A chemist isolated a gas in a glass bulb with a volume of 255 mL at a temperature of 25.0 degree C and a pressure ( in the bulb ) of 10.0 torr. The gas weighed 12.1 mg. What is the molar mass of this gas?

1 Answer
Ernest Z.
Mar 30, 2015

The molar mass of the gas is 88.2 g/mol.

We can solve this problem by using the Ideal Gas Law:

#PV = nRT#

#n = "mass"/"molar mass" = m/M#

So, #PV = (m/M)RT# and

#M = (mRT)/(PV)#

#P = 10.0 cancel("torr") × "1 atm"/(760 cancel("torr")) = "0.013 16 atm"#

#T = "(25.0 + 273.15) K" = "298.15 K"#

#M = (mRT)/(PV) = ("0.0121 g × 0.082 06" cancel("L·atm·K⁻¹")"mol⁻¹" × 298.15 cancel("K"))/(0.013 16 cancel("atm") × 0.255 cancel("L")) = "88.2 g/mol"#

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