# Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 4

Q: 9. Vapour pressure of pure water at is mm Hg. 50 g of urea is dissolved in of water. Calculate the vapour pressure of water for this solution and its relative lowering.

It is given that vapour pressure of water,

Weight of water taken,

Weight of urea taken,

Molecular weight of water,

Molecular weight of urea,

Now, we have to calculate vapour pressure of water in the solution. We take vapour pressure as .

Now, from Raoult's law, we have:

Hence, the vapour pressure of water in the given solution is mm of Hg and its relative lowering is .

Q: 10. Boiling point of water at mm Hg is . How much sucrose is to be added to 500 g of water such that it boils at C.Molal elevation constant for water is .

Here, elevation of boiling point

Molar mass of sucrose

Molal elevation constant,

We know that:

(Approximately)

Hence, g of sucrose is to be added.

Note: There is a slight variation in this answer and the one given in the NCERT textbook.

Q: 11. Calculate the mass of ascorbic acid (Vitamin ) to be dissolved in of acetic acid to lower its melting point by

Mass of acetic acid,

Molar mass of ascorbic acid

Lowering of melting point,

We know that:

(Approx)

of ascorbic acid is needed to be dissolved.

None: There is a slight variation in this answer and the one given in the NCERT textbook.

Q: 12. Calculate the osmotic pressure in Pascal’s exerted by a solution prepared by dissolving 1.0 g of polymer of molar mass in of water at .

It is given

Volume of water,

Temperature,

Number of moles of the polymer,

We know that:

Osmotic pressure,

(Approximately)