Question

A 0.175 m weak acid acid solution has ph of 3.25 find ka for the acid

Answer 1

The acid dissociation constant or Ka is a value used to measure the strength of a specific acid in solution. For a general dissociation of an acid solution,

HA = H+ + A-

we express Ka as follows:

Ka = [H+] [A-] / [HA]

Where the terms represents the concentrations of the acid and the ions. Assuming that the weak acid in the problem is HA, we first calculate for the concentration of H+ from the pH.

pH = - log [H+]
3.25 = - log [H+]
[H+] = 0.0005623 M

By the ICE table, we can calculate the equilibrium concentrations,
        HA      =      H+            +        A-
I      0.175           0                         0
C      -x               +x                      +x
 --------------------------------------------------
E  .174438    0.0005623       0.0005623

Ka = (0.0005623) (0.0005623) / .174438
Ka = 1.81x10^-6