Question

The vapour pressure of water at $20\text{℃}$ is 17.54 mm Hg. When 20 g of a non-ionic, substance is dissolved in 100 g of water, the vapour pressure is lowered by 0.30 mm Hg. What is the molecular mass of the substance?

• A. 200.8 g/mole
• B. 207.087 g/mole
• C. 210.5 g/mole
• D. 215.2 g/mole

Answer 1

Answer: (B)

$\frac{{\text{p}}^{\circ }-{\text{p}}_{\text{s}}}{{\text{p}}^{\circ }}={\text{X}}_{\text{s}\text{o}\text{l}\text{u}\text{t}\text{e}}$
or
$\frac{{\text{p}}^{\circ }-{\text{p}}_{\text{s}}}{{\text{P}}_{\text{s}}}=\frac{\text{n}}{\text{N}}$
$\text{∴}\frac{0.3}{17.24}=\frac{\frac{20}{M}}{\frac{100}{18}+\frac{20}{M}}\{M=molarmassofsolute\}$
$So,M=207.087g/mol$