Question

If a buffer solution is 0.160 M in a weak acid (Ka = 3.4 × 10-5) and 0.510 M in its conjugate base, what is the pH? If a buffer solution is 0.260 M in a weak base (Kb = 6.9 × 10-5) and 0.550 M in its conjugate acid, what is the pH?

Please show work that way it's actually learning and not just giving an answer. Thank you!

Answer 1

This question has two parts one is finding the pH of acidic buffer and other is finding the pH of basic buffer

Part1:

0.160 M in a weak acid (Ka = 3.4 × 10-5) and 0.510 M in its conjugate base is an example of acidic buffer, so according to Hasselbach-Handerson equation its pH will be given by:

pH = pKa + log [Conjugate base] / [acid]

Where, [Conjugate base], [Acid] = concentration of conjagate base and acid.

pKa = - log (ka)

= - log (3.4 * 10^-5)

= 4.46

Putting the values in above equation,

pH = 4.46 + log (0.510) / (0.160)

= 4.46 + log ( 3.1875)

= 4.46 + 0.50

= 4.96

Part 2:

Similarly 0.260 M in a weak base (Kb = 6.9 × 10-5) and 0.550 M in its conjugate acid is a basic buffer

pkb= - log (6.9 * 10^-5)

= 4.161

So, pOH = pKb + log [Conjugate acid] / [base]

= 4.161 + log (0.550) / (0.260)

= 4.48

Since, pH + pOH = 14

pH = 14 - pOH

pH = 14 - 4.48

pH = 9.52