Concentration of acetylcholine (ACh) determined from pH change after hydrolysis. Sample incubated with acetylcholinesterase is quatitatively converted to choline + acetic acid which then dissociates to acetate + H+. A 15mL solution has unknown amount of ACh with pH 7.65, and when incubated decreases to 6.87. Determine number of moles of ACh.
[HA] = [H30][A-]/Ka
The Attempt at a Solution
initial molarity of H3O
10^-7.65 = 2.2387211e-8
molarity after incubation
10^-6.87 = 1.3489629e-7
difference in molarity gives increase in concentration of H3O
1.3489629e-7-2.2387211e-8 = 1.1250908e-7
acetic acid dissociates to equal amounts of H3O + Acetate and Ka is 1.8e-5
1.1250908e-7^2/1.8e-5 = 7.032385e-10
therefore the concentration of acetic acid is 7.032385e-10. If I add this to the H3O concentration to give the total concentration of the acid before it dissociated I get 1.1321232e-7 which when multiplied by the volume of the sample in litres gives 1.6981848e-9 which is the molar amount acetic acid, and since all of the Ach is converted it is also the amount of Ach.
I had initially done this wrong, however after writing it out here I figured it out :). Since I have written it all out though, is there any better way of doing it or does that look ok?