The document describes the continuous variation method used to construct ionic equations for insoluble salts. It involves carrying out experiments where the volume of one reactant is kept fixed while the volume of the other reactant is varied. The mole ratio of the reactants that results in all of the first reactant reacting indicates the ratio in the ionic equation. An example using lead (II) chromate (PbCrO4) precipitation is provided to illustrate the method.

1. NURUL ASHIKIN BT. ABD RAHMAN PART 5

2. LEARNING OUTCOMES
 Construct ionic equations through
the continuous variation method.

3. Continuous variation method is a method that is
use to construct ionic equations for the
formation of insoluble salt.
X (ion A) + Y (ion B) AXBY

4. Step involved in the continuous
variation Method
To determie the ionic equation of the reaction between
A ions and B ions
Carry out an experiment : fix volume solution A but
different and varying of solution B
Determine the volume B ions that react with all of the A
ions
Calculate the no. of mole A ions and B ions
Determine the simplest mole ration of A ions to the Y
ions in the reaction to construct the ionic equation

5. Fixed volume of reactant A are added to varying volumes
of a second reactant B in different test tubes.
Experiment :
To construct a balanced ionic equation for the precipitation
of lead (II) chromate (VI), PbCrO4 using the continuous
variation method

6. Example:
18.0 cm3 of 0.2 mol dm-3 solution of Xm+ ions
reacts completely with 12.0 cm3 of 0.1 mol
dm-3 solution of Yn- ions to form a salt XnYm.
Write the ionic equation and hence determine
the empirical formula of the salt in this
reaction.

7. Solution:
No. of mole Xn+ ions = MV
0.2×18.0
=
1000
=0.0036 mol
Number of mole Y n ions = MV
0.1×12.0
1000
=0.0012 mol

8. Mol ratio of Xm+ ions: Yn- ions
= 0.0036 : 0.0012
0.0036 0.0012
:
0.0012 0.0012
3:1
3 moles Xm+ react with 1 mole Yn-
Ionic equation : 3Xm+ + 1Yn-  X3Y
Empirical formula of the salt: X3Y