Information in chemical formulae

A chemical formula tells us:

• the number ratio of all the different atoms in the chemical formula.
• the number ratio of the ions in the chemical formula.

Number ratio of the atoms

A chemical formula gives the number ratio of the different kinds of atoms present in the compound. This means that the ratios are the same if you count in individual atoms, dozens of atoms, or moles of atoms.

Take a look at the ratio of the number of atoms in sodium carbonate, Na₂CO₃.

The chemical formula of Na2CO3	consists of:
	number of Na atoms	number of C atoms	number of O atoms
one formula unit	two Na atoms	one C atom	three O atoms
12 formula units	2(12) = 24 Na atoms	1(12) = 12 C atoms	3(12) = 36 O atoms
one dozen formula units	two dozens (= 24) 2(12) Na atoms	one dozen (= 12) 1(12) C atoms	three dozens (= 36) 3(12) O atoms
6.022 x 1023 formula units	2(6.022 x 1023) = 12.044 x 1023 Na atoms	1(6.022 x 1023) = 6.022 x 1023 C atoms	3(6.022 x 1023) = 18.066 x 1023 O atoms
one mole formula units	2 moles (= 12.044 x 1023) Na atoms	1 mole (= 6.022 x 1023) C atoms	3 moles (= 18.066 x 1023) O atoms

Number ratio of the ions

A chemical formula gives the number ratio of the ions present in the ionic compound. This means that the ratios are the same if you count in individual ions, dozens of ions, or moles of ions.

Take a look at the ratio of the ions in sodium carbonate, Na₂CO₃.

The chemical formula of Na2CO3	consists of:
	number of Na+ ions	number of CO32- ions
one formula unit	two Na+ ions	one CO32- ion
one dozen formula units	two dozens Na+ ions	one dozen CO32- ion
one mole formula units	two moles Na+ ions	one mole CO32- ion

Content suitability

BCIT courses: CHEM 0011