VJC Prelim P3_Questions

2  VJC 2017 9729/03/PRELIM/17 [Turn over 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] (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] (iii) Calculate the volume of ethanoic acid and that of aqueous sodium ethanoate required to prepare 60 cm3 of the above buffer. [1] (iv) The boiling points of the components in the buffer are as shown. Ethanoic acid 118 oC Sodium ethanoate 881 oC With reference to the structure and bonding present in the compounds, account for the difference in the boiling points. [3] (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. [1] When the solution in (b) is evaporated and the resulting solid calcium ethanoate is heated strongly in a test-tube, an organic compound Q is formed, which condenses to a colourless liquid. The residue in the tube contains calcium carbonate. When 0.10 g of compound Q was injected into a gas syringe at a temperature of 383 K and a pressure of 101 kPa, 55 cm3 of vapour were produced. (ii) Calculate the relative molecular mass of Q. [2]

3  VJC 2017 9729/03/PRELIM/17 [Turn over (iii) Compound Q is neutral and water -soluble. Q does not react with sodium metal nor with Fehling’s solution but it does react with alkaline aqueous iodine. Suggest a structural formula for Q. Justify your answer by reference to these properties of Q. [4] (iv) Construct a balanced eq