Chemistry, Third edition

IONIC EQUATIONS 83

Example 6.1

What ions are produced when magnesium nitrate dissolves in

water?

Answer

The formula of magnesium nitrate is Mg(NO 3 ) 2. Its ionic formula is Mg^2 ,2NO 3 . This generates

magnesium (Mg^2 (aq)) and nitrate (NO 3 (aq)) ions in water:

H 2 O

Mg^2 ,2NO 3 (s)Mg^2 (aq)2NO 3 (aq)

BOX 6.1

Composition of natural waters

It has been estimated that there is 1.4

 1021 dm^3 of water on the Earth, with

97% of the water being present in the

oceans. About 8  1018 dm^3 is tied up

as fresh water rivers and

3  1019 dm^3 as ice in ice-caps and

the glaciers. Table 6.1 shows the

average concentrations of ions in

rain, riverwater and ocean water. The

units of concentration are milligrams

per dm^3.

The main ions to be found in the

oceans are sodium and chloride, i.e.

seawater contains a relatively high

concentration of common salt. Note

that fluoride ions are not found in fresh

water.

Ion Rain River water Sea water

Na 2.0 6.3 10 770

K 0.3 2.3 398

Mg^2  0.3 4.1 1 290

Ca^2  0.6 15.0 412

Cl 3.8 7.8 19 500

SO 42  2.0 11.2 900

HCO 3  0.1 58.4 28

F 0.0 0.0 1.3

Source:Encyclopaedia of Physical Science and

Technology, Academic Press (1987) Vol 5, p. 260.

Table 6.1Average composition (mg dm^3 ) of

rain, river water and sea water

Dissolution of salts in water

Write equations to show the dissolution in water of:

(i) calcium nitrate (iii) sodium carbonate decahydrate (Na 2 CO 3 · 10H 2 O)

(ii)potassium sulfate

Exercise 6A

Ionic equations

Reaction of zinc metal with copper ions

The reaction between copper(II) sulfate solution and zinc metal makes zinc(II) sulfate

solution and copper metal:

CuSO 4 (aq)Zn(s)ZnSO 4 (aq)Cu(s)

6.2