# molar mass of unknown gas?

A sample of an unknown gas weighs 0.419 grams and produced 5.00L of gas at 1.00atm (unknown gas only) and 298.15K, what is the molar mass (g/mole) of this unknown gas? Remember to adjust your calculated answer for significant figures.

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• let's assume the gas is ideal and let's use dimensional analysis to setup and solve this problem • Login to reply the answers
• Molar mass of a gas ....

PV = nRT ............ ideal gas equation

PV = mRT / M ..... n = m / M ...... m = mass, M = molar mass

M = mRT / PV ..... solve for M

M = 0.419g x 0.08206 Latm/molK x 298.15K / 1.00 atm / 5.00L

M = 2.05 g/mol .... to three significant digits

That's close to the molar mass of hydrogen (H2), which, to three significant digits, has a molar mass of 2.02 g/mol

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• Use the Ideal Gas Eq'n

PV = nRT

n = PV/RT

P = 1 atm = 101532 Pa

V = 5.0L = 0.005 m^3

R = 8.314 The Gas Constant

T = 298.15 K

Sbstitute

n = (101532 X 0.005) / ( 8.314 X 298.15)

n = 0.2047 moles.

Hence

Mr = 0.419 g / 0.2017 moles

Mr = 2.38 g/mole

• Dr W
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