Chemistry Form 4: UPSI/SLISS 2012 Chapter 8: Salt (Part 5)

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