Volumetric/Quantitative Analysis – SS2 Chemistry Lesson Note (Third Term)

Volumetric analysis is a type of quantitative analysis based on measuring volume. In a titration, a standard solution, which is a solution of known concentration, reacts with a solution of unknown concentration, and the volumes used are measured to find the unknown concentration.

Behavioural objectives

By the end of the lesson, students should be able to:

1. state the two types of quantitative analysis;
2. describe how a standard solution is prepared;
3. prepare 0.1 mol/dm³ NaOH and 0.1 mol/dm³ HCl by calculation;
4. choose a suitable indicator for an acid/base titration;
5. relate molar concentration to mass concentration;
6. calculate the average titre from a titration report.

What is titration?

There are two types of quantitative analysis, namely volumetric and gravimetric analysis. Volumetric analysis is based on volume measurement, while gravimetric analysis involves direct mass measurement.

Volumetric analysis is carried out using titration. In a titration, a standard solution (a solution of known concentration) is used to react with a solution of unknown concentration.

How is a standard solution prepared?

A standard solution is a solution of which the concentration is known. It is prepared by weighing a pure solute, dissolving it in a suitable solvent, usually water, and making up the solution to a definite volume in a volumetric flask.

For instance, a solution known to contain exactly 10.6 g of anhydrous sodium trioxocarbonate (IV), Na₂CO₃, in 1 dm³ of solution is a standard solution.

Preparation of 0.1 mol/dm³ NaOH

1. 40 g of NaOH dissolved in 1 dm³ of water gives a 1.0 mol/dm³ solution.
2. X g of NaOH will be dissolved in 1 dm³ of water to give 0.1 mol/dm³.

Xg = (40 g × 0.1 mol/dm³) / 1.0 mol/dm³ = 4 g

Therefore, 4 g of sodium hydroxide pellets is measured, dissolved in water and made up to the 1 dm³ mark to obtain 0.1 mol/dm³ NaOH.

Preparation of 0.1 mol/dm³ HCl

To prepare 0.1 mol/dm³ HCl, the dilution formula is used to find the volume of the stock acid that will be measured and dissolved in water to obtain the desired concentration.

The dilution formula is C₁V₁ = C₂V₂

• C₁ = concentration of stock acid = 11.6 mol/dm³ (for HCl)
• V₁ = volume of stock acid
• C₂ = desired concentration of acid = 0.1 mol/dm³
• V₂ = final volume of solution = 1000 cm³ (1 dm³)

V₁ = C₂V₂ / C₁ = (0.1 × 1000) / 11.6 = 8.6 cm³

Thus, 8.6 cm³ of the stock acid is measured using a measuring cylinder and added to water, then made up to 1 dm³ to obtain 0.1 mol/dm³ HCl.

Which indicators are used for acid/base titration?

Acid-base indicators are dyes that change colour according to the pH of the medium. The table below shows some titrations and their suitable indicators.

What is concentration?

The concentration of a solution is the amount of solute in a given volume of the solution. It can be expressed as mol/dm³ or g/dm³.

Molar concentration

The molar concentration of a compound is the number of moles of the compound in 1 dm³ of the solution. A solution containing one mole (the molar mass) of the compound in 1 dm³ has a molar concentration of 1 mol/dm³. The unit of molar concentration is mol/dm³.

Mass concentration

The mass concentration of a compound is the mass of the compound contained in 1 dm³ of solution. The unit is g/dm³.

What is the relationship between molar concentration and mass concentration?

Concentration = number of moles / volume, that is, C = n/V ……(i)

Number of moles, n = C × V ……(ii)

But number of moles, n = m/M, where M = molar mass and m = mass.

Substituting n = m/M into (ii), we have m/M = C × V.

That is, m/V = C × M.

But m/V = mass concentration.

Therefore, mass concentration = molar concentration × molar mass = C × M.

How is a titration report written?

The average volume of acid used = (23.20 + 23.20 + 23.10) / 3 = 23.17 cm³

Evaluation

1. What is a standard solution?
2. Calculate the mass of NaOH needed to prepare 1 dm³ of 0.1 mol/dm³ solution. (Molar mass of NaOH = 40 g/mol)
3. Which indicator is suitable for a titration between a strong acid and a weak base?
4. Show that mass concentration = molar concentration × molar mass.
5. Calculate the average titre from the 1st, 2nd and 3rd titrations in the titration report.

Assignment

For questions 1 to 5, write the correct option only.

1. The indicator used when titrating a weak acid against a strong base is A. methyl orange B. phenolphthalein C. methyl red D. any indicator
2. The colour of phenolphthalein in acids is A. blue B. red C. colourless D. yellow
3. Which of the following formulae is correct for amount n? A. n = C/p B. n = M/m C. n = C × V D. n = C × m
4. The mass concentration of a substance can be expressed as A. mass/density B. molar concentration/molar mass C. mass/volume D. number of moles × volume
5. At the end point there is A. a colour change B. no change of colour C. decrease in mass D. an increase in mass
6. Define the following terms:
   1. molar concentration
   2. equivalence point
7. 160 cm³ of distilled water is added to 40 cm³ of 0.500 mol/dm³ H₂SO₄ solution. Determine the concentration of the diluted solution.