Chromic acid (H2CrO4) is an acid that is used in ceramic glazes and colored glass. The pH of a 0.078 M solution of chromic acid is the same as the pH of a 0.059 M HCl solution. Calculate Ka for chromic acid.

Ka = 0.1815

Explanation:

Chromic acid

pH = ?

Concentration = 0.078 M

Ka = ?

HCl

conc. = 0.059M

pH = -log(H+)

pH = -log(0.059) = 1.23

pH of chromic acid = 1.23

Step 1 - Set up Initial, Change, Equilibrium table;

H2CrO4 ⇄  H+   +   HCrO4−

Initial - 0.078M 0   0

Change : -x    +x       +x

Equilibrium : 0.078-x    x      x

Step 2- Write Ka as Ratio of Conjugate Base to Acid

The dissociation constant Ka is [H+] [HCrO4−] / [H2CrO4].

Step 3 - Plug in Values from the Table

Ka = x * x / 0.078-x

Step 4 - Note that x is Related to pH and Calculate Ka

[H+] = 10^-pH.

Since x = [H+] and you know the pH of the solution,

you can write x = 10^-1.23.

It is now possible to find a numerical value for Ka.

Ka =  (10^-1.23))^2 / (0.078 - 10^-1.23) = 0.00347 / 0.0191156

Ka = 0.1815

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