Question

(a) 30 g of urea (m = 60 g $mo{l}^{-1}$) is dissolved in 846 g of water. Calculate the vapour pressure of water for this solution if vapour pressure of pure water at 298 K is 23.8 mm Hg.
(b) Write two differences betwen ideal solutions and non- ideal solutions.

Answer 1

(a) Mass of urea $\left(w_1\right)=30$ g
Mass of water $\left(w_2\right)=846$ g
Molar mass of urea $\left(M_1\right)=60gmo{l}^{-1}$
Molar mass of water $\left(M_2\right)=18gmo{l}^{-1}$
So
$X_{H_{2}O}=\frac{P^{\circ }-P_s}{P^{\circ }}$
$\Rightarrow \frac{23.8-P_3}{23.8}=\frac{w_1\left(urea\right)\times M_2\left(H_{2}O\right)}{M_1\left(urea\right)\times w_2\left(H_{2}O\right)}\frac{23.8-P_s}{23.8}=\frac{30\times 18}{60\times 846}23.8-P_s=0.0106\times 23.8=0.2523P_s=23.8-0.2523=23.55$ mm Hg
(b) $$\begin{array}{cc}\text{Ideal solutions}& \text{Non - ideal solutions}\\ \text{- Obey Raoult's Law}& \text{- Do not obey Raoult's Law}\\ \text{- Heat is neither absorbed nor evolved}& \text{- Heat can be evolved or absorbed}\end{array}$$