ACID/BASE REACTIONS

CONTENT

• Preparation of Standard Solutions.
• Indicators
• Calculations based on Acid-Base Titration.

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 art using Titration.  In titration, a standard solution (one of known concentration must be using be used to react with a solution of unknown of concentration)

PREPARATION OF A STANDARD SOLUTION

A standard solution is a solution of which the concentration is known .A standard solution is prepared by weighing a pure solute, for instance, and 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 10.6g of anhydrous sodium trioxocarbonate (IV), Na₂CO₃, in 1 dm3 of solution is a standard solution.

Preparation of 0.1mol/dm³NaOH

         40g NaOH dissolved in 1 dm³ of the water gives 1.0mol/dm³ solution

            XgNaOH will be dissolved in 1 dm3 of water to give 0.1mol/dm³

            Xg = 40g x 0.1mol/dm³ /1.0mol/dm³

= 4g

Therefore, 4g of sodium hydroxide pellet is measured, dissolved in water and made up to 1dm³ mark to obtain 0.1mol/dm³NaOH

Preparation of 0.1mol/dm³HCl

To prepare 0.1mol/dm³HCl, the dilution formula is used to determine 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

Where C₁ = concentration of stock acid = 11.6mol/dm³ (for HCl)

            V₁ = volume of stock acid     

            C₂ = desired concentration of acid = 0.1mol/dm³

            V₂ = volume of water = 1000cm³ (1dm³)

            V₁ = C₂V₂ / C₁= 0.1 x 1000/ 11.6 = 8.6cm³

Thus, 8.6cm³ of the stock acid is measured using a measuring cylinder and added to water, then made up to 1dm³ to obtained 0.1 mol/dm³HCl.

EVALUATION

1. Describe how to prepare 0.05mol/dm³ H₂SO₄
2. List the apparatuses used during volumetric analysis(titration)

INDICATORS FOR ACID/BASE TITRATION

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

Acid / base                             Indicator

Strong acid and strong base    methyl orange or phenolphthalein

Strong acid and weak base      methyl orange

Weak acid and strong base     phenolphthalein

Weak acid and weak base       No suitable indicator

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 one which contains one mole or the molar mass of the compound in 1dm³ of the solution. 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

Relationship between Molar concentration and Mass concentration

Concentration = number of moles / volume  = n/V………………(i)

Number of moles, n = C x 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 x V

That is, m/V = C x M

But m/V = mass concentration

Therefore, mass concentration = molar concentration x molar mass = C x M

TITRATION REPORT

Average volume of acid used =  (23.20 + 23.20 + 23.10) / 3

                                                = 23.17cm³

GENERAL EVALUATION/REVISION

1. Which substance is added to the base during titration experiment?
2. What is the colour of methyl orange in a base?
3. What is the point at which the titration experiment appears to complete called?
4. State three differences between electrolytic cell and electrochemical cell
5. Calculate the standard e.m.f of a cell given that the e.m.f of Zn²⁺/Zn and Cu²⁺/Cu are -0.76V and +0.34V respectively

READING ASSIGNMENT: New School Chemistry SSS by O. Y. Ababio (6^th edition) pages 165-168

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1.         The indicator used when titrating a weak acid against a strong base isA. methyl orange B. phenolphthalein C. methyl redD. any indicator

2.         The colour of phenolphthalein in acids is   A. blue    B. red    C.colourlessD. yellow

3.         Which of the following formulae is direct for amount n? A. n=C/p B. n=M/m C. n=C x V D. n=C x m

4.         The mass concentration of a substance can be expressed as A. mass/densityB. molar concentration/molar mass     C. mass/volume D. number of moles x volume

5.         At the end point there is A. a colour change     B. no change of colourC. decrease in mass     D. an increase in mass

SECTION B

1. Define the following terms (a) molar concentration (b) Equivalent point
2. 160cm³ of distilled water is added to 40cm³ of 0.500mol/dm³ H₂SO₄ solution. Determine the concentration of the diluted solution.