VJC Prelim P3_Answers

1  VJC 2017 9729/03/PRELIM/17 [Turn over Victoria Junior College 2017 H2 Chemistry Prelim Exam 9729/3 Suggested Answers Section A Answer all the questions in this section. 1 (a) Ethanoic acid and its salt, sodium ethanoate, is often used as an acid buffer to help extend the shelf-life of food products such as meat, fish and dairy. A buffer solution of pH 5.5 was prepared by mixing 0.200 mol dm−3 of ethanoic acid and 0.200 mol dm−3 aqueous sodium ethanoate. (Ka of ethanoic acid = 1.74 x 10−5 mol dm−3) (i) What do you understand by the term buffer solution? [1] A buffer solution can maintain a fairly constant pH when a small amount of acid or base is added to it. (ii) Calculate the ratio of acid][ethanoic ][ethanoate required to prepare the above buffer solution. Hence deduce whether the above buffer is more effective in buffering against added acids or bases. Write an equation to illustrate your answer. [3] Ka = [HA] ]A][[H - = 1.74 x 10 −5 where A − is ethanoate and HA is ethanoic acid. [HA] ]A[ - = 5.5- -5. 10 10741  = 6- -5 . . 10163 10741   = 1 515. As higher [ ethanoate] is present, buffer is better at removing added acids. CH3CO2− + H+  CH3CO2H (iii) Calculate the volume of ethanoic acid and that of aqueous sodium ethanoate required to prepare 60 cm3 of the above buffer. [1] Let x cm 3 be volume of ethanoate and (60 – x) cm 3 to be volume of ethanoic acid. x x 60 = 1 515. x = 50.8 cm3 Hence 50 .8 cm3 of sodium ethanoate and (60- 50.8) = 9.20 cm3 of ethanoic acid will be required.

2  VJC 2017 9729/03/PRELIM/17 [Turn over (iv) The boiling points of the components in the buffer are as shown. Ethanoic acid 118oC Sodium ethanoate 881oC With reference to the structure and bonding present in the compounds, account for the difference in the boiling points. [3] Ethanoic acid has a simple molecular structure with hydrogen bonding between the polar molecules. Sodium ethanoate has a giant ionic structure with very strong electrostatic attractions between the oppositely charged ions. Hence a lot of heat energy is required to overcome the ionic attractions compared to the much weaker hydrogen bonding between acid molecules. (b) The limestone that collects in kettles in hard water areas is mainly calcium carbonate. It can be removed fairly harmlessly by using a warm solution of vinegar, which contains ethanoic acid. The limestone dissolves with fizzing and a solution of calcium ethanoate remains. (i) Write a balanced equation for the reaction between ethanoic acid and calcium carbonate.