Question

What amount of a non-volatile solute (urea)should be dissolved in $100g$ of water in order to decrease the vapour pressure of water by $25\%$?
Assume the solution to be non-dilute

• A. $1.01g$
• B. $1.1g$
• C. $111.1g$
• D. $11.1g$

Answer 1

The correct option is C $111.1g$

Using Raoult's law in the following form for non dilute solution:
$\frac{P_A^o-P_s}{P_s}=\frac{w_B\times M_A}{w_A\times M_B}$
$w_A=\text{mass of A (Solvent)}$
$w_B=\text{mass of B (Solute)}$
$M_A=\text{molar mass of A}$
$M_B=\text{molar mass of B}$

If $P_A^o=100mm$, then $P_s=75mm$
$\frac{100-75}{75}=\frac{w_B\times 18}{100\times 60}{w}_B=111.1g$