Question

22. (8 points) The “gas density” method is a classical method used to measure the molecular mass of a pure sample of an unknown gas. The flask has a known volume; P and I are measured, and n is calculated. The mass of the flask with the gas sample is measured and compared to the mass of the empty flask. A 0.1006 gram sample of a pure gas is collected in a flask that has a volume of 22.1 mL, and P= 1.02 atm. The temperature is measured to be 20.0°C. What is the molar mass of the compound? If the compound is known to have an empirical formula of SFA, what is its molecular formula?

Answer 1

Using the ideal gas equation

PV = nRT

where n, number of moles = mass/ molar mass

Given

P =1.02 atm

V= 22.1 mL = 22.1x10^-3 L

T=20 C = 20+273= 293 K

mass = 0.1006g

Substituting the known values

1.02 atm x 22.1x10^-3 L = [0.1006g/ molar mass] x 0.0821 L.atm/mol.K x 293 K

molar mass = 107.35 g/mol

2)

If the empirical formula =SF4

empirical formula mass = 108g/mol

then molar mass is approximately equal to empirical formula mass, thus molecular formula = SF4