Chemistry, Third edition

142 9 · CALCULATING CONCENTRATIONS

Example 9.4 (continued)

Express this ratio as a fraction:

amount of HCl in moles 1

amount of KOH in moles



1

Since the amount of substance in moles volumemolar concentration, we can

substitute:

volmolar conc HCl 1

volmolar conc KOH



1

Substitute the values given in the equation:

25.0/1000molar concentration of HCl



1

37.5/10000.500 1

Therefore, the molar concentration of HCl is

37.50.500

0.750 mol dm^3

25

Calculation of unknown concentrations by titration

Use approximate atomic masses for these calculations.

(i) How much dilute sulfuric acid of concentration 1.0 mol dm^3 will react completely with

25 cm^3 of potassium hydroxide solution of concentration 4.0 mol dm^3?

H 2 SO 4 (aq) + 2KOH(aq) K 2 SO 4 (aq) + 2H 2 O(I)

(ii)A solution of sodium hydroxide contains 4.0 g dm^3. An endpoint is reached when 25 cm^3

of this solution react with 35 cm^3 of dilute nitric acid. Calculate the concentration of the

nitric acid.

NaOH(aq) + HNO 3 (aq)NaNO 3 (aq) + H 2 O(I)

(iii)A standard solution was prepared by dissolving 2.6061 g of anhydrous sodium carbonate

in deionized water and the solution diluted to 250 cm^3. A 25.0 cm^3 portion of this solution

was titrated against hydrochloric acid, using a suitable indicator. The endpoint was

reached after 18.7 cm^3 of acid had been added. Calculate the concentration of the

acid.

Na 2 CO 3 (aq) + 2HCl(aq) 2NaCl(aq) + H 2 O(I) + CO 2 (g)

(iv)How many cubic centimetres of a solution of hydrochloric acid, of concentration

2.0 mol dm^3 , are required to react completely with 50 cm^3 of a solution of Ba(OH) 2 that

contains 68.4 g dm^3?

2HCl(aq) + Ba(OH) 2 (aq)BaCl 2 (aq) + 2H 2 O(I)

Exercise 9E

Redox titrations

The technique of titration may also be used to determine the concentration of an

unknown solution, when the solutions react together in a redoxreaction. Potassium

permanganate (or potassium manganate(VII), KMnO 4 ) is a powerful oxidizing

agent and is used to estimate the concentration of many reducing agents. Potassium

permanganate solution is purple in colour, but is virtually colourless when reduced

to Mn^2 – it can therefore act as a self-indicator. As it is added to the reducing

solution it turns colourless, but at the endpoint a very small amount of excess