Chemistry Year 12 - Module 6 - Lesson 8
pH and pOH — Calculations for Strong Acids & Bases
1. Key Ideas
When a person hyperventilates, they expel CO₂ faster than their body produces it — blood pH rises above 7.45 within minutes, causing dizziness, tingling, and muscle cramps. Every calculation in this lesson is the mathematics behind why a shift of 0.1 pH units in blood has clinical consequences.
- pH = −log₁₀[H₃O⁺] and pOH = −log₁₀[OH⁻]
- Why the pH scale is logarithmic (each unit = 10× change in [H₃O⁺])
2. Success Criteria
By the end, you should be able to:
- pH = −log₁₀[H₃O⁺] and pOH = −log₁₀[OH⁻]
- pH + pOH = 14.00 at 25°C only
- Strong acids assumed 100% ionised; diprotic acids contribute 2 H⁺ per molecule
3. Key Terms
4. Activity: Build the Lesson Map
Use the lesson to complete the table. Keep answers brief but specific.
| Prompt | Your answer |
|---|---|
| Main concept | |
| Important example | |
| Common mistake to avoid | |
| How this links to the next lesson |
5. Short Answer Questions
1. Explain this lesson goal in your own words: "pH = −log₁₀[H₃O⁺] and pOH = −log₁₀[OH⁻]". Use one specific example from the lesson.
2. Apply this idea to a new example: "pH + pOH = 14.00 at 25°C only". Show your reasoning clearly.
3. Analyse why this idea matters for understanding pH and pOH — Calculations for Strong Acids & Bases: "Strong acids assumed 100% ionised; diprotic acids contribute 2 H⁺ per molecule".
6. Extend: Apply the Idea
A student gives a memorised answer about pH and pOH — Calculations for Strong Acids & Bases but does not use evidence or reasoning.
Improve the answer by writing a stronger response that uses accurate terminology, a relevant example and a clear explanation.
7. Multiple Choice
1. What is the best first step when answering a question about pH and pOH — Calculations for Strong Acids & Bases?
A. Identify the key concept being tested
B. Write every fact from memory
C. Ignore the command word
D. Skip examples and evidence
2. Which answer would show stronger understanding of pH and pOH — Calculations for Strong Acids & Bases?
A. An answer with accurate terms and reasoning
B. A copied definition only
C. A single-word response
D. An answer with no example
3. What should you do if a question asks you to explain?
A. Link the idea to a reason or cause
B. List unrelated facts
C. Only draw a diagram
D. Write the shortest possible answer
8. Success Criteria Proof
Finish with evidence that you can do each success criterion.