10.2. Mass, Volume, and the Mole http://www.ck12.org
10.2 Mass, Volume, and the Mole
Lesson Objectives
- Be able to convert between mass and moles of a substance.
- Explain how Avogadro’s hypothesis relates to volumes of gases at standard temperature and pressure.
- Convert between moles and volume of a gas at STP.
- Calculate the mass of a gas at STP when given its volume.
- Make conversions between mass, number of particles, and gas volumes.
Lesson Vocabulary
- standard temperature and pressure (STP): A pressure of one atmosphere and a temperature of 0°C.
Check Your Understanding
- What is the value of Avogadro’s number, and what does it represent?
Introduction
As we discussed in the previous lesson, there are many different ways to measure how much of something you
have. Usually, there is a particular unit of measurement that is easiest to use, depending on what you are trying to
quantify. In this lesson you will learn the significance of using moles in converting between measurements and in
understanding how much of something you have.
Mass and Moles
When we need materials for a chemical reaction, counting out a certain number of atoms or molecules is obviously
impractical, so we weigh out a certain mass of each substance instead. As we will see in a later chapter, chemical
equations tell us the molar ratios in which chemicals react with one another. This information can be used to
determine how much of one chemical is needed to fully react with a set amount of another substance.
Example 10.6
In a certain reaction, we want to use two moles of silver nitrate, AgNO 3. We need to know how many grams of
silver nitrate will be needed. First, we determine the molecular weight of the chemical:
Ag: 1 x 107.9 g/mol = 107.9 g/mol