1

### AIPMT 2005

The vapour pressure of two liquids P and Q are 80 and 60 torr, respectively. The total vapour pressure of solution obtained by mixing 3 mole of P and 2 mole of Q would be
A
72 torr
B
140 torr
C
68 torr
D
20 torr

## Explanation

Hence total vapour pressure

= [(Mole fraction of P) × (Vapour pressure of P)] + [(Mole fraction of Q) × Vapour pressure of Q)]

= $\left( {{3 \over 5} \times 80 + {2 \over 5} \times 60} \right)$

= 48 + 24 = 72 torr
2

### AIPMT 2005

The mole fraction of the solute in one molal aqueous solution is
A
0.009
B
0.018
C
0.027
D
0.036

## Explanation

One molal solution means one mole solute present in 1 kg (1000 g) solvent.

$\therefore$ mole of solute = 1

Mole of solvent (H2O) = ${{1000} \over {18}}$

Xsolute = ${1 \over {1 + {{1000} \over {18}}}}$ = 0.018
3

### AIPMT 2002

2.5 litre of 1 M NaOH solution is mixed with another 3 litre of 0.5 M NaOH solution. Then find out molarity of resultant solution.
A
0.80 M
B
1.0 M
C
0.73 M
D
0.50 M

## Explanation

From molarity equation

M1V1 + M2V2 = MV

$\Rightarrow$ 1× 2.5 + 0.5 × 3 = M × 5.5

$\Rightarrow$ M = ${4 \over {5.5}}$ = 0.73M
4

### AIPMT 2002

A solution contains non volatile solute of molecular mass M2. Which of the following can be used to calculate the molecular mass of solute in terms of osmotic pressure?
A
${M_2} = \left( {{{{m_2}} \over \pi }} \right)VRT$
B
${M_2} = \left( {{{{m_2}} \over V}} \right){{RT} \over \pi }$
C
${M_2} = \left( {{{{m_2}} \over V}} \right)\pi RT$
D
${M_2} = \left( {{{{m_2}} \over V}} \right){\pi \over {RT}}$

## Explanation

For dilute solution, the van’t Hoff equation is

$\pi = {n \over V}RT$

$\Rightarrow$ $\pi V = nRT$

$\Rightarrow$ $\pi V = {{{m_2}} \over M}RT$

$\Rightarrow$ ${M_2} = \left( {{{{m_2}} \over V}} \right){{RT} \over \pi }$