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Worked solutions to student book questions

Chapter 4 Analysing acids and bases
Q1. Antacid tablets should normally be chewed before they are swallowed. Why? A1. Antacid tablets are normally chewed to provide a larger surface area for faster reaction with stomach acids. Q2. A laboratory test to determine how much hydrochloric acid is neutralised by a brand of antacid does not give a complete picture of its effectiveness in the stomach. What other factors might be important? A2. Other factors to consider when deciding an antacid’s effectiveness include the neutralising action of the antacid over a prolonged period (30 minutes, for example), and whether or not the antacid upsets the acid balance in the stomach. The presence and nature of food in the stomach may also affect the neutralisation reaction. Furthermore, some brands claim to have a coating action on the stomach wall which might be unrelated to the neutralising action. Q3. Examine the range of antacids shown in Figure 4.1 on page 37 of the student book. If you were to choose one of these from all the others, what features, apart from its ability to neutralise acid, might influence your choice? A3. The consumer may have a preference for tablets, gels or solutions. He or she could also be influenced by price, attractiveness of packaging and whether or not the medication can be conveniently carried. Q4. Many antacids fizz when dissolved in a glass of water. One such brand lists among its ingredients, sodium hydrogen carbonate and citric acid. Write an ionic equation for the reaction between HCO3–(aq) and H3O+(aq) responsible for the ‘fizz’. A4. HCO3–(aq) + H3O+(aq) → CO2(g) + 2H2O(l) Q5. In each of the following equations: i identify the acids and bases ii name the conjugate acid–base pairs a NH3(aq) + H2O(l) → NH4+(aq) + OH–(aq) b HSO4–(aq) + H2O(l) → H3O+(aq) + SO42–(aq) c NH4+(aq) + S2–(aq) → NH3(aq) + HS–(aq) d CH3COO–(aq) + H3O+(aq) → H2O(l) + CH3COOH(aq)

Heinemann Chemistry 2 4th edition Enhanced Copyright © Pearson Australia 2010 (a division of Pearson Australia Group Pty Ltd)

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Worked solutions to student book questions

Chapter 4 Analysing acids and bases
A5. (Acid is listed first.) a i H2O; NH3 ii NH4+, NH3 and H2O, OHb i HSO4–; H2O ii HSO4–, SO42– and H3O+, H2O c i NH4+; S2– ii NH4+, NH3 and HS–, S2– d i H3O+; CH3COO– ii CH3COOH, CH3COO– and H3O+, H2O Q6. The graphs in Figure 4.7 show the pH curves for titrations involving combinations of acids and bases of various strengths. You have a choice of phenolphthalein and methyl orange indicator. Phenolphthalein changes colour over a pH range 8.2 to 10.0. Methyl orange changes colour between pH 3.2 and 4.4. Decide which indicator(s) would be suitable to identify the equivalence point for each reaction. Provide reasons for your selections. a b

c

d

Figure 4.7 Change in pH during a titrations of: a a strong acid with a strong base; b a strong acid with a weak base; c weak acid with a strong base; d weak acid with a weak base.

Heinemann Chemistry 2 4th edition Enhanced Copyright © Pearson Australia 2010 (a division of Pearson Australia Group Pty Ltd)

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Worked solutions to student book questions

Chapter 4 Analysing acids and bases
A6. a The equivalence point occurs in the range pH 3 to pH 11. Both indicators will change colour over this pH range. Both indicators will provide a sharp end point, i.e. they will change colour at the equivalence point with the addition a small volume, 1 drop, of acid. b The equivalence point occurs in the pH range 3 to 7. Methyl orange provides a sharper end point over this pH range. c The equivalence point occurs in the pH range 7 to 11. Phenolphthalein provides the sharper end point. d Both indicators will provide a broad end point and neither would be suitable. Q7. The ethanoic acid content of white vinegar was determined by titrating a 20.00 mL aliquot of the vinegar with 0.9952 M sodium hydroxide solution. The phenolphthalein indicator changed...
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