Question

What is the ratio by mass of NaF and NaI which when dissolved in water produces the same osmotic effects as 0.1 molar solution of urea in water? The mass of residue obtained on evaporation of the salt solution is 0.48 g per 100 mL of solution evaporated. Assume complete dissociation of the salts. Write answer after multiplying with 10 to the nearest integer.

Answer 1

Let NaF and NaI be a and b respectively, then for 100 mL solution
$a+b=0.48$ .....(i)
Thus, for NaF+NaI solution in water
$\pi \times \frac{100}{1000}=(\frac{a}{42}+\frac{b}{150})\times 2\times 0.0821\times T$ .....(ii)
For urea in water, $\pi \times 1=0.1\times 0.0821\times T$ .....(iii)
By eqs. (ii) and (iii)
$\frac{(\frac{a}{42}+\frac{b}{150})\times 2}{0.1}=\frac{100}{1000}$
$\therefore 150a+42b=31.5$ .....(iv)
By Eqs. (i) and (iv),
$a=0.105$ and $b=0.375$
Thus, $\frac{a}{b}=\frac{0.105}{0.375}=0.28$
Thus, the ratio by mass of NaF and NaI is 0.28:1.