Question

An aqueous solution of NaCl has boiling point of 102.42 oC. What is the corresponding freezing point of this solution?
a) -2.42 °C
b) -8.77 °C
c) 8.60 °C
d) 2.42 °C

Answer 1

An aqueous solution of NaCl has boiling point, T_b = 102.42 ^oC and also we know that the boiling point constant, K_b of water is 0.512 ^oC/molal.

Using the formula, we can calculate the molality of the given NaCl solution.

  , where, T_b^*= 100 ^oC

vant's Hoff factor, i = 2, NaCl completely dissociates into two ions

Now by putting the values,

  

  

  

We can now calculate freezing point of the solution by using the formula of depression in freezing point,i.e.,

  

, where, T_f^*= 0 ^oC

vant's Hoff factor, i = 2 and K_f = 1.86^oC/molal

Therefore option b) -8.77 ^oC is correct.