PS7_Key

# PS7_Key - Chemistry 2080 Spring 2009 Problem Set#7 due date...

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Chemistry 2080 Spring 2009 Problem Set #7 - due date: Friday, March 27th at 2 pm Name:____ _KEY________ 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) + H 2 O(l) A (aq) + H 3 O + (aq) The relative concentrations of HA and its conjugate base A depend on the acid dissociation constant K a 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 K a and [H 3 O + ] (not the pH) of the solution. You could set [acid total ] = [HA] + [A ] then the fraction you are looking for is [HA] / ([acid total ]) . Derive the similar expression that gives the fraction of acid present as A . K a = [A ][H 3 O + ] / [HA] so we can rearrange this to read: [A ] = K a [HA] / [H 3 O + ] We can substitute this value of [A ] in the expression: [acid total ] = [HA] + [A ] and we get: [acid total ] = [HA] + K a [HA] / [H 3 O + ] i.e. [acid total ] = [HA](1 + K a / [H 3 O + ]) This can be rearranged to read [acid total ] = [HA] ( [H 3 O + ] + Ka) / [H 3 O + ] Rearranging this gives us what we want: [HA] / ([acid total ]) = [H 3 O + ] / ( [H 3 O + ] + Ka) By a similar process, we can substitute [HA] = [A ][H 3 O + ] / K a in the expression for [acid total ] , and we get the expresson: [A ] / ([acid total ]) = K a / ( [H 3 O + ] + Ka) (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 (i.e. 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. Blue: 1.0 p -nitrophenol 0.8 0.6 Fraction 0.4 0.2 Red: p -nitrophenoxide 0 3 4 5 6 7 8 9 10 11 pH

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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 K a for acetic acid is 1.8 x 10 –5 at 25°C). You may have to take into account the autoionization of water.
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PS7_Key - Chemistry 2080 Spring 2009 Problem Set#7 due date...

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