A student prepares an acetic acid / acetate buffer at pH = 4.74 (pKa of CH3COOH = 4.74). Available stock solutions are 0.500 M CH3COOH and 0.500 M CH3COONa. The student combines 25.0 mL of acetic acid with 25.0 mL of sodium acetate to a total of 50.0 mL. The measured pH is 4.74.
The student adds 5.0 mL of 0.10 M HCl to the 50.0 mL buffer. What is the new pH (approximately)?
- A
5.00
- Bcheck_circle
4.71
- C
4.04
- D
1.30
Explanation
Initial moles: HA = A- = 0.0125. Add 5.0e-4 mol HCl converts A- to HA: A- = 0.0120, HA = 0.0130. pH = 4.74 + log(0.0120/0.0130) = 4.74 - 0.035 = 4.71.