Ionic compounds and formula mass

There is a problem. Sodium chloride has no molecules — so how can it have a molecular mass at all?

What an ion lattice is

Ionic compounds such as sodium chloride (NaCl) exist not as molecules but as an ion lattice. Its formula is written by taking the simplest ratio between the Na⁺ and Cl^- ions in that lattice, and a formula written that way is called the empirical formula.

Formula mass

Since ionic compounds have no molecules, what is treated as the molecular mass is the mass belonging to the formula. It is called the relative formula mass, or simply the formula mass.

The calculation is unchanged

Sodium chloride

NaCl = 23 + 35.5

= 58.5

Relative atomic masses: Na = 23, Cl = 35.5

So what differs is the name, not the arithmetic. Adding up works exactly as before; for an ionic compound the result is called a formula mass rather than a molecular mass.

Exercise 02

Calculate the formula masses of the following compounds.

Magnesium oxide (MgO) — A_r: O = 16, Mg = 24
Calcium carbonate (CaCO₃) — A_r: C = 12, O = 16, Ca = 40
Potassium sulphate (K₂SO₄) — A_r: O = 16, S = 32, K = 39

Answers

MgO = 24 + 16 = 40

CaCO₃ = 40 + 12 + (3 × 16) = 100

K₂SO₄ = (2 × 39) + 32 + (4 × 16) = 174

In K₂SO₄ the subscript 2 applies to the potassium alone, not to the sulphate. Without brackets, a subscript acts only on the symbol immediately before it.