Exactly $0.5 \mathrm{~g}$ of a volatile liquid, upon complete vaporisation, yields $112 \mathrm{~mL}

Question

Exactly $0.5 \mathrm{~g}$ of a volatile liquid, upon complete vaporisation, yields $112 \mathrm{~mL}$ of vapour at STP. What is the vapour density of the substance?

Professor PC Thomas Classes · Answer

(b) : The number of moles of the vapour at STP = Volume at STP / $22400 \mathrm{~mL} = 112 / 22400 = 0.005 \mathrm{~mol}$. Molar mass of the substance = Mass / Moles = $0.5 / 0.005 = 100 \mathrm{~g/mol}$. The vapour density is defined as exactly half of the molar mass. Therefore, Vapour density = $100 / 2 = 50$.