7.2 Molecular Mass, Avogadro’s Number and The Mole
7.2 Molecular Mass, Avogadro’s Number and The Mole
Learning Objectives
By the end of this section, you will be able to:
- Calculate molecular masses (or numerically equivalent molar masses) for covalent and ionic compounds
- Explain the relation between mass, moles, and numbers of atoms or molecules, and perform calculations deriving quantities from one another
We can argue that modern chemical science began when scientists started exploring the quantitative as well as the qualitative aspects of chemistry. For example, Dalton’s atomic theory was an attempt to explain the results of measurements that allowed him to calculate the relative masses of elements combined in various compounds. Understanding the relationship between the masses of atoms and the chemical formulas of compounds allows us to quantitatively describe the composition of substances.
Molecular Mass for Substances
The formula represents the numbers and types of atoms composing a single molecule of the substance; therefore, the formula mass may be correctly referred to as a molecular mass. This value of the formula mass can also be used to consider a substance’s molar mass (numerically equivalent to molecular mass per 1 mole of substance as discussed in section 7.1). Consider chloroform (CHCl3), a covalent compound once used as a surgical anesthetic and now primarily used in the production of the “anti-stick” polymer, Teflon. The molecular formula of chloroform indicates that a single molecule contains one carbon atom, one hydrogen atom, and three chlorine atoms. The average molar mass of a chloroform molecule is therefore equal to the sum of the average atomic masses of these atoms. Figure 7.2a outlines the calculations used to derive the molecular mass of chloroform, which is 119.37 amu. The molecular mass of 119.37 amu expressed as a molar mass is 119.37 g/mol.
Likewise, the molecular mass of an aspirin molecule, C9H8O4, is the sum of the atomic masses of nine carbon atoms, eight hydrogen atoms, and four oxygen atoms, which amounts to 180.15 amu (Figure 7.2b) or 180.15 g/mol (molar mass).
What is the molecular mass (amu) for this compound? What is the molar mass?
Solution
Molecules of this compound are comprised of 13 carbon atoms, 18 hydrogen atoms, and 2 oxygen atoms. Following the approach described above, the average molecular mass for this compound is, therefore:
The molecular mass is 206.27 amu. The molar mass is 206.27 g/mol.
Exercise 7.2a
Acetaminophen, C8H9NO2, is a covalent compound and the active ingredient in several popular nonprescription pain medications, such as Tylenol. What is the molecular mass (amu) for this compound? What is the molar mass?
Check Your Answer[1]
Example 7.2b
Computing Molecular Mass for an Ionic Compound
Aluminum sulfate, Al2(SO4)3, is an ionic compound that is used in the manufacture of paper and in various water purification processes. What is the molecular mass (amu) of this compound? What is the molar mass?
Solution
The formula for this co