You have 250ml of a buffer containing a weak acid with a pka = 3.5 . The buffer
ID: 888679 • Letter: Y
Question
You have 250ml of a buffer containing a weak acid with a pka = 3.5 . The buffer is present in the solution at a concentration of 0.4M (mol/L). The starting pH of the buffer = 3. How much 10M NaOH will be required to increase the pH of the buffer up to 5?
a. What is the ratio of the conjugate base and weak acid in the buffer when its pH = 3?
b. How many moles of the A- and HA are present in this buffer when the pH = 3? Show individual amounts of A-, HA and total moles of buffer (A- + HA).
c. What is the ratio of the conjugate base and weak acid in this buffer when the pH=5?
d. How many moles of the A- and HA are present in this buffer when the pH = 5? Show individual amounts of A-, HA and total moles of buffer (A- + HA).
Explanation / Answer
(a) pKa = 3.5
pH = 3
pH = pKa + log [A- / HA]
3 = 3.5 + log [A- / HA]
A- / HA = 0.316
ratio of conjugate base and acid = 0.316 : 1
(b)
total buffer moles = 250 x 0.4 / 1000 = 0.1
conjugate base moles + acid moles = 0.1
A- + HA = 0.1
0.316 H A + HA = 0.1
HA moles = 0.076
A - moles = 0.1 - 0.076
A - moles = 0.024
(c)
pH = pKa + log [conjugate base / acid]
5 = 3.5 + log [conjugate base / acid]
conjugate base / acid = 31.62 : 1
(d)
total moles = 0.1
conjugate base + acid = 0.1
31.62 acid+ acid = 0.1
acid (HA)= 3.065 x 10^-3 moles
conjugate base moles (A-)= 0.0969 moles
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