PS7 - Chemistry 2080 Spring 2009 Problem Set#7 — due date...

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Unformatted text preview: Chemistry 2080 Spring 2009 Problem Set #7 — due date: Friday, March 27th at 2 pm Name: Lab TA Name: , Lab Day/Time: Practice problems (not graded). Chapter 17: 7, 15, 23, 25 1) (a) The acid HA ionizes thus: HA(aq) + H200) ‘——‘ A*(aq) + H3O+(aq) The relative concentrations of HA and its conjugate base A— depend on the acid dissociation constant Ka of HA, and on the pH of the solution. Derive an expression that gives the fraction of acid in the solution that is present as HA, as a function of Ka and [H3041 (not the pH) of the solution. You could set [acidtota'] 2 [HA] + [A‘] then the fraction you are looking for is [HA] f ([acidtotaID . Derive the similar expression that gives the fraction of acid present as A‘. (b) The compound p—nitrophenol has a pKa of 7.15 at 25°C. The conjugate base of this phenol is p- nitrophenoxide, and has a bright yellow color. On the graph below, plot the fraction of “total p—nitrophenol” that is present as p—nitrophenol (Le. the acid form) and then do the same for the fraction that is present as the conjugate base (phenoxide), versus the pH of the solution, from pH 3 to pH 11. 2) (a) What is the pH of a 1.4 x 10"7 M solution of acetic acid in water at 25°C? (The value of Ka for acetic acid is 1.8 x 10"5 at 25°C). You may have to take into account the autoionization of water. (b) What is the % ionization of the acetic acid in (a) above? 3) This question is added as an interesting challenge, and will NOT be graded. What is the pH (at 25°C) of a solution that is made by adding 100 mL OH). 100 Macetic acid (Ka = 1.8 x 105) to 150 mL of 0.120 M methylamine (Ka = 2.40 x 10—11)? (Assume that the final volume is 250 mL). ...
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