Question

Calculate the vapour pressure of a solution at $100^{\circ }C$ containing $3g$ of cane sugar in $33g$ of water. (At wt. $C=$ $12,H=1,O=16)$

• A. 760 mm
• B. 756.90 mm
• C. 758.30 mm
• D. None

Answer 1

Answer: (B)

Vapour pressure of pure. water (solvent) at $100^{\circ }C$ ,
$p^{\circ }=760mm.$
Vapour pressure of solution, $p=$ ?
Wt. of solvent, $W=33g$
Wt. of solute, $w=3g$
Mol. wt. of water $\left(H_{2}O\right),M=18$
Mol. wt. of sugar $\left(C_{12}{H}_{22}{O}_{11}\right)$
$m=12\times 12+22\times 1+11\times 16=342$
According to Raoult's law,
$\frac{p^{\circ }-p}{p^{\circ }}=\frac{wM}{Wm}$
$\therefore p=p^{\circ }-\frac{w\times M}{m\times W}\times p^{\circ }$
$p=760-\frac{3\times 18}{342\times 33}\times 760$
$(\therefore p^{\circ }$ for $H_{2}O=760mm)$
$=760-3.19=756.90mm$