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SQA National 5 Chemistry Formulae

Formulae

This page covers the following topics:

1. Predicting formulae

There are many compounds that are regularly used in chemistry, so their chemical formula and name are presumed to be common knowledge. Some of them are provided in the list below:

Hβ‚‚O is water,
NH₃ - ammonia,
CHβ‚„ - methane,
COβ‚‚ - carbon dioxide,
SiOβ‚‚ - sand.

The formulae of ionic compounds can be predicted by cross-multiplying the charges of the anion and cation and dividing them by a common factor. The name for ionic compounds is derived by combining the names of the two ions, using a suitable suffix for the anion; β€œide” for single-atom ions like sulphide, β€œate”/β€œite” for group ions like sulphate.

When predicting the formulae of covalent molecules, the valency of each atom must be considered. The number of bonds an atom can make depends on the number of unpaired electrons present in the outer shell. Each atom in such compound make a fixed number of covalent bonds with some of them being a part of a double or a triple bond.

Predicting formulae

1

What is the molecular formula of a covalent compound that boron and fluorine would form?

What is the molecular formula of a covalent compound that boron and fluorine would form?

2

What is the name of LiF?

What is the name of LiF?

3

Work out the formula of aluminium sulphate. Sulphate ion has a formula SO₄²⁻.

Work out the formula of aluminium sulphate. Sulphate ion has a formula SO₄²⁻.

4

Provide the molecular formula of water.

Provide the molecular formula of water.

5

Work out the formula of calcium oxide.

Work out the formula of calcium oxide.

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