1

### AIPMT 2007

Calculate the pOH of a solution at 25oC that contains 1 $\times$ 10$-$10 M of hydronium ions, i.e. H3O+.
A
4.000
B
9.000
C
1.000
D
7.000

## Explanation

Given, [H3O+.] = 1 $\times$ 10$-$10

$\Rightarrow$ pH = 10

Also we know, pH + pOH = 14

$\Rightarrow$ pOH = 14 - pH = 14 - 10 = 4
2

### 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

### 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
4

### 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.