Question

1.0g of a protein is dissolved in 100 ml of distilled water. The resulting solution has an osmotic pressure of .001 atm. What is the molecular weight of the protein? Use r = .0821 l atm-1 mol-1.

Answer 1

The osmotic pressure π, is related to concentration i.e. molarity (M) through the following equation:

π = MRT ------(1)

where R = gas constant = 0.0821 L atm/mol-K

T = temperature (let us assume 25 C = 25 + 273 = 298 K)

M = π/RT = 0.001/0.0821 * 298 = 4.09 * 10^-5 M

Now,

molarity = moles of solute/volume of solvent in Liters

            = (mass of protein/molar mass)/volume

4.09 * 10^-5 moles/L = (1.0 g/molar mass)/0.1 L

Molar mass of protein = 2.445 * 10^5 g/mole