A. 35 m solution of weak acid, HX, has a h + of 4.15*10^-3. what is the value of ka for this acid

Ka has the formula:

Ka = [H+] [X-] / [HX]

We have the following given:

[H+] = 4.15x10^-3

[HX] = 0.35

Since the acid dissociates in a ratio of 1 [H+] to 1 X,

[H+] = [X-]

Plugging in the values,

Ka = (4.15x10^-3) (4.15x10-3) / 0.35

Ka = 4.92x10^-5