In: Chemistry
In the laboratory, a general chemistry student measured the pH of a 0.532 M aqueous solution of acetylsalicylic acid (aspirin), HC9H7O4 to be 1.885. Use the information she obtained to determine the Ka for this acid.
Ka(experiment) =
pH = 1.885
concentration is given by
[HA] = 0.532 M
note that aspirin is a weak acid
HC9H7O4
will dissociate partially as follows
HC9H7O4(aq) <--> C9H7O4-(aq) + H+(aq)
now, this has an equilibirum value
Ka = [C9H7O4-][H+]/[HC9H7O4]
due to sotichioemtry:
[C9H7O4-] = 0
[H+] = 0
[HC9H7O4] = 0.532
in equilibirum:
[C9H7O4-] = 0 + x
[H+] = 0 + x
[HC9H7O4] = 0.532 - x
and we know
[H+] = 10^-pH = 10^-1.885 = 0.013031 M
now, substitute
[C9H7O4-] = 0 + x = 0.013031
[H+] = 0 + x = 0.013031
[HC9H7O4] = 0.532 - 0.013031 = 0.518969
Substitute in Ka
Ka = [C9H7O4-][H+]/[HC9H7O4]
Ka = (0.013031 )(0.013031 )/0.518969
Ka = 0.000327200
KA = 3.27*10^-4