pH of a buffer (Henderson equation): Class 11 practice
For an acidic buffer pH = pKa + log([salt]/[acid]). Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Logarithms
The idea behind ph of a buffer (henderson equation)
For an acidic buffer pH = pKa + log([salt]/[acid]). Equal concentrations give pH = pKa. A basic buffer uses pOH = pKb + log([salt]/[base]).
Step-by-step method
- Find pKa = −log Ka (or use the given value).
- Compute log([salt]/[acid]).
- Add to pKa (or use pOH then pH = 14 − pOH).
Worked example 1 (Basic)
A buffer has pKa = 3.75 and [salt]/[acid] = 1. Find its pH.
- pH = 3.75 + log 1
- = 3.75
- Answer: 3.75
Worked example 2 (Standard)
A buffer has pKa = 4.2 and [salt]/[acid] = 0.5. Find its pH.
- pH = 4.2 + log 0.5
- = 3.9
- Answer: 3.9
Worked example 3 (Challenge)
An NH₃/NH₄Cl buffer has [NH₄⁺]/[NH₃] = 1 and pKb = 4.75. Find its pH.
- pOH = 4.75 + log 1 = 4.75
- pH = 14 − 4.75 = 9.25
- Answer: 9.25
Common mistakes and misconceptions
- Inverting the ratio
- Forgetting 14 − pOH for basic buffers
Practice questions
- (Basic) A buffer has pKa = 9.25 and [salt]/[acid] = 10. Find its pH.
- (Basic) A buffer has pKa = 4.2 and [salt]/[acid] = 2. Find its pH.
- (Standard) A buffer has pKa = 3.75 and [salt]/[acid] = 0.5. Find its pH.
- (Standard) A buffer has pKa = 4.2 and [salt]/[acid] = 10. Find its pH.
- (Challenge) An NH₃/NH₄Cl buffer has [NH₄⁺]/[NH₃] = 0.1 and pKb = 4.75. Find its pH.
- (Challenge) An NH₃/NH₄Cl buffer has [NH₄⁺]/[NH₃] = 10 and pKb = 4.75. Find its pH.
Answer key with full working
- Q1: pH = 9.25 + log 10 → = 10.25 → Answer: 10.25
- Q2: pH = 4.2 + log 2 → = 4.5 → Answer: 4.5
- Q3: pH = 3.75 + log 0.5 → = 3.45 → Answer: 3.45
- Q4: pH = 4.2 + log 10 → = 5.2 → Answer: 5.2
- Q5: pOH = 4.75 + log 0.1 = 3.75 → pH = 14 − 3.75 = 10.25 → Answer: 10.25
- Q6: pOH = 4.75 + log 10 = 5.75 → pH = 14 − 5.75 = 8.25 → Answer: 8.25
Syllabus note
This skill is listed under "Equilibrium" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is ph of a buffer (henderson equation) in?
"Equilibrium", Class 11 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.