Question

The ph of a 0.30 m solution of an acid, ha, is 5.20. calculate ka of ha.

Answer 1

When the acid is hydrolyzed, the reaction is

HA --> H⁺ + A⁻

So,

Ka = [H⁺][A⁻]/[HA]

Apply the ICE approach:

            HA --> H⁺ + A⁻
I          0.3        0      0
C         -X         +X   +X
E      0.3-X      X       X

Ka = [X][X]/[0.3-X]

X = H⁺ = A⁻
The formula for pH is:
pH = -log[X]
5.2 = -log[X]
Solving for X,
X = 6.31*10⁻⁶

Therefore,
Ka = [6.31*10⁻⁶][6.31*10⁻⁶]/[0.3-6.31*10⁻⁶]
Ka = 1.33×10⁻¹⁰