Question

What mass of non-volatile solute (urea) needs to be dissolved in $100g$ of water in order to decrease the vapour pressure by $25\%$. What will be the molality of solution?

• A. 18.52 m
• B. 20.22 m
• C. 11.45 m
• D. 66.52 m

Answer 1

Answer: (A)

Using Raoult's law in the following form,
$\frac{p_{\circ }-p_o}{p_s}=\frac{wM}{Wm}$

$P{}_o=Vapourpressureofpuresolvent$

$P{}_S=Vapourpressureofsolution$

$w=weightofnon-volatilesubstance$

$M=Molarmassofsolvent$

$W=Weightofsolvent$

If $p_{\circ }=100mm$, then $p_s=75mm$
$\frac{100-75}{75}=\frac{w\times 18}{100\times 60}$
$w=83.33g$
$\text{Molality}=\frac{w\times 1000}{m\times W}=\frac{83.33\times 1000}{60\times 100}=18.52m$