Question

The vapour pressure of pure water at 26${}^o$C is 25.21 torr. What is the vapour pressure of a solution which contains 20.0 g of glucose (C${}_6$H${}_{12}$O${}_6$), in 70 g water?

• A. $24.5$
• B. $24.8$
• C. $25.3$
• D. $25.5$

Answer 1

The correct option is A $24.5$

The relationship between the relative lowering of vapour pressure and the molecular weight of the solute and solvent is as shown below.

$\frac{\Delta P}{P}=\frac{W}{M}\times \frac{m}{w}$

The molecular weight of glucose is 180 g/mol.

Substitute values in the above equation.

$\frac{\Delta P}{25.21}=\frac{20}{180}\times \frac{18}{70}$

$\Delta P=0.72torr$.

Thus $25.21-p=0.72torr$ or $p=24.5torr$

Hence, the correct option is $\text{A}$