# Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 8

Q: 13. The partial pressure of ethane over a solution containing g of ethane is bar. If the solution contains g of ethane, then what shall be the partial pressure of the gas?

Molar mass of ethane

∴ Number of moles present in g of ethane

Let the number of moles of the solvent be x.

According to Henry's law,

Number of moles present in of ethane =

Again,

Thus, partial pressure of the gas is 0.764 bar.

Q: 14. What is meant by positive and negative deviations from Raoult's law and how is the sign of Hrelated to positive and negative deviations from Raoult's law?

According to Raoult's law, the partial vapour pressure of each volatile component in any solution is directly proportional to its mole fraction. The solutions which obey Raoult's law over the entire range of concentration are known as ideal solutions. The solutions that do not obey Raoult's law (non-ideal solutions) have vapour pressures either higher or lower than that predicted by Raoult's law. If the vapour pressure is higher, then the solution is said to exhibit positive deviation, and if it is lower, then the solution is said to exhibit negative deviation from Raoult's law.

Vapour pressure of a two-component solution showing positive deviation from Raoult's law

Vapour pressure of a two-component solution showing negative deviation from Raoult's law

In the case of an ideal solution, the enthalpy of the mixing of the pure components for forming the solution is zero.

In the case of solutions showing positive deviations, absorption of heat takes place.

In the case of solutions showing negative deviations, evolution of heat takes place.

Q: 15. An aqueous solution of non-volatile solute exerts a pressure of bar at the normal boiling point of the solvent. What is the molar mass of the solute?

Here,

Vapour pressure of the solution at normal boiling point bar

Vapour pressure of pure water at normal boiling point

Mass of solute,

Mass of solvent

Molar mass of solvent

According to Raoult's law,

Hence, the molar mass of the solute is