Question

A solution containing 60 g of a non-volatile solute in 250 g of water freezes at 270.67 K. Calculate the molar mass of the solute. (K_f of water = 1.86 K kg mol⁻¹).

Answer 1

Molar mass of solute `(M_B) = (K_f xx W_B xx 1000)/(\DeltaT_f xx W_A)`

W_B = Weight of solute = 60 g

W_A = Weight of solvent = 250 g

K_f (Cryoscopic constant of water) = 1.86 K kg/mol

ΔT_f = Depression in freezing point = `T_f^0 - T_f`

= 273 − 270.67 

= 2.33

So, `M_B = (1.86 xx 60 xx 1000)/(2.33 xx 250)`

= 191.58 g/mol