Chemistry, Third edition

144 9 · CALCULATING CONCENTRATIONS

Reactions of solids with solutions

Calculations involving the reaction of a solid with a solution can be worked out in a

similar manner.

Redox titrations

(i)ii25 cm^3 of a solution of an iron(II) salt needed 13.5 cm^3 of a 0.020 mol dm^3 solution of

potassium dichromate for complete oxidation to iron(III). Write the balanced overall redox

equation for the reaction and calculate the concentration of Fe^2 (aq) in the original

solution.

(ii)iCerium(IV) ions are reduced by oxalate ions in solution:

(ii)i2Ce^4 (aq) + C 2 O 42 (aq)2Ce^3 (aq) + 2CO 2 (g)

(ii)iFerroin indicator is used in the titration. Calculate the volume of 0.050 mol dm^3 cerium(IV)

solution needed to exactly react with 25 cm^3 of 0.050 mol dm^3 oxalate ion solution.

(iii)Calculate the volume of a 0.020 mol dm^3 solution of potassium permanganate that is

needed for the complete oxidation of 0.0070 mol of iron(II) oxalate solution. (Be careful! –

there are two oxidations occurring here – the oxidation of iron(II) to iron(III) and the

oxidation of the oxalate ion to carbon dioxide.)

Exercise 9F

Example 9.6

What mass of zinc will react completely with 50.0 cm^3 of

0.100 mol dm^3 sulfuric acid?

Answer

Write the balanced equation for the reaction:

Zn(s) + H 2 SO 4 (aq)ZnSO 4 (aq) + H 2 (g)

Work out the molar relationship between the two reactants:

1 mol Zn 1 mol H 2 SO 4

Express this ratio as a fraction:

amount of Zn in moles 1

amount of H 2 SO 4 in moles



1

Since, for a solution, the number of moles volumemolar concentration, we

can substitute this expression for H 2 SO 4 only(zinc is a solid, not a solution):

amount of Zn in moles 1

volmolar concentration H 2 SO 4



1

Substitute the values given in the equation:

amount of Zn in moles



1

50.0/10000.100 1

Therefore, the moles of zinc needed is

50.0

1000

0.1000.005 00 mol

and the mass of zinc required 0.005 00  65 0.325 g.