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Unformatted text preview: University of Cape Town Department of Molecular & Cell Biology Course Code: MCB2014F Course Name: Macromolecules Test 1 April 2008 (Total = 65 marks) A. Water and pH (5 marks) The following questions (1 to 5) are of the multiple choice type (choose the most appropriate answer): 1. Lemon juice at pH 2.2 contains about _____ times as much H + as orange juice at pH 4.3. a. 10-2 b. 0.9 c. 2.1 d. 21 e. 126 (1) 2. If equal amounts of NaH 2 PO 4 and Na 2 HPO 4 are mixed in water, calculate the resulting pH. The pK a s of phosphoric acid are 2.1, 7.2, and 12.4. a. 2.1 b. 4.5 c. 7.2 d. 9.8 e. 12.4 (1) 3. Which of the following pairs would be the best buffer at pH 10.0? a. Acetic acid and sodium acetate (pK a = 4.76) b. H 2 CO 3 and NaHCO 3 (pK a s are 3.77 and 10.4) c. Lactic acid and sodium lactate (pK a = 3.86) d. NaH 2 PO 4 and Na 2 HPO 4 (pK a s are 2.1, 7.2, 12.4) e. Sodium succinate and succinic acid (pK a = 4.21) (1) 4. Buffers have all of the following characteristics EXCEPT: a. they have relatively flat titration curves at the pH(s) where they buffer. b. they resist changes in their pH as acid or base is added. c. they are typically composed of a weak acid and its conjugate base. d. they buffer best for polyprotic acids half-way between the two pK a values. e. buffer where the amounts of conjugate base are nearly equivalent to the amounts of weak acid. (1) 5. When preparing an acetate buffer at pH 4.5 with 0.01 M solutions of acetic acid (pK a = 4.8) and sodium acetate, the volume of acetic acid needed would be _________________ the volume of sodium acetate solution. a. equal to b. less than half of c. more than half of d. about six times e. about twice (1) B. Thermodynamics of Biological Systems (5 marks) The following questions (1 to 5) are of the multiple choice type (choose the most appropriate answer): 1. Calorimetry measures ______________ by a biochemical process a. heat absorbed or given off, H, b. pressure change inside calorimeter created c. water pressure created d. entropy change, S, e. volume change, V, created (1) 1 2. Which equation defines a system at equilibrium? a. G > 0 b. G = G c. G = 0 d. G = 0 e. G = RT ln ([products]/[reactants]) (1) 3. Thermodynamic parameters (entropy, enthalpy, free energy, and internal energy) are given for an unknown enzyme. Explain which results would be expected for the breaking of hydrogen bonds and the exposure of hydrophobic groups from the interior during the unfolding process of a protein. a. Entropy change, S, is zero b. Entropy change, S, is negative c. The reaction is spontaneous d. Enthalpy change, H, is negative e. Enthalpy change, H, is positive (1) 4....
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