Question

1. 0.680 g of a compound is dissolved in 15.0 g of benzene, and the freezing point of the solution is lowered by 1.44°C.
   Calculate the experimental molecular mass of the compound.
   (K_f for benzene = 5.12 K kg mol⁻¹)
2. If the theoretical molecular mass of the compound referred to above is 80.5 g/mol⁻¹, suggest whether it is undergoing association or dissociation.

Answer 1

(i) Given: Weight of solute (w₂) = 0.680 g

Weight of solvent (benzene, w₁) = 15.0 g

Freezing point depression (ΔT_f) = 1.44 K (or °C)

Cryoscopic constant (K_f) for benzene = 5.12 Kg mol⁻¹

Formula: `M_2 = (1000 xx K_f xx w_2)/(ΔT_f xx w_1)`

= `(1000 xx 5.12 xx 0.680)/(1.44 xx 15.0)`

= `3481.6/21.6`

= 161.18 g/mol

(ii) Conclusion: The compound is undergoing association.

Justification: Since the Experimental Molecular Mass (161.18 g/mol) is greater than the Theoretical Molecular Mass (80.5 g/mol), it indicates that the solute particles are joining together (associating) in the solution.