Question

$0.39g$ of a liquid on vapourization gave $112$ ml of vapor at STP. Its molecular weight is:

• A. $39$
• B. $18.5$
• C. $112$
• D. None of the above

Answer 1

Answer: (D)

None of the above

At STP, 1 mole of vapour $=22400ML$
At STP, 112 mL of vapour $=\frac{112mL}{22400mL/mol}=0.005mol$

0.005 moles of vapour $=$ 0.005 moles of liquid.
Density of liquid $=\frac{\text{mass}}{\text{volume}}=\frac{0.39g}{0.005mol}=78g/mol$

Hence, the molecular weight of liquid is 78 g/mol.