Question

Enter your answer in the provided box. How much HCl must be added to a liter of buffer that is 1.4 M in acetic acid and 0.70 M in sodium acetate to result in a buffer pH of 4.01?

__mol

Answer 1

Sol :-

Use Henderson-Hasselbalch equation, we have

pH = pKa + log[salt /acid] ..................(1)

Given, pH = 4.01

pKa of acetia acid = 4.76

So,

4.01 = 4.76 + [salt / acid]

When we add acid in this buffer it will increase the concentration of acetic acid by the same amount and will decrease the concentration of salt by the same amount: So,

CH₃COO^- + HCl --------> CH₃COOH + Cl^-

Let we need to add y mole of HCl

4.01 = 4.76 + log [0.70 -y / 1.40 +y]

-0.75 = log [0.7 -y / 1.4 +y]

Taking antiolg on both side

0.1778 = (0.7 -y) / (1.4 +y)

0.2489 + 0.1778 y = 0.7 - y

1.1778 y = 0.4511

y = 0.383 mol /L will be required