6.1.4Additional lessons
The pH Scale — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 119 of 172
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Part 1 of 5Definition
The pH scale turns very small hydrogen ion concentrations into convenient numbers.
Formula
- pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol dm⁻³.
Reverse
- The concentration is found from pH using [H⁺] = 10⁻ᵖᴴ mol dm⁻³.
Neutral
- At 298 K, a neutral solution has pH 7, an acid has pH below 7 and an alkali has pH above 7.
Range
- Typical values run from 0 to 14, though very strong acids and bases can be outside these limits.
Part 2 of 5A Logarithmic Scale
A Logarithmic Scale
Tenfold change
- A decrease of one pH unit means the hydrogen ion concentration is ten times greater.
Two units
- A solution of pH 3 has 100 times the [H⁺] of a solution of pH 5.
Significant figures
- The number of decimal places in a pH equals the number of significant figures in [H⁺].
Measuring
- pH is measured with a pH meter, or approximated with universal indicator which gives a colour for each range.
| [H⁺] / mol dm⁻³ | pH | Nature |
|---|---|---|
| 1.0 × 10⁻¹ | 1.0 | Strongly acidic |
| 1.0 × 10⁻⁷ | 7.0 | Neutral |
| 1.0 × 10⁻¹³ | 13.0 | Strongly alkaline |
Part 3 of 5Calculations
Calculators and logarithms are used for the following steps.
1
To find pH from concentration, enter the value of [H⁺] and take −log₁₀ with a calculator.
2
To find [H⁺] from pH, calculate 10 raised to the power of minus the pH.
3
For a strong monoprotic acid, [H⁺] is equal to the acid concentration because it dissociates completely.
4
For a strong diprotic acid, remember that two moles of H⁺ can be released per mole of acid.
5
Always round the final answer to the number of decimal places that matches the data.
Part 4 of 5Worked Example
Question Calculate the pH of 0.025 mol dm⁻³ HCl, and the [H⁺] of a solution with pH 3.5.
- HCl is a strong monoprotic acid, so [H⁺] = 0.025 mol dm⁻³.
- pH = −log₁₀(0.025) = 1.60.
- For pH 3.5: [H⁺] = 10⁻³·⁵ = 3.2 × 10⁻⁴ mol dm⁻³.
Answer pH = 1.60; [H⁺] = 3.2 × 10⁻⁴ mol dm⁻³
Part 5 of 5Exam Checklist & Tips
6.1.4 – Key Points for the Exam
- pH The negative logarithm to base 10 of the hydrogen ion concentration.
- Neutral solution
- One in which [H⁺] equals [OH⁻];
- at 298 K the pH is 7.
- One in which [H⁺] equals [OH⁻];
- Hydrogen ion concentration The amount of H⁺ per dm³ of solution in mol dm⁻³.
- Logarithmic scale A scale in which each unit corresponds to a tenfold change.
Common Mistakes
- Saying that pH 3 is twice as acidic as pH 6; it is 1000 times as acidic.
- Taking the pH of a strong acid to be −log of the acid concentration even for a diprotic acid.
- Forgetting the negative sign in the formula.
- Quoting too many decimal places for a pH.
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