What is the mass, in grams, of ch4 gas in a 5.00 l cylinder at a pressure of 781 torr and a temperature of 25.0 °c?

We assume that this gas is an ideal gas which allows us to use the ideal gas equation expressed as PV=nRT. We calculate as follows:

PV = nRT

781 torr (1atm / 760 torr) (5 L) = n (0.08206 atm L / mol K) (298.15 K)

n = 0.21 mol CH4 gas

m = n (molar mass) = 0.21 mol (16.05 g / mol) = 3.37 g CH4

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