http://www.ck12.org Chapter 10. The Mole
10.2 Mass, Volume, and the Mole
Lesson Objectives
- Use molar mass to make conversions between mass and moles of a substance.
- Explain Avogadro’s hypothesis and how it relates to the volume of a gas at standard temperature and pressure.
- Convert between moles and volume of a gas at STP.
- Calculate the density of gases at STP.
- Use the mole road map to make two-step conversions between mass, number of particles, and gas volume.
Lesson Vocabulary
- Avogadro’s hypothesis
- molar volume
- standard temperature and pressure (STP)
Check Your Understanding
Recalling Prior Knowledge
- What is a mole? How is the mole related to the number of particles in a sample of matter?
- What is the molar mass of a substance and how is it calculated?
Chemistry is about the study of chemical reactions. Quantitatively, chemicals react with one another in very specific
ratios based upon the number of reacting particles. A more in depth study of chemical reactions requires the ability
to make conversions between mass, volume, and moles.
Mole-Mass Relationship
You now know that the molar mass of any substance is the mass in grams of one mole of representative particles
of that substance. The representative particles can be atoms, molecules, or formula units of ionic compounds. This
relationship is frequently used to make calculations in the laboratory. Suppose that you need 3.00 moles of calcium
chloride (CaCl 2 ) for a certain experiment. Since calcium chloride is a solid (Figure10.5), it would be convenient to
measure out this amount by using a balance, but, to do so, you would need to know what mass of CaCl 2 is equivalent
to 3.00 moles. The molar mass of CaCl 2 is 110.98 g/mol, so a conversion factor can be constructed based on the fact
that 1 mol CaCl 2 = 110.98 g CaCl 2. Dimensional analysis will then allow you to calculate the mass of CaCl 2 that
you should use.