Buffer chemistry

Question

A buffer was prepared by dissolving 0.100 mol of a weak acid, HA (pKa = 5.00) plus 0.050 mol of the salt of its conjugate base, NaA, in 1.00 L of solution. What is the pH of the buffer?

Answer 1

Use the Henderson Hasselbalch equation:

pH = pKa + log [salt] / [acid]

pH = ?

pKa = 5.00

[salt] = 0.050 M

[acid] = 0.100 M

pH = 5.00 + log (0.050 / 0.100) = 5.00 + log 0.5

pH = 5.00 -0.30

pH = 4.70