Question

Arrange the following in increasing order of osmotic pressure and give reasons in support of your answer.

1. 34.2 g per litre of sucrose (MW = 342)
2. 90.0 g per litre of glucose (MW = 180)
3. 5.85 g per litre of sodium chloride (MW = 58.5)

Answer 1

34.2 g per litre of sucrose < 5.85 g per litre of sodium chloride < 90.0 g per litre of glucose

Reason:

Osmotic pressure π is given by:

π = iCRT

Where:

i = Van’t Hoff factor (depends on dissociation)

C = molar concentration = `w/(M * V)`

R, T = constants

Molar concentration (C) for:

Sucrose = `C = 34.2/(342 xx 1) = 0.1` mol/L, i = 1

Glucose = `C = 90/(180 xx 1) = 0.5` mol/L, i = 1

NaCl = `C = 5.85/(58.5 xx 1) = 0.1` mol/L, i = 2

Effective iC for sucrose = 1 × 0.1 = 0.1

Effective iC for glucose = 1 × 0.5 = 0.5

Effective iC for NaCl = 2 × 0.1 = 0.2

Since π ∝ iC, osmotic pressure order:

Sucrose < NaCl < Glucose

Sucrose has the lowest concentration and does not dissociate. NaCl has the same molar concentration as sucrose but dissociates into two particles. Glucose has the highest concentration and therefore exhibits the highest osmotic pressure. Hence, a greater number of solute particles results in a higher osmotic pressure.