6.2: Formula mass
16
Harper College Chemistry Department
Summary
By the end of this section, you will be able to:
- Calculate formula masses for covalent and ionic compounds
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.
Formula Mass
In an earlier chapter, we described the development of the atomic mass unit, the concept of average atomic masses and the use of chemical formulas to represent the elemental makeup of substances. These ideas can be extended to calculate the formula mass of a substance by adding the average atomic masses of all the atoms represented in the substance’s formula.
Formula Mass for Covalent Substances
For covalent 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. Consider chloroform (CHCl 3 ), 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 molecular mass of a chloroform molecule is therefore equal to the sum of the average atomic masses of these atoms. Figure 1 outlines the calculations used to derive the molecular mass of chloroform, which is 119.37 amu.
Likewise, the molecular mass of an aspirin molecule, C 9 H 8 O 4 , 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 2 ).
Computing Molecular Mass for a Covalent Compound:
Ibuprofen, C 13 H 18 O 2 , is a covalent compound and the active ingredient in several popular nonprescription pain medications, such as Advil and Motrin. What is the molecular mass (amu) for this compound?
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:
Check Your Learning
Acetaminophen, C 8 H 9 NO 2 , 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?
Formula Mass for Ionic Compounds
Ionic compounds are composed of discrete cations and anions combined in ratios to yield electrically neutral bulk matter. The formula mass for an ionic compound is calculated in the same way as the formula mass for covalent compounds: by adding the average atomic masses of all the atoms in the compound’s formula. Keep in mind, however, that the formula for an ionic compound does not represent the composition of a discrete molecule, so it may not correctly be referred to as the “molecular mass.”
As an example, consider sodium chloride, NaCl, the chemical name for common table salt. Sodium chloride is an ionic compound composed of sodium cations, Na + , and chloride anions, Cl − , combined in a 1:1 ratio. The formula mass for this compound is computed as 58.44 amu ( Figure 3 ).
Note that the average masses of neutral sodium and chlorine atoms were used in this computation, rather than the masses for sodium cations and chlorine anions. This approach is perfectly acceptable when computing the formula mass of an ionic compound. Even though, a sodium cation has a slightly smaller mass than a sodium atom (since it is missing an electron), this difference will be offset by the fact that a chloride anion is slightly more massive than a chloride atom (due to the extra electron). Moreover, the mass of an electron is negligibly small with respect to the mass of a typical atom. Even when calculating the mass of an isolated ion, the missing or additional electrons can generally be ignored, since their contribution to the overall mass is negligible, reflected only in the non-significant digits that will be lost when the computed mass is properly rounded. The few exceptions to this guideline are very light ions derived from elements with precisely known atomic masses.
Computing Formula Mass for an Ionic Compound:
Aluminum sulfate, Al 2 (SO 4 ) 3 , is an ionic compound that is used in the manufacture of paper and in various water purification processes. What is the formula mass (amu) of this compound?
Solution:
The formula for this compound indicates it contains Al 3 + and SO 4 2 − ions combined in a 2:3 ratio. Three sulfate ions have 3 sulfurs and 12 oxygens (3 ions × 4 oxygens per ion). For purposes of computing a formula mass, it is helpful to rewrite the formula in the simpler format, Al 2 S 3 O 12 . Following the approach outlined above, the formula mass for this compound is calculated as follows:
Check Your Learning
Calcium phosphate, Ca 3 (PO 4 ) 2 , is an ionic compound and a common anti-caking agent added to food products. What is the formula mass (amu) of calcium phosphate?
Key Concepts and Summary
The formula mass of a substance is the sum of the average atomic masses of each atom represented in the chemical formula and is expressed in atomic mass units. The formula mass of a covalent compound is also called the molecular mass. Due to the use of the same reference substance in defining the atomic mass unit and the mole, the formula mass (amu) and molar mass (g/mol) for any substance are numerically equivalent (for example, one H 2 O molecule weighs approximately 18 amu and 1 mole of H 2 O molecules weighs approximately 18 g).
Chemistry End of Chapter Exercises
1. What is the total mass (amu) of carbon in each of the following molecules?
(a) CH 4
(b) CHCl 3
(c) C 12 H 10 O 6
(d) CH 3 CH 2 CH 2 CH 2 CH 3
2.What is the total mass of hydrogen in each of the molecules?
(a) CH 4
(b) CHCl 3
(c) C 12 H 10 O 6
(d) CH 3 CH 2 CH 2 CH 2 CH 3
3. Calculate the molecular or formula mass of each of the following:
(a) P 4
(b) H 2 O
(c) Ca(NO 3 ) 2
(d) CH 3 CO 2 H (acetic acid)
(e) C 12 H 22 O 11 (sucrose, cane sugar).
4. Determine the molecular mass of the following compounds:
(a)
(b)
(c)
(d)
5. Determine the molecular mass of the following compounds:
(a)
(b)
(c)
(d)
6. Which molecule has a molecular mass of 28.05 amu?
(a)
(b)
(c)
Glossary
- Avogadro’s number ( N A )
- experimentally determined value of the number of entities comprising 1 mole of substance, equal to 6.022 × × 10 23 mol −1
- formula mass
- sum of the average masses for all atoms represented in a chemical formula; for covalent compounds, this is also the molecular mass
- molar mass
- mass in grams of 1 mole of a substance
- mole
- amount of substance containing the same number of atoms, molecules, ions, or other entities as the number of atoms in exactly 12 grams of 12 C