Buffer Solutions Expected pH

Question

Calculate of the expected pH of the solution in beaker 3 after the addition of 20 mL of 1.0 M HCl.

Beaker 3 volume: 100mL

Beaker 3 (1.0M CH3CO2H & 1.0M NaCH3CO2) pH init: 4.73

Breaker 3 (1.0M CH3CO2H & 1.0M NaCH3CO2) pH final: 4.56



Answer 1

Since you are adding acid, HCl to the buffer, the added H⁺ will react with the NaCH₃CO₂ as follows:

NaCH₃CO₂ + H⁺ ==> CH₃CO₂H + NaCl thus reducing the amount of NaCH₃CO₂ and increasing CH₃CO₂H

The amount of acid added = 20 ml x 1 L/1000 ml x 1.0 mol/L = 0.02 moles H⁺

We can use an ICE table for this, and the final volume will be 120 ml = 0.120 L

CH₃CO₂^- + H⁺ ==> CH₃CO₂H

0.1 mole........0.02 mol.....0.1 mole ......Initial

-0.02 mol....-0.02 mol......+0.02 mol...Change

0.08 mol......0 mol............0.12 mol....Equilibrium

Final [CH3CO2-] = 0.08 mol/0.120 L = 0.667 M

Final ]CH3CO2H = 0.12 mol/0.120 L = 1.0 M

From the Henderson Hasselbalch equation and the original pH, we can find the pKa of the weak acid:

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

4.73 = pKa + log [1.0 M]/[1.0 M] = pKa + 0

pKa = 4.73

Now, use HH equation again and the new final concentrations to find the pH after addition of H⁺:

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

pH = 4. 73 + log (0.667/1.0)

pH = 4.73 + (-0.176)

pH = 4.55