# Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 16

Q: 38. Benzene and toluene form ideal solution over the entire range of composition. The vapour pressure of pure benzene and toluene at are and respectively. Calculate the mole fraction of benzene in vapour phase if of benzene is mixed with of toluene.

Molar mass of benzene

Molar mass of toluene

Now, no. of moles present in 80 g of benzene

And, no. of moles present in 100 g of toluene

∴Mole fraction of benzene,

And, mole fraction of toluene,

It is given that vapour pressure of pure benzene,

And, vapour pressure of pure toluene,

Therefore, partial vapour pressure of benzene,

And, partial vapour pressure of toluene,

Hence, mole fraction of benzene in vapour phase is given by:

Q: 39. The air is a mixture of a number of gases. The major components are oxygen and nitrogen with approximate proportion of is to by volume at . The water is in equilibrium with air at a pressure of 10 atm. At K if the Henry's law constants for oxygen and nitrogen are mm and respectively, calculate the composition of these gases in water.

Percentage of oxygen in air

Percentage of nitrogen in air

Also, it is given that water is in equilibrium with air at a total pressure of 10 atm, that is,

Therefore,

Partial pressure of oxygen,

Partial pressure of nitrogen,

Now, according to Henry’s law:

For oxygen

For nitrogen:

Hence, the mole fractions of oxygen and nitrogen in water are 4respectively.