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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: HCN

Relative atomic mass: 27

 

b) Formula: CO2

Relative atomic mass: 44

 

c) Formula: Na2CO3

Formula mass: 106

 

d) Formula: NH4OH

Formula mass: 35

 

e) Formula: BaSO4

Formula mass: 233

 

f) Formula: Cu(NO3)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:

         
Na+Mg2+Al3+NH4+H+

Br-

Name:

NaBr

Sodium bromide

MgBr2

Magnesium bromide

AlBr3

Aluminium bromide

NH4Br

Ammonium bromide

HBr

hydrogen bromide

S2-

Name:

Na2S

sodium sulphide

MgS

magnesium sulphide

Al2S3

aluminium sulphide

(NH4)2S

ammonium sulphide

H2S

hydrogen sulphide

P3-

Name:

Na3P

sodium phosphide

Mg3P2

magnesium phosphide

AlP

aluminium phosphide

(NH4)3P

ammonium phosphide

H3P

hydrogen phosphide

MnO4-

Name:

NaMnO4

sodium permanganate

Mg(MnO4)2

magnesium permanganate

Al(MnO4)3

aluminium permanganate

NH4MnO4

ammonium permanganate

HMnO4

hydrogen permanganate

Cr2O72-

Name:

Na2Cr2O7

sodium dichromate

MgCr2O7

magnesium dichromate

Al2(Cr2O7)3

aluminium dichromate

(NH4)2MnO4

ammonium dichromate

H2Cr2O7

hydrogen dichromate

HPO42-

Name:

Na2HPO4

sodium hydrogen phosphate

Mg(HPO4)2

magnesium hydrogen phosphate

Al2(HPO4)3

aluminium hydrogen phosphate

(NH4)2HPO4

ammonium hydrogen phosphate

H3PO4

phosphoric acid