Question

The vapour pressure of water is 17.54 mm Hg at 293 K. The vapour pressure of 0.5 molal solution of a solute in it is:

• A. 17.38 mm Hg
• B. 76.26 mm Hg
• C. 23.72 mm Hg
• D. 12.82 mm Hg

Answer 1

The correct option is A 17.38 mm Hg

molal solution means 1 mole of solute dissolved in 1 kg of the solvent

pressure of solution = mole fraction of solvent X vapour pressure of pure solvent

$\text{Moles of solvent}=\frac{\text{mass}}{\text{molar mass}}$

$=\frac{1000g}{18g/mol}$

= 55.55 moles

so, total moles = 55.5 + 1

= 56.1 moles

So mole fraction of water $=\frac{\text{no of moles of water}}{\text{total no of moles of solution}}=\frac{55.5}{56.5}$

$=0.98$

So vapour pressure of solution $=0.98\times 17.54$

= 17.38 mm of Hg

Hence, the correct option is $A$