Question

If the sum of elevation in boiling point and depression

in freezing point of an aqueous solution of urea is
10, find molaliry of the solution
(1) 2.1
(3) 4.2
(2) 2.38
(4) 5

Answer 1

The molality of the solution, given that the sum of elevation in boiling point and depression in freezing point is 10, is 4.2 (option 3)

How to calculate the molality of the solution?

First, we shall obtain an expression justifying the question given above. This is shown below:

For freezing point, we have

• ΔTf = Kf × m

For boiling point, we have:

• ΔTb = kb × m

Now, we were told that the sum of elevation in boiling point and depression in freezing point is 10. Thus, we have:

`\Delta T_f\ +\ \Delta T_b\ = 10\\\\mK_f\ +\ mK_b\ = 10\\\\m(K_f\ +\ K_b)\ = 10\\\\m = (10)/((K_f\ +\ K_b)) \\\\`

Now, we shall calculate the molality of the solution. Details below:

• Freezing point depression (Kf) = 1.86 °C/m
• Boiling point elevation (Kb) = 0.512 °C/m
• Molality of solution (m) =?

`m = (10)/(K_f\ +\ K_b)\\\\m = (10)/(1.86\ +\ 0.512)\\\\m = 4.2\ m\\`

Thus, the correct answer is option 3