### JEE Mains Previous Years Questions with Solutions

4.5
star star star star star
1

### AIEEE 2010

If 10–4 dm3 of water is introduced into a 1.0 dm3 flask to 300 K, how many moles of water are in the vapour phase when equilibrium is established ?
(Given : Vapour pressure of H2O at 300 K is 3170 Pa ; R = 8.314 J K–1 mol–1)
A
5.56 x 10–3 mol
B
1.53 x 10–2 mol
C
4.46 x 10–2 mol
D
1.27 x 10–3 mol

## Explanation

From the ideal gas equation :

$PV = nRT\,\,$

or $\,\,\,n = {{PV} \over {RT}} = {{3170 \times {{10}^{ - 3}}} \over {8.314 \times 300}}$

$= 1.27 \times {10^{ - 3}}$
2

### AIEEE 2007

Equal masses of methane and oxygen are mixed in an empty container at 25oC. The fraction of the total pressure exerted by oxygen is
A
2/3
B
1/2
C
1/3 $\times$ 273/298
D
1/3

## Explanation

Let the mass of methane and oxygen $=m$ $gm.$

Mole fraction of ${O_2}$

$= {{Moles\,\,of\,\,{O_2}} \over {Moles\,\,of\,\,{O_2}\, + \,Moles\,\,of\,\,C{H_4}}}$

$= {{m/32} \over {m/32 + m/16}}$

$= {{m/32} \over {3m/32}} = {1 \over 3}$

Partial pressure of ${O_2}$

$=$ Total pressure $\times$ mole fraction of ${O_2},$

${P_{{O_2}}} = P \times {1 \over 3} = {1 \over 3}P$
3

### AIEEE 2005

Which one of the following statements is NOT true about the effect of an increase in temperature on the distribution of molecular speeds in a gas?
A
The most probable speed increases
B
The fraction of the molecules with the most probable speed increases
C
D
The area under the distribution curve remains the same as under the lower temperature

## Explanation

Distribution of molecular velocities at two different temperature is given shown below.

NOTE : At higher temperature more molecules have higher velocities and less molecules have lower velocities.

As evident from fig. thus it is clear that With the increase in temperature the most probable velocity increase but the fraction of such molecules decreases.
4

### AIEEE 2004

In Van der Waals equation of state of the gas law, the constant ‘b’ is a measure of
A
intermolecular repulsions
B
intermolecular collisions per unit volume
C
Volume occupied by the molecules
D
intermolecular attraction

## Explanation

In van der Waals equation $'b'$ is for volume correction.