Atomic Foundations of Matter · Lesson 13 of 14
Formula Unit Mass of Ionic Compounds
“Ionic crystals use formula units, so their mass is calculated from the simplest whole-number ion ratio.”
• Define a formula unit. • Define formula unit mass. • Explain why the term molecule is not used for the ionic crystal description in this chapter. • Calculate formula unit mass for simple ionic formulae. • Expand brackets correctly in formulae containing polyatomic ions. • Calculate formula unit mass of calcium nitrate and magnesium hydroxide.
Ionic compounds form repeating three-dimensional crystals rather than separate molecules. Their formula therefore represents the simplest whole-number ratio of ions in the crystal. The corresponding mass calculation is called formula unit mass.
Formula Unit Mass of Ionic Compounds
The simplest whole-number ratio of ions represented by the chemical formula of an ionic compound.
The sum of the atomic masses of all atoms represented in one formula unit of an ionic compound.
Simple Formula Unit
Problem
Find the formula unit mass of Na₂O using Na = 23 u and O = 16 u.
- 1.Na₂O contains 2 sodium atoms in the represented ratio and 1 oxygen atom.
- 2.Sodium contribution = 2 × 23 u = 46 u.
- 3.Oxygen contribution = 1 × 16 u = 16 u.
- 4.Add the contributions: 46 u + 16 u = 62 u.
- 5.Formula unit mass of Na₂O = 62 u.
Problem
Find the formula unit mass of KCl using K = 39 u and Cl = 35.5 u.
- 1.KCl represents one potassium and one chlorine.
- 2.Potassium contribution = 39 u.
- 3.Chlorine contribution = 35.5 u.
- 4.Add: 39 + 35.5 = 74.5 u.
- 5.Formula unit mass = 74.5 u.
Formulae With Brackets
Brackets multiply every atom inside the bracket. In Ca(NO₃)₂, the subscript two applies to the whole nitrate group. One nitrate contains one nitrogen and three oxygen atoms, so two nitrates contain two nitrogen and six oxygen atoms.
Problem
Calculate the formula unit mass of Ca(NO₃)₂ using Ca = 40 u, N = 14 u and O = 16 u.
- 1.Count atoms carefully: 1 Ca, 2 N and 6 O.
- 2.Calcium contribution = 1 × 40 = 40 u.
- 3.Nitrogen contribution = 2 × 14 = 28 u.
- 4.Oxygen contribution = 6 × 16 = 96 u.
- 5.Add: 40 + 28 + 96 = 164 u.
- 6.Formula unit mass of Ca(NO₃)₂ = 164 u.
Problem
Calculate the formula unit mass of Mg(OH)₂ using Mg = 24 u, O = 16 u and H = 1 u.
- 1.Mg(OH)₂ contains 1 Mg and two complete OH groups.
- 2.The two OH groups contain 2 O and 2 H.
- 3.Magnesium contribution = 24 u.
- 4.Oxygen contribution = 2 × 16 = 32 u.
- 5.Hydrogen contribution = 2 × 1 = 2 u.
- 6.Total = 24 + 32 + 2 = 58 u.
- 7.Formula unit mass = 58 u.
The calculation method resembles molecular-mass addition, but the structural meaning is different: a covalent formula describes a molecule, while an ionic formula represents the simplest ion ratio in a crystal.
Quiz
Which description best matches Formula Unit?
Which description best matches Formula Unit Mass?
Which term matches this description: The simplest whole-number ratio of ions represented by the chemical formula of an ionic compound.
Which term matches this description: The sum of the atomic masses of all atoms represented in one formula unit of an ionic compound.
Which statement is a key takeaway from this lesson?
Practice Problems
- Calculate the formula unit mass of KCl using the masses given above.
- Calculate the formula unit mass of Mg(OH)₂.
- Count the atoms represented in Al₂(SO₄)₃ before attempting a mass calculation.
- Explain why brackets must be expanded before multiplying atomic masses.
Key Takeaways
• Ionic compounds are represented by formula units rather than individual molecules. • Formula unit mass is calculated by adding the atomic masses represented in the simplest ion ratio. • Subscripts and brackets must be interpreted carefully when counting atoms. • The calculation is similar to molecular mass, but the meaning of the chemical formula is different.