Question

Calculate the osmotic pressure of each of the following aqueous solutions at 20C. a. 24.5 mL of glycerol in 250 mL of aqueous solution (density of glycerol is 1.265g/mL) b. 22.5g of CaCl2 in 350 mL of solution.

Answer 1

A. Osmotic pressure is calculated by following equation

Pi = i M R T
where, i is the dimensionless van 't Hoff factor
M is the molarity
R=0.082 057 L atm K−1 mol−1 is the gas constant
T is the thermodynamic (absolute) temperature

For Glycerol, Molar mass of C3H8O3 is 92.09 g/mol
molarity of the solution = 24.5 g / 92.09 g mol = 0.266 L = 1.064 M

here Pi = 1 * 1.064 * 0.082 * 293
= 25.56 atmospheres

B.

For Ca(OH)2, Molar mass of Ca(OH)2 is 74.09 g/mol
molarity of the solution = 22.5 g / 74.09 g mol = 0.303 L = 0.865 M

i = Ca(OH)2 ----> Ca^2+, 2 OH- =3

here Pi = 3 * 0.865 * 0.082 * 293
= 62.39 atmospheres