Molar Volume of Gases

​Moles and Volume

By Stacey Kohler

We usually use mass to measure out a mole

​But what about gases?

You can't really weigh a helium balloon! Gases like helium present some special problems, so using mass to measure them is difficult. Instead, we use volume. At Standard Temperature and Pressure (STP), 1 mole of any gas will occupy a volume of 22.4 L. You should write that down on your stoichiometry page

What is STP?

​Gases can expand or contract

So we have to pick a standard temperature and pressure to measure them at. STP is 0 degrees C and 1 atmosphere of pressure. If you see a problem that asks about a gas at STP, you know that 1 mole of a gas at STP will occupy 22.4 L. If you see STP, be happy, because it means that everything is normal and you don't have to do anything weird in the problem.

Multiple Choice

Standard temperature and pressure (STP) mean

• A temperature of 0°C and a pressure of 1 atmosphere (atm).

• A temperature of 100°C and a pressure of 1 atmosphere (atm).

• A temperature of 0°K and a pressure of 10 atmosphere (atm).

Multiple Choice

What is the volume of 5 moles of oxygen gas at STP?

• 112 L

• 22.4 L

• 120 L

• 24 L

Multiple Choice

How many moles of methane gas molecules, CH₄, are in 11.2 liters of methane at STP?

• 11.2 moles

• 0.5 moles

• 179.2 moles

• 27.2 moles

Multiple Choice

A 2.00 L sample of pure oxygen gas (O₂) is measured at STP.  How many moles of oxygen is this?

• 24.4 mol
• 0.007 mol
• 1.44 mol
• 0.09 mol

Multiple Choice

What is the volume of 3 moles of hydrogen gas at STP?

• 22.4 L

• 24 L

• 67.2 L

• 89.6 L