Determine the molecular weight of the vapour.

Question

A 256.0 cm^3 flask is filled with a vapour at 97.325 KPa and 100 C. The mass of the vapour is 1.675 g. Determine the molecular weight of the vapour. Use R = 8.314 JK^-1mol^-1.

Answer 1

Molecular weight (molar mass) is defined as grams / moles. Therefore, to determine molar mass we need to find the moles since we are given the grams (1.675 g). To do this, we will use the ideal gas law: PV = nRT

P = pressure = 97.325 kPa

V = volume in liters = 256.0 cm3 x 1 L / 1000 cm3 = 0.2560 L

n = mols = ?

R = gas constant = 8.314 L-kPa / K-mole (note to use the R value that has kPa in the units)**

T = temperature in Kelvin = 100ºC + 273 = 373K

Solving for n (moles), we have...

n = PV/RT = (97.325)(0.256) / (8.314)(373)

n = 0.0080343 moles

Molecular weight (molar mass) = g / moles

molar mass = 1.675 g / 0.0080343 mols

molar mass = 208.48 g / mol

** NOTE that the units of R given in the problem are J/Kmol which are correct units if calculating energy (joules) but incorrect for use in the gas laws.