You may be asked to determine the volume, in liters, of a certain gas at STP (standard temperature and pressure.) When converting from liters to moles or moles to liters, you will use the equality 1 mole=22.4 L for a gas at STP. This number can be found on the Constants and Conversions section of the STAAR reference material.

Source: STAAR Reference Material, Texas Education Agency

This equality can be written as a set of two conversion factors. They are as follows:

1 mole over 22.4 L 1 mole 22.4 L     or     22.4 L over 1 mole 22.4 L 1 mole

Remember these conversion factors can only be used at standard temperature and pressure.

Let’s practice a few mole-molar conversions. You will use the steps for dimensional analysis to help solve these problems.

  1. Write the given information as a fraction by placing it over 1. (Placing it over 1 makes it a fraction but does not change its value.)
  2. Write a conversion factor that has the unit you want to remove in the denominator and the unit you want to end up with in the numerator. After you fill in your units, add the numbers. (Usually 1 of the numbers is a 1, but it can be in either the denominator or the numerator.) Note: In some cases you may need to repeat this step a number of times in order to get the unit you want to end up with in the numerator.
  3. Mark through the units to double check that they all cancel and that you are left with the units you want.
  4. Multiply the numbers in the numerators, and then multiply the numbers in the denominators.
  5. Divide the numerator by the denominator. Be sure to add your units to your final answer.

Practice Problem 1

What is the volume, in liters, occupied by 0.030 moles of a gas at STP ?

Practice Problem 2

How many moles of argon are present in 11.2 L of argon gas at STP?

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