If a buffer solution is 0570 M in a weak acid Ka 21 106 an

If a buffer solution is 0.570 M in a weak acid (Ka = 2.1 × 10-6) and 0.140 M in its conjugate base, what is the pH?

Solution

Ka = 2.1*10^-6

pKa = - log (Ka)
= - log(2.1*10^-6)
= 5.678

use:
pH = pKa + log {[conjugate base]/[acid]}
= 5.678+ log {0.140/0.570}
= 5.068

Answer: 5.068