Question

2. (a)(i) Define each of the following terms: I. normal salt; II. acid salt. (ii) Tetraoxosulphate (VI) acid and sodium hydroxide react to produce salt and water. Write a balanced chemical equation for the formation of: I. a normal salt; II. an acid salt.
(b)(i) Explain briefly the term acid-base indicator. (ii) Copy and complete the following table.
Indicator | Colour in acidic medium | Colour in basic medium
Methyl orange | ? | ?
Phenolphthalein | ? | ?
(iii) For each of the following titrations, state the most suitable indicator: I. strong acid against strong base; II. strong acid against weak base; III. weak acid against strong base.
(c) Baking soda and hydrochloric acid react according to the equation: NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(g). Calculate the mass of baking soda that would produce 10g of carbon (IV) oxide. [H=1.00, C=12.0, O=16.0, Na=23.0]
(d) Give a reason why a given mass of sodium hydroxide pellets cannot be used to prepare a standard solution.

Accepted Answer

(a)(i) I. A normal salt is formed when all the replaceable hydrogen ions of an acid are replaced by metallic or ammonium ion. Examples are NaCl, KNO3, Na2SO4 etc. II. An acid salt is formed when only part of the replaceable hydrogen ions are replaced by metallic or ammonium ion. Examples are NaHSO4, NaHSO3, Na2HPO4 etc.
(ii) I. 2NaOH+H2SO4 → Na2SO4+2H2O (normal salt). II. NaOH+H2SO4 → NaHSO4+H2O (acid salt).

(b)(i) An acid-base indicator is a dye which changes colour according to the pH of the medium.
(ii) Methyl orange: Red (acidic medium), Yellow (basic medium). Phenolphthalein: Colourless (acidic medium), Red/Pink (basic medium).
(iii) I. Strong acid against strong base: any indicator (methyl orange, litmus, phenolphthalein etc.). II. Strong acid against weak base: Methyl orange. III. Weak acid against strong base: Phenolphthalein.

(c) Mass of CO2 = 10g. Molar mass of CO2 = 12+16(2) = 44 g/mol. Mole of CO2 = 10g/44gmol⁻¹ = 0.2273 mol. According to the equation, 1 mole of CO2 is generated from 1 mole of baking powder (NaHCO3). Thus, 0.2273 mol of CO2 will come from 0.2273 mol of NaHCO3. Mass of NaHCO3 = (0.2273×84)g = 19.09g.

(d) Sodium hydroxide pellets cannot be used to prepare a standard solution because it is deliquescent (it absorbs moisture from the atmosphere and its mass changes, so an accurately weighed amount does not remain accurate).

WAEC Chemistry 2013 Theory — Question 2

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