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Example Problem: Molar Mass of a Gas from its weight and P,V,T

Problem: A sample of natural gas is collected at 25.0 oC in a 250.0 ml flask. If the sample had a mass of 0.118 g at a pressure of 550.0 Torr, what is the molecular weight of the gas? Plan: Use the Ideal gas law to calculate n, then calculate the molar mass.

Solution: P = 550.0 Torr x

1mm Hg 1 Torr

x

1.00 atm 760 mm Hg

= 0.724 atm

V = 250.0 ml x

1.00 L 1000 ml

= 0.250 L

n=

PV RT

T = 25.0 oC + 273.15 K = 298.2 K

n=

(0.724 atm)(0.250 L)

= 0.007393 mol

(0.0821 L atm/mol K) (298.2 K) M = mass / n = 0.118 g / 0.007393 mol = 16.0 g/mol

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