Question

20.

A)In the laboratory, a general chemistry student measured the pH of a 0.475 M aqueous solution of acetylsalicylic acid (aspirin), HC₉H₇O₄ to be 1.910.
Use the information she obtained to determine the K_a for this acid.
K_a(experiment) =

B)In the laboratory, a general chemistry student measured the pH of a 0.475 M aqueous solution of hypochlorous acidto be 3.903.
Use the information she obtained to determine the K_a for this acid.
K_a(experiment) =

Answer 1

A)

use:

pH = -log [H+]

1.91 = -log [H+]

[H+] = 1.23*10^-2 M

HC9H7O4 dissociates as:

HC9H7O4 -----> H+ + C9H7O4-

0.475 0 0

0.475-x x x

Ka = [H+][C9H7O4-]/[HC9H7O4]

Ka = x*x/(c-x)

Ka = 1.23*10^-2*1.23*10^-2/(0.475-1.23*10^-2)

Ka = 3.271*10^-4

Answer: 3.27*10^-4

B)

use:

pH = -log [H+]

3.903 = -log [H+]

[H+] = 1.25*10^-4 M

HClO dissociates as:

HClO -----> H+ + ClO-

0.475 0 0

0.475-x x x

Ka = [H+][ClO-]/[HClO]

Ka = x*x/(c-x)

Ka = 1.25*10^-4*1.25*10^-4/(0.475-1.25*10^-4)

Ka = 3.292*10^-8

Answer: 3.29*10^-8