Question

substance a tetramerise in water to the extent of 80percentage. a solution of 2.5g of a in 100 g of water lowers the freezing point by 0.3c. the molar mass of a is kf for water 1.86k kg /mol.

Answer 1

Solution 'a' tetramerise in water to the extent of 80%
Means, Van Hoff's factor , i = 80/100 = 0.8
Lowers the freezing point, ∆Tf = 0.3°C
Kf = 1.86 kg/mol

We know, ∆Tf = i × m × Kf
Where m is molality.
so, 0.3 = 0.8 × m × 1.86
m = 0.3/(1.86 × 0.8) = 0.2

we also know, molality = mass of solute × 1000/{mass of solvent × molar mass of solute }
Given, mass of solute = 2.5g
mass of solvent = 100g
molar mass of solute , 'a' = ? Let molar mass of solute , 'a' = M
∴ 0.2 = 2.5 × 1000/(100 × M}
⇒ M = 25/0.2 = 125 g/mol

Hence, molar mass of a = 125 g/mol