PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 369

Example

Let us consider a solution of aluminium sulfate of the molality 0.2 mol kg−^1. Calculate the

mean molality of Al 2 (SO 4 ) 3 and write the relations for the mean activity and the mean activity

coefficient.

Solution

For the molality of ions we have

m+=ν+m= 2× 0 .2 = 0. 4 , m−=ν−m= 3× 0 .2 = 0. 6.

From relation (11.38) we obtain

m±= (0. 42 × 0. 63 )^1 /^5 = 0. 51.

We would obtain the same result from relation (11.39). Check it!

From relations (11.40) we have

a±=

[(

a+

) 2 (

a−

) 3 ] 1 / 5

,

γ±=

[(

γ+

) 2 (

γ−

) 3 ]^1 /^5

11.4.3 Ionic strength of a solution.

The ionic strength of a solutionIis defined by the relation

I=

1

2

∑

i

mizi^2 , (11.41)

wheremiis the molality of ionsiin a solution, andziis the ion charge. Summation is done
over allkinds of ions present in the solution. If the solution composition is given in molar
concentrations, the ionic strength is approximated using the relation

I=

1

2

∑

i

cizi^2. (11.42)