pH of strong acids and bases: Class 10 practice
pH = −log[H⁺]. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Powers of 10
- Logarithms of 10ⁿ
The idea behind ph of strong acids and bases
pH = −log[H⁺]. For a strong acid [H⁺] equals its concentration (× number of H per formula for H₂SO₄). For a strong base find pOH = −log[OH⁻], then pH = 14 − pOH at 25 °C.
Step-by-step method
- Find [H⁺] or [OH⁻].
- pH = −log[H⁺] or pH = 14 + log[OH⁻].
- Below 7 acidic, above 7 basic.
Worked example 1 (Basic)
Find the pH of 10⁻1 M HCl.
- [H⁺] = 10⁻1
- pH = 1
- Answer: 1
Worked example 2 (Standard)
Find the pH of 10⁻4 M NaOH.
- [OH⁻] = 10⁻4, pOH = 4
- pH = 14 − 4 = 10
- Answer: 10
Worked example 3 (Challenge)
Find the pH of 5e-2 M H₂SO₄ (fully dissociated).
- [H⁺] = 2 × 5e-2 = 1e-1
- pH = 1
- Answer: 1
Common mistakes and misconceptions
- Forgetting the 14 − pOH step
- Ignoring the 2 H⁺ from H₂SO₄
Practice questions
- (Basic) Find the pH of 10⁻5 M HCl.
- (Basic) Find the pH of 10⁻4 M HCl.
- (Standard) Find the pH of 10⁻2 M NaOH.
- (Standard) Find the pH of 10⁻5 M NaOH.
- (Challenge) Find the pH of 5e-5 M H₂SO₄ (fully dissociated).
- (Challenge) Find the pH of 5e-3 M H₂SO₄ (fully dissociated).
Answer key with full working
- Q1: [H⁺] = 10⁻5 → pH = 5 → Answer: 5
- Q2: [H⁺] = 10⁻4 → pH = 4 → Answer: 4
- Q3: [OH⁻] = 10⁻2, pOH = 2 → pH = 14 − 2 = 12 → Answer: 12
- Q4: [OH⁻] = 10⁻5, pOH = 5 → pH = 14 − 5 = 9 → Answer: 9
- Q5: [H⁺] = 2 × 5e-5 = 1e-4 → pH = 4 → Answer: 4
- Q6: [H⁺] = 2 × 5e-3 = 1e-2 → pH = 2 → Answer: 2
Syllabus note
This skill is listed under "Acids Bases and Salts" for Class 10 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 strong acids and bases in?
"Acids Bases and Salts", Class 10 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.