http://www.ck12.org Chapter 10. The Mole
If we know the volume of a gas at STP, we can combine this information with its molar mass to determine how much
mass is present in the sample.
Example 10.12
What is the mass of 86.7 liters of N 2 at STP?
The molar mass of N 2 is 28.0 grams/mole, and because one mole of a gas at STP takes up 22.4 L of space, we can
perform the following calculation.
( 86 .7 L N 2 )( 22 1 mol N.4 L N^22 )(^28 1 mol N.0 g N 22 ) =108 g N 2
The conversion factors are arranged so that all units cancel except for grams, which are the units of our final answer.
When studying chemical reactions, we frequently need to convert back and forth between mass, volume, moles,
and number of particles. This will be expanded upon in our chapter on stoichiometry.Figure10.3 summarizes the
relationships that we have studies so far between these different quantities.
FIGURE 10.3
Mole relations for mass, volume, and par-
ticles.Lesson Summary
- Molar masses can be used to determine the mass of a given quantity of material.
- At standard temperature and pressure (1 atmosphere of pressure and 0°C), one mole of a gas occupies 22.4
liters. - The mass of a sample can be calculated for a given volume of a known gas at STP using Avogadro’s number.
Lesson Review Questions
- Calculate the number of moles in 56.3 grams of CaCO 3.