Question

Part A.) What is the pH of a buffer prepared by adding 0.708 mol of the weak acid HA to 0.406 mol of NaA in 2.00 L of solution? The dissociation constant Ka of HA is 5.66×10−7.

Part B.) What is the pH after 0.150 mol of HCl is added to the buffer from Part A? Assume no volume change on the addition of the acid.

Part C.) What is the pH after 0.195 mol of NaOH is added to the buffer from Part A? Assume no volume change on the addition of the base.

Answer 1

Part A.)

pKa = -logKa = 6.25

pH = pKa + log [NaA/HA]

pH = 6.25 + log [0.406 /0.708]

pH = 6.00

part B )

HCl moles = 0.150

on addition of C moles acid to acidic buffer acid moles increases and salt moles decreases

pH = pKa + log [NaA - C /HA + C]

pH = 6.25 + log [0.406 -0.150 /0.708 + 0.150 ]

pH = 5.7

part C)

on addition of C moles base to acidic buffer acid moles decreases and salt moles increases

pH = pKa + log [NaA + C / HA - C]

pH = 6.25 + log [0.406 + 0.195 / 0.708 - 0.195 ]

pH = 6.32