1

### 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
2

### 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.
3

### 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

### 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),    ${\left( {{1 \over {{K_1}}}} \right)^2} = {K_2}$