OCR A Jan 2011 Paper 5 Q3

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3 Glycolic acid, HOCH2COOH, and thioglycolic acid, HSCH2COOH, are weak acids. (a) Glycolic acid reacts with bases, such as aqueous sodium hydroxide, NaOH(aq), to form salts. A student pipetted 25.0 cm3 of 0.125 mol dm3 glycolic acid into a conical flask. The student added NaOH(aq) from a burette. A pH meter and data logger were used to measure continuously the pH of the contents of the conical flask. The pH curve that the student obtained is shown below. pH 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 10 20 35 15 volume NaOH(aq) added / cm3 25 30 40 45 50 1 mol of glycolic acid reacts with 1 mol of sodium hydroxide. (i) Write the equation for the reaction that takes place in the titration.[1] (ii) Determine the concentration, in mol dm3, of the NaOH. concentration of NaOH =mol dm3 [2]OCR 2011<br />
 (iii) The student decided to carry out this titration using an acidbase indicator. What important factor does the student need to consider when deciding on the most suitable indicator to use for this titration?[1] (b) The 0.125 mol dm3 glycolic acid had a pH of 2.37. (i) What is the expression for the acid dissociation constant, Ka, of glycolic acid? (ii) Calculate Ka for glycolic acid. [1] (iii) Calculate the percentage molar dissociation of the glycolic acid. Ka =units[3] percentage dissociation =% [1]OCR 2011 Turn over<br />
 (c) A buffer of glycolic acid and ammonium glycolate is used in a facial cleanser. 10 Explain, using equations,how a solution containing glycolic acid and glycolate ions can act as a buffer how this buffer could be prepared from ammonia and glycolic acid. In your answer you should explain how the equilibrium system allows the buffer solution to control the pH.[7]OCR 2011<br />
 11 (d) Ammonium thioglycolate, HSCH2COONH4, is the ammonium salt of thioglycolic acid, HSCH2COOH. When ammonium thioglycolate is dissolved in water, an acidbase equilibrium is set up. The equilibrium lies well to the left-hand side. HSCH2COO(aq) NH4 +(aq) HSCH2COOH(aq) NH3(aq)In the spaces above,label one conjugate acidbase pair as Acid 1 and Base 1 label the other conjugate acidbase pair as Acid 2 and Base 2. [2] (e) Ammonium thioglycolate is used by hairdressers to perm hair. Hair is a protein and its shape is largely the result of cross-linked disulfide bonds, SS. The formula of the protein in hair can be represented as RSSR. Perming of hair involves two stages. Stage 1Hair is first wound around curlers and a solution of ammonium thioglycolate is applied to the hair. In this process, each disulfide bond is broken by two thioglycolate ions to form two molecules containing thiol groups, SH, and one other product. Stage 2After 1530 minutes, the hair is rinsed with a weak solution of hydrogen peroxide, H2O2. The hydrogen peroxide reforms disulfide bonds that lock the hair in the shape of the curlers. The hair is now permed. Suggest equations for the two processes that take place during perming. In your equations, use RSSR to represent the protein in hair. Stage 1 Stage 2OCR 2011 [2] [Total: 20] Turn over<br />

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