THE MOLE: Formulae and Chemical Equations: Titration

2.0 THE MOLE: Empirical and Molecular formula

2.11 Titration

Volumetric analysis is actually done through the process called titration. Titration involves measuring a known volume, say 25 ml of an alkalis (Vb) using a pipette. A few (2 to 3) drops of an indicator are added. Then an acid is run into the mixture in small quantities from a burette till the colour of the indicator just changes or disappears (end-point).

The volume obtained in the first titration, also called trial run, may be higher than the rest because the end-point is normally overshot. However, include it in the average if comparable to the rest. From Vb and volume of acid used, Va, we can calculate the amounts of reactants.

A typical table for titration results is as follows.

Table 2.11:Sample titration results

These results were obtained when 25.00 ml of dilute sodium hydroxide solution of unknown concentration was titrated using a standard 0.4M sulphuric acid, with phenolphthalein as the indicator. NB: Acid-base indicators are generally unstable at high concentrations of above 1M.

Questions 2.11

1. Complete the table to show the volume of acid used in each titration
2. Determine the average volume of the titre (sulphuric) acid used.
3. How many moles of the acid were in this average volume?
4. Write a balanced equation for the reaction between sodium hydroxide and sulphuric acid solutions.
5. From the equation, how many moles of sodium hydroxide does 1 mole of the acid require?
6. How many moles of sodium hydroxide were in the 25 ml used?
7. Determine the molarity of sodium hydroxide solution.
8. In a different experiment, 20 cm³ of 0.5M hydrochloric acid completely neutralized 25.0 cm³ of dilute potassium hydroxide solution. Determine the concentration in mol/dm³ of the potassium hydroxide solution.