Correct Answer)

### AIPMT 2006

The hydrogen ion concentration of a 10$-$8 M, HCl aqueous solution at 298 K (Kw = 10$-$14) is
A
1.0 $\times$ 10$-$8 M
B
1.0 $\times$ 10$-$6 M
C
1.0525 $\times$ 10$-$7 M
D
9.525 $\times$ 10$-$8 M

## Explanation

In HCl solution [H+] = 10-8 M

Also from water, [H+] = 10-7 M

$\therefore$ Total [H+] = 10-8 M + 10-7 M

= 1.1 ×10–7 M

$\simeq$ 1.0525 $\times$ 10$-$7 M

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