Question

34.05 mL of phosphorous vapour weighs 0.0625 g at 546°C and 0.1 bar pressure. What is the molar mass of phosphorous?

Answer 1

Mass of phosphorous vapour

W = 0.0625 g

Volume of phosphorous vapour

V = 34.05 mL

Temperature, T = 546°C + 273° = 819 K

Pressure, p = 0.1 bar

For the gas equation,

pV = (W/M)RT

or, M = (W/pV)RT

or, M = {0.0625}/{0.1 bar x 34.05 mL} x 82.1 mL bar K-1 mol-1 x 819 K

= 1247.7 g