DHS_H2_CHEM_P3 ANS

Answer all questions 1(a) Pyruvic acid, CH 3COCOOH, occurs naturally in th e body and is an end product of the metabolism of sugar. When 10.0 cm 3 of a solution of pyruvic acid were titrated against 0.01 mol dm –3 sodium hydroxide, with a data logger, the following curve was obtained. pH Volume of NaOH / cm3 7 Equivalence point 2.5 30 (i) Suggest a suitable indicator, if the titr ation were to be repeated without the use of a data logger. Explain your reasoning. Phenolphthalein. The pH trans ition range of the indicator lies within the sharp pH change over its equivalence point. (ii) Calculate the value of Ka for pyruvic acid. No. of mol of NaOH = 01 01000 30 . = 3 x 10–4 No. of mol of pyruvic acid = 3 x 10–4 [pyruvic acid] = 1000 10 10 34 = 0.03 mol dm–3 From graph, pH = 2.5 Hence [H +] = 10–2.5 = 3.162 x 10–3 Ka = ] _ [ ] [ acidpyruvic H 2 = 3 2 3 10 162 3 03 0 10 162 3      . . ). ( = 3.73 x 10–4 mol dm–3 2 [Turn Over

(iii) Explain, with the aid of an appropriate equation, why the pH at equivalenc e point is greater than 7. [7] CH3COCOO– (aq) + H2O (l) CH3COCOOH (aq) + OH– (aq) CH3COCOO– undergoes salt hydrolysis. [OH] > [H+]. (b) Pyruvic acid is converted to the amino acid , alanine, in just one single step in the body by the enzyme alanine transaminas e. However, to achieve the same conversion in a laboratory, multiple steps are required. Given the chemical structure of alanine below, show how pyruvic acid can be converted to alanine in not more than 4 steps. Indicate the r eagents, conditions and all intermediates in your answer. The alanine formed from yo ur suggested synthesis steps may be in its cationic, anionic or zwitterionic forms. C C H 3 H NH2 C O OH Alanine [5] C C C H H H O O OH CCH H H OH O OHH CCC Cl O ClH H H H I : H 2 with Ni, heat OR NaBH 4 in ethanol / methanol (room temp) (LiA lH4 not acceptable) II: PCl 5, room temperature III: NH3 in sealed tube, heat. IV: HCl / H2SO4, heat OR NaOH, heat CCCH H H NH2 O OHH CCCH H H NH2 O NH2H C I II III IV 3 [Turn Over

Alternative answer: I : H2 with Ni, heat OR NaBH 4 in ethanol / methanol (room temp) (LiA lH4 not acceptable) II: concentrated H2SO4, 170 oC III: HCl (g) IV: NH3 in sealed tube, heat. (c) Alanine, together with the 19 other standard amino acids are the building blocks of proteins. State the two factors that cause protein denaturation and explain how the two stated factors denature the protein.