Part A What is the pH of a buffer prepared by adding 0.708 mol of the weak acid
ID: 1030218 • Letter: P
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 5.66×107.
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.
Express the pH numerically to three decimal places.
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.
Express the pH numerically to three decimal places.
Explanation / Answer
Solution:-
[HA] = 0.708/ 2.00 = 0.354 M
[A-] = 0.406/2.00 =0.203 M
pKa = 6.25
pH = 6.25 + log 0.203 / 0.354 =6.01
A- + H+ >> HA
moles A- = 0.406 - 0.150 =0.256
[A-]= 0.256/ 2.00 =0.128 M
moles HA = 0.708 + 0.150 =0.858
[HA] = 0.858/2.00 = 0.429 M
pH = 6.25 + log 0.128/0.429 = 5.72
HA + OH- >> A- + H2O
moles HA = 0.708 - 0.195 =0.513
[HA]= 0.513/2.00 =0.257 M
moles A- = 0.406 + 0.195 =0.601
[A-]= 0.601/2.00 = 0.301 M
pH = 6.25 + log 0.301/0.257
=6.32