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PS6 - Chemistry 2080 Spring 2009 Problem Set#6 due date...

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Unformatted text preview: Chemistry 2080 Spring 2009 Problem Set #6 - due date: Friday, March 13th at 2 pm Name: Lab TA Name: Lab DayfTime: Practice problems (not graded). Chapter 16: 17, 19, 25, 27, 41, 44 1) The pKa for acetic acid (CH3C02H) is 4.74 at 25°C. The pKa for the conjugate acid of the weak base ammonia is 9.25 at 25°C. - a) - Calculate the value of the equilibrium constant Kc for the following aqueous equilibrium: CH3C02H(aq) + NH3(aq) : CH3c02‘(aq) + NH4+(aq) b) Remember that at equilibrium, the rates of the forward reaction and the back (reverse) reaction are equal. Express the equilibrium constant Kc for the above equilibrium in terms of the forward and the reverse. rate constants, kf and kr respectively. c) The rate constant for the forward reaction (kf) is probably approximately diffusion controlled (say around 1010 Lmol—lsec—l in water at 25°C). What then is the rate constant for the reverse reaction (kr)? 2) What is the pH of a 0.20 M aqueous solution of sodium acetate at 25°C? (The Ka for acetic acid is 1.8 x 10‘5 at 25°C). - 3) The value of Kb for the weak base ethylamine is 5.6 x 10“4 at 25°C. Calculate the pH for each of the following two solutions of ethylamine in water at 25°C: a) An initial concentration of ethylamine of 0.45 molar b) An initial concentration of ethylamine of 0.045 molar 4) The pH of Coca Cola “classic” is about 2.5 at 25°C. A good deal of the acid in Coca Cola is believed to be the triprotic acid, phosphoric acid. The pKa’s of H3PO4 are 2.15, 7.20, and 12.38 at 25°C. What mass of phosphoric acid would you have to dissolve in 100 mL of water to give a solution that had a pH of 2.5 at 25°C? ...
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