1

### AIPMT 2006

Which of the following pairs constitutes a buffer?
A
HCl and KCl
B
HNO2 and NaNO2
C
NaOH and NaCl
D
HNO3 and NH4NO3

## Explanation

HNO2 is a weak acid and NaNO2 is its salt thus these two constitute to form an acidic buffer.
2

### 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 = ${{{P_{C{O_2}}}} \over {{P_{C{H_4}}} \times {{\left[ {{P_{{O_2}}}} \right]}^2}}}$
3

### AIPMT 2005

Equilibrium constants K1 and K2 for the following equilibriam:

are related as
A
K2 = 1/K12
B
K2 = K12
C
K2 = 1/K1
D
K2 = K1/2

## Explanation

NO(g) + O2(g) ⇌ NO2(g) ,    K1

Reverse the above equation

NO2(g) ⇌ NO(g) + O2(g),     ${1 \over {{K_1}}}$

Multiply the above equation by 2, we get

NO2(g) ⇌ NO(g) + O2(g),    ${\left( {{1 \over {{K_1}}}} \right)^2} = {K_2}$
4

### AIPMT 2005

At 25oC, the dissociation constant of a base, BOH, is 1.0 $\times$ 10$-$12. The concentration of hydroxyl ions in 0.01 M aqueous solution of the base would be
A
1.0 $\times$ 10$-$5 mol L$-$1
B
1.0 $\times$ 10$-$6 mol L$-$1
C
2.0 $\times$ 10$-$6 mol L$-$1
D
1.0 $\times$ 10$-$7 mol L$-$1

## Explanation

BOH B+ + OH-
Initially C 0 0
At equilibbrium C - C$\alpha$ C$\alpha$ C$\alpha$

[OH-] = C$\alpha$

$\Rightarrow$ [OH-] = $\sqrt {{K_b}C}$

$\Rightarrow$ [OH-] = $\sqrt {1 \times {{10}^{ - 12}} \times {{10}^{ - 2}}}$

= 1.0 $\times$ 10$-$7 mol L$-$1