Question

what mass of Nacl must be dissolved in
65g of water to lower the freezing point
of water by 7.50°C? The freezing point
depression constant (kf) for water is
1. 86°c/m Assume vant hoff factor for Nacl is
1.87 ( Molan mass of Nacl: 58.5 g/mol)
​

Answer 1

Answer:

The formula for depression on freezing point is given as

ΔTf=iKfm

Where ΔTf is the depression of freezing point i is the Van't Hoff factor , m is the molality of the solution and Kf is the freezing point depression constant .

We are given that ΔTf=7.50C,Kf=1.86∘C/m

Let z grams of NaCl be present in 65 g of water.

Therefore

ΔTf=iKf×[(z/58.5)×(1000/65)]

(as molarity = number of moles of solute / Mass of solvent in kg)

=>7.5=1.87×1.86×[(z/58.5)×(1000/65)]

z=8.2g