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Q.
Calculate the vapour pressure of a solution at $100^{\circ} C$ containing $3\, g$ of cane sugar in $33 \,g$ of water. (At wt. $C =$ $12, H =1, O =16)$
Solutions
Solution:
Vapour pressure of pure. water (solvent) at $100^{\circ} C$ ,
$ p ^{\circ}=760\,mm .$
Vapour pressure of solution, $p =$ ?
Wt. of solvent, $W=33 \,g$
Wt. of solute, $w=3\, g$
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,
$\frac{p^{\circ}-p}{p^{\circ}}=\frac{w M}{W m}$
$\therefore p = p ^{\circ}-\frac{w \times M}{m \times W} \times p ^{\circ}$
$p =760-\frac{3 \times 18}{342 \times 33} \times 760$
$(\therefore p ^{\circ}$ for $H _{2} O =760 \,mm )$
$=760-3.19=756.90 \,mm$