Question

An acid with a K_a= 0.000343, is titrated with a strong base, what would the pH be after half the volume of base needed to reach the equivalenc point has been added?

Answer 1

This will be a BUFFER:

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,

the pH after:

half the volume of base is used

this implies, this is now the HALF equivalence point

there is equal amount of weak acid = conjugate base

so

the pH for a buffer:

pH = pKa + log(A-/HA)

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

so

pH = pKa

pKa = -log(Ka) = -log(0.000343) = 3.464

so

pH = pKa = 3.64