Solutions — Practice

Question 1

Calculate the molality of a solution of 10 g glucose (M = 180) in 250 g water.

Accepted Answer

Moles = 10/180 = 0.0556. Molality = 0.0556/0.25 = 0.222 m.

Question 2

Pure water has vapour pressure 23.76 mm Hg at 25 C. Find the vapour pressure of a solution of 10 g urea (M = 60) in 100 g water.

Accepted Answer

Moles urea = 0.167, moles water = 5.556. x(water) = 5.556/5.723 = 0.971. p = 23.76 x 0.971 = 23.07 mm Hg.

Question 3

0.5 g of a non-electrolyte in 50 g water gives a freezing point of -0.186 C (Kf = 1.86). Find the molar mass.

Accepted Answer

delta-Tf = Kf m: 0.186 = 1.86 x (0.5/M)/0.05 = 1.86 x 10/M. M = 18.6/0.186 = 100 g/mol.

Question 4

Calculate the osmotic pressure of 0.1 M glucose solution at 27 C.

Accepted Answer

pi = CRT = 0.1 x 0.0821 x 300 = 2.46 atm.

Question 5

A 0.1 M KCl solution has delta-Tf = 0.335 C (Kf = 1.86). Find the van't Hoff factor.

Accepted Answer

delta-Tf = i Kf m: 0.335 = i x 1.86 x 0.1, so i = 0.335/0.186 = 1.80 (less than 2 due to ion pairing).

Practice — Solutions