Question

What is the Ka of an acid is the pH at 1/2 the volume needed to reach the equivalence point is 3.86?

Answer 1

A buffer is any type of substance that will resist pH change when H+ or OH- is added.

This is typically achieved with equilibrium equations. Both type of buffer will resist both type of additions.

When a weak acid and its conjugate base are added, they will form a buffer

The equations:

The Weak acid equilibrium:

HA(aq) <-> H+(aq) + A-(aq)

Weak acid = HA(aq)

Conjugate base = A-(aq)

Neutralization of H+ ions:

A-(aq) + H+(aq) <-> HA(aq); in this case, HA is formed, H+ is neutralized as well as A-, the conjugate

Neutralization of OH- ions:

HA(aq) + OH-(aq) <-> H2O(l) + A-(aq) ; in this case; A- is formed, OH- is neutralized as well as HA.

Then

in the HALF (1/2) equivalence point

A- = HA, since 50% has been neutralized and only 50% is left

apply Henderson hasslebach equation

pH = pKa + log(A-/HA)

since

A- = HA, then log(1) = 0

pH = pKa

if pH = 3.86

then

pKa = 3.86

so

Ka = 10^-pKa

Ka = 10^-3.86

Ka = 0.000138038