What pH measures
pH is a compact way to describe hydronium concentration. Lower pH means higher [H3O+] and a more acidic solution.
Unit 8: Acids and Bases
pH, pOH, and Strong Acid/Base Calculations
How do chemists measure how acidic or basic a solution is?
24 min
High School Chemistry / AP Chemistry
5 source chapters
Learning Objectives
- Define pH and pOH.
- Use pH = -log[H3O+] for strong acid solutions.
- Use pOH = -log[OH-] for strong base solutions.
- Use pH + pOH = 14.00 at 25 °C.
- Interpret pH changes on a logarithmic scale.
Why pH is logarithmic
The pH scale is logarithmic, so a change of 1 pH unit means a tenfold change in [H3O+]. A pH of 2 has ten times the hydronium concentration of pH 3.
Reading the pH scale
The scale runs from acidic to basic, with pH 7 neutral at 25 °C. Moving one unit lower means [H3O+] increases by a factor of 10.
Horizontal pH scale from acidic to basic with logarithmic callouts.
pH and pOH equations
These equations connect concentration to a logarithmic scale. The 14.00 relationship applies at 25 °C.
pH = -log[H3O+]
pOH = -log[OH-]
pH + pOH = 14.00 at 25 °C
- 1For strong acids, use complete dissociation to find [H3O+] first.
Worked Example
Strong acid pH
Calculate the pH of 0.010 M HCl.
Worked Example
Strong base pH
Calculate the pH of 0.0010 M NaOH at 25 °C.
Quick Check
If [H3O+] increases by a factor of 10, what happens to pH?
Quick Check
At 25 °C, a solution has pOH = 4.25. What is the pH?
Misconception Check
Misconception: A pH difference of 1 is a small linear change.
Correction: A pH difference of 1 represents a tenfold difference in [H3O+]. The pH scale is logarithmic, not linear.
Key Terms
acid
酸A substance that increases hydronium concentration in water.
base
碱A substance that increases hydroxide concentration or accepts protons.
hydronium ion
水合氢离子H3O+, the ion used to represent acidic aqueous solutions.
hydroxide ion
氢氧根离子OH-, the ion central to basic aqueous solutions.
pH
pHA logarithmic measure of hydronium concentration.
pOH
pOHA logarithmic measure of hydroxide concentration.
strong acid
强酸An acid treated as completely dissociated in water.
strong base
强碱A base treated as completely dissociated in water.
Summary
- pH measures hydronium concentration on a logarithmic scale.
- Strong acid shortcuts assume complete dissociation.
- Strong base calculations often go through pOH first.
- pH + pOH = 14.00 only at 25 °C.
- A 1-unit pH change means a tenfold [H3O+] change.
Practice bridge
Calculate the pH of 0.00010 M HNO3.
Calculate the pH of 0.020 M KOH at 25 °C.
Explain why the strong-acid shortcut should not be used for a weak acid without equilibrium work.
Related Resources
Marker chapters
- Chemistry 11e By Steven Zumdahl, DeCoste/Chapter 14- Acids and Bases
- Chemistry 14e by Raymond Chang Jason Overby/Chapter 15 - Acids and Bases
- Chemistry 11e By Steven Zumdahl, DeCoste/Chapter 15- Acid-Base Equilibria
- Chemistry 14e by Raymond Chang Jason Overby/Chapter 16 - Acid-Base Equilibria and Solubility Equilibria
- Chemistry The Central Science in SI Units 15th Global Edition/Chapter 17- Additional Aspects of Aqueous Equilibria
Optional downloads
- ph_poh_strong_acids_bases_student_handout.md
- ph_poh_strong_acids_bases_teacher_guide.md
- ph_poh_strong_acids_bases_qa_report.md
Practice Bridge
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