Finding molecular weight

Question

A liquid was vaporized which had a total of 165mL at 100°C, and 754 torr. The mass of the vapor was .38g. What’s the molecular weight of this liquid?

Answer 1

Use the ideal gas law to estimate the number of moles of liquid that was present.

PV = nRT

P = pressure =754 torr x 1 atm / 760 torr = 0.992 atm

V = volume in L = 165 ml x 1 L / 1000 ml = 0.165 L

n = mols = ?

R = gas constant = 0.0821 Latm/Kmol

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

Solving for n (moles):

n = PV/RT = (0.992 atm)(0.165 L) / (0.0821 Latm/Kmol)(373K)

n = 0.00534 mols

molar mass = molecular weight = g / mol

molar mass = 0.38 g / 0.00534 mols

molar mass = 71.1 g / mol = 71 g/mol (2 sig. figs.)