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## Writing formulae

In Chapter 2 you learnt about the writing of chemical formulae. Table 1 shows some of the common anions and cations that you should know.

Table 1: Table showing common compound ions and their formulae
 Name of compound ion formula Name of compound ion formula Acetate (ethanoate) $CH 3 COO -$ Manganate $MnO 4 2-$ Ammonium $NH 4 +$ Nitrate $NO 3 -$ Carbonate $CO 3 2-$ Nitrite $NO 2 -$ Chlorate $ClO 3 -$ Oxalate $C 2 O4 2-$ Chromate $CrO 4 -$ Oxide $O 2 -$ Cyanide $CN -$ Permanganate $MnO 4 -$ Dihydrogen phosphate $H 2 PO 4 -$ Peroxide $O 2 2-$ Hydrogen carbonate $HCO 3 -$ Phosphate $PO 4 3-$ Hydrogen phosphate $HPO 4 3-$ Phosphide $P 3-$ Hydrogen sulphate $HSO 4 -$ Sulphate $SO 4 2-$ Hydrogen sulphite $HSO 3 -$ Sulphide $S 2-$ Hydroxide $OH -$ Sulphite $SO 3 2-$ Hypochlorite $ClO -$ Thiosulphate $S 2 O 3 2-$

## Chemical compounds: names and masses

In Chapter 4 you learnt about atomic masses. In this chapter we have learnt that atoms can combine to form compounds. Molecules are formed when atoms combine through covalent bonding, for example ammonia is a molecule made up of three hydrogen atoms and one nitrogen atom. The relative molecular mass (M) of ammonia $(NH 3 )$ is:

$M=relativeatomicmassofonenitrogen+relativeatomicmassofthreehydrogens=14,0+3(1,01)=17,03$(1)

One molecule of $NH 3$ will have a mass of 17,03 units. When sodium reacts with chlorine to form sodium chloride, we do not get a molecule of sodium chloride, but rather a sodium chloride crystal lattice. Remember that in ionic bonding molecules are not formed. We can also calculate the mass of one unit of such a crystal. We call this a formula unit and the mass is called the formula mass. The formula mass for sodium chloride is:

$M=relativeatomicmassofonesodiumatom+relativeatomicmassofonechlorineatom=23,0+35,45=58,45$(2)

The formula mass for $NaCl$ is 58,45 units.

### Exercise 1: Chemical formulae

Write the chemical formulae for each of the following compounds and calculate the relative molecular mass or formula mass:

1. hydrogen cyanide

2. carbon dioxide

3. sodium carbonate

4. ammonium hydroxide

5. barium sulphate

6. copper (II) nitrate

a) Formula: $\text{HCN}$

Relative atomic mass: 27

b) Formula: ${\text{CO}}_{2}$

Relative atomic mass: 44

c) Formula: ${\text{Na}}_{2}{\text{CO}}_{3}$

Formula mass: 106

d) Formula: ${\text{NH}}_{4}\text{OH}$

Formula mass: 35

e) Formula: ${\text{BaSO}}_{4}$

Formula mass: 233

f) Formula: ${\text{Cu(NO}}_{3}{\text{)}}_{2}$

Relative atomic mass: 187,5

Complete the following table. The cations at the top combine with the anions on the left. The first row is done for you. Also include the names of the compounds formed and the anions.

Table 2
 $Na +$ $Mg 2+$ $Al 3+$ $NH 4 +$ $H +$ $Br -$ name: $NaBr$ $MgBr 2$ $AlBr 3$ $(NH 4 )Br$ $HBr$ sodium bromide magnesium bromide aluminium bromide ammonium bromide hydrogen bromide $S 2-$ name: $P 3-$ name: $MnO 4 -$ name: $Cr 2 O 7 2-$ name: $HPO 4 2-$ name:
${\text{Na}}^{+}$${\text{Mg}}^{2+}$${\text{Al}}^{3+}$${\text{NH}}_{4}^{+}$${\text{H}}^{+}$

${\text{Br}}^{-}$

Name:

$\text{NaBr}$

Sodium bromide

${\text{MgBr}}_{2}$

Magnesium bromide

${\text{AlBr}}_{3}$

Aluminium bromide

${\text{NH}}_{4}\text{Br}$

Ammonium bromide

$\text{HBr}$

hydrogen bromide

${\text{S}}^{2-}$

Name:

${\text{Na}}_{2}\text{S}$

sodium sulphide

$\text{MgS}$

magnesium sulphide

${\text{Al}}_{2}{\text{S}}_{3}$

aluminium sulphide

${\text{(NH}}_{4}{\text{)}}_{2}\text{S}$

ammonium sulphide

${\text{H}}_{2}\text{S}$

hydrogen sulphide

${\text{P}}^{3-}$

Name:

${\text{Na}}_{3}\text{P}$

sodium phosphide

${\text{Mg}}_{3}{\text{P}}_{2}$

magnesium phosphide

$\text{AlP}$

aluminium phosphide

${\text{(NH}}_{4}{\text{)}}_{3}\text{P}$

ammonium phosphide

${\text{H}}_{3}\text{P}$

hydrogen phosphide

${\text{MnO}}_{4}^{-}$

Name:

${\text{NaMnO}}_{4}$

sodium permanganate

${\text{Mg(MnO}}_{4}{\text{)}}_{2}$

magnesium permanganate

${\text{Al(MnO}}_{4}{\text{)}}_{3}$

aluminium permanganate

${\text{NH}}_{4}{\text{MnO}}_{4}$

ammonium permanganate

${\text{HMnO}}_{4}$

hydrogen permanganate

${\text{Cr}}_{2}{\text{O}}_{7}^{2-}$

Name:

${\text{Na}}_{2}{\text{Cr}}_{2}{\text{O}}_{7}$

sodium dichromate

${\text{MgCr}}_{2}{\text{O}}_{7}$

magnesium dichromate

${\text{Al}}_{2}{\text{(Cr}}_{2}{\text{O}}_{7}{\text{)}}_{3}$

aluminium dichromate

${\text{(NH}}_{4}{\text{)}}_{2}{\text{MnO}}_{4}$

ammonium dichromate

${\text{H}}_{2}{\text{Cr}}_{2}{\text{O}}_{7}$

hydrogen dichromate

${\text{HPO}}_{4}^{2-}$

Name:

${\text{Na}}_{2}{\text{HPO}}_{4}$

sodium hydrogen phosphate

${\text{Mg(HPO}}_{4}{\text{)}}_{2}$

magnesium hydrogen phosphate

${\text{Al}}_{2}{\text{(HPO}}_{4}{\text{)}}_{3}$

aluminium hydrogen phosphate

${\text{(NH}}_{4}{\text{)}}_{2}{\text{HPO}}_{4}$

ammonium hydrogen phosphate

${\text{H}}_{3}{\text{PO}}_{4}$

phosphoric acid