I'm trying to calculate the % ionization of the carboxylic acid R group in aspirin given a pKa of 3.5 and a pH of 7.4.
I'm very confused about how to apply the Henderson Hasselbalch equation in this context and how to get the concentration of the conjugate acid and base. Any help would be greatly appreciated.
Keeshan M
Favorite Answer
10 to the negative pH value will give you the concentration of the ions in the solution of the acid. This is how much it has dissociated by. This over the original concentration of the acid times 100 will give you a %. If you don't have the original concentration, then substitute x in the Ka expression and solve for it.
Anonymous
AH + H2O ==== A- + H3O+ (monoprotic acids) Ka = (A-)(H3O+) / (AH)(H2O), this is showing activities of compounds, translating into concentrations, Ka = [A-] [H+]/[AH] pKa = -log Ka = log {[AH]/[A-]} - log [H+] = log [AH] - log [A-] - log [H+] Balance of mass a-x ---- x + x, in equilibrium, so [H+] = [A-], and knowing -log [H+] = pH pKa = log {[AH]o - [H+]} + 2 pH [H+] = 10^-3.4 = 3.98 * 10^-4 pKa = log (0.25 - 3.98*10^-4) + 6.8 = 6.197 Ka = 6.35 * 10^-7