Ideal gas calculation

Question

Consider a sample of helium in a 5.0 liter tank with a pressure of 785 mm Hg at a temperature of 18 degree Celsius.

What is the mass of the gas sample?

Answer 1

Another Gas Law is the Ideal gas Law PV = nRT where n represents the number of moles of the gas and R represents the gas constant. The # of moles, n you should remember is calculated using the mass divided by the molar mass of the gas.

The molar mass of the gas CAN be given as the value or indicated through the name of the gas. You should know whether the gas is a monoatomic or diatomic gas { Helium is He }.

Using the information given in the problem, identify the parameter... The value used for R depends on the unit for pressure! This is a constant usually given in exams...

P = 786 mm Hg [= 785 torr] ; T = 18 +273 = 291K ; R = 62.4 L.torr/mol.K

Using the information you can calculate the # of moles of the gas ; then the mass of the gas.

785 torr X 5.0 L = n X 62.4 L. torr/mol.K X 291 K

n = 0.216 mol Helium = mass / molar mass (4.0 g/mol)

Mass of gas = 0.86 grams

Note: you were given 2 significant figures in the volume, therefore answer needs to be given to 2 significant figures.

Again these are gas law formulas that students needs to know and understand how to use them.

Another way of stating the formula above is

PV =g/M(RT) where g/M = n and M = molar mass of the gas