Exam01_KEY - MCB 120L 23 January 2009 Name: KEY Exam 1...

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MCB 120L Name: KEY 23 January 2009 Exam 1 Chocolate is made from the seed pods of Theobroma cacao . A major component of chocolate (and mature tea leaves) is the alkaloid theobromine (from which caffeine is made in plants). Caffeine is a fast-acting stimulant while theobromine is a slower acting that results in a feeling of well-being (yeah for chocolate!). theobromine (FW 180.16) caffeine (FW 194.19) 1. The extraction and solubilization of these alkaloids in an aqueous solution, and subsequent biochemical study, requires an acidic buffered solution at pH 5.75. The lab has the following solutions available to make a buffer each at 1 M stock concentration: sodium acetate, MES-base, Tricine-base, CAPSO-base, and ammonium bicarbonate; and 1 M NaOH and 1 M HCl. The pKa values for these solutions are: HA pKa acetic acid 4.76 MES 6.50 Tricine 8.05 CAPSO 9.60 NH 4 HCO 3 10.4 a) Choose an appropriate buffer: MES (its pKa is closer to the required pH, thus greatest buffering capacity) b) What volumes of the stock buffer solution, HCl or NaOH, and water are theoretically needed to prepare a 500 ml of 0.2 M buffer pH 5.75? Show all work. (500 ml)(0.2 M buffer) = (1 M Mes-base)( X ml) X = 100 ml of the 1 M stock MES-base 5.75 = 6.5 + log (A/HA) 0.1778 = A/HA [HA] = (1/1.1778)(0.2 M) = 0.0302 M = final concentration of HCl to titrate the MES-base
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Exam01_KEY - MCB 120L 23 January 2009 Name: KEY Exam 1...

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