pH = –log₁₀[H⁺], with [H⁺] in mol dm⁻³.

How do you find [H⁺] from a pH value?

[H⁺] = antilog(–pH), the inverse of –log₁₀.

Why is [H⁺] equal to the acid concentration for HCl?

HCl is a strong monoprotic acid and is fully dissociated.

What is the effect of a pH change of 1 on [H⁺]?

[H⁺] changes by a factor of ten.

1.00 × 10⁻¹⁴ mol² dm⁻⁶.

How do you find the pH of a strong base?

Find [OH⁻], then [H⁺] = Kw / [OH⁻], then pH = –log₁₀[H⁺].

How does Kw change as temperature rises, and why?

It increases, because dissociation of water is endothermic and the equilibrium shifts right.

Is pure water at 333 K acidic?

No. It is neutral because [H⁺] = [OH⁻], although its pH is below 7.

[OH⁻] in 0.020 mol dm⁻³ Ba(OH)₂?

0.040 mol dm⁻³, because each formula unit releases two OH⁻.

pH and the ionic product of waterAQA A-Level Chemistry: Flashcards

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Define pH.

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Define pH.: pH = –log₁₀[H⁺], with [H⁺] in mol dm⁻³.
How do you find [H⁺] from a pH value?: [H⁺] = antilog(–pH), the inverse of –log₁₀.
What is the pH of 0.0100 mol dm⁻³ HCl?: 2.00.
Why is [H⁺] equal to the acid concentration for HCl?: HCl is a strong monoprotic acid and is fully dissociated.
What is the effect of a pH change of 1 on [H⁺]?: [H⁺] changes by a factor of ten.
Write the equation for the dissociation of water.: H₂O ⇌ H⁺ + OH⁻.
Write the expression for Kw.: Kw = [H⁺][OH⁻].
What are the units of Kw?: mol² dm⁻⁶.
What is Kw at 298 K?: 1.00 × 10⁻¹⁴ mol² dm⁻⁶.
How do you find the pH of a strong base?: Find [OH⁻], then [H⁺] = Kw / [OH⁻], then pH = –log₁₀[H⁺].
How does Kw change as temperature rises, and why?: It increases, because dissociation of water is endothermic and the equilibrium shifts right.
Is pure water at 333 K acidic?: No. It is neutral because [H⁺] = [OH⁻], although its pH is below 7.
[OH⁻] in 0.020 mol dm⁻³ Ba(OH)₂?: 0.040 mol dm⁻³, because each formula unit releases two OH⁻.

Exam questions on pH and the ionic product of water

A student dilutes 10.0 cm³ of 0.200 mol dm⁻³ hydrochloric acid with distilled water to a total volume of 250 cm³. All measurements are at 298 K.
Calculate the pH of the diluted solution and the change in pH caused by the dilution.2 marks
A cleaning product contains sodium hydroxide at a concentration of 0.0250 mol dm⁻³. At 298 K the ionic product of water, Kw, is 1.00 × 10⁻¹⁴ mol² dm⁻⁶.
Use Kw to calculate the hydrogen ion concentration in the original sodium hydroxide solution.2 marks
Pure water is slightly dissociated: H₂O(l) ⇌ H⁺(aq) + OH⁻(aq). A data book gives Kw = 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K and Kw = 9.61 × 10⁻¹⁴ mol² dm⁻⁶ at 333 K.
Show how the expression Kw = [H⁺][OH⁻] is derived from the equilibrium constant for the dissociation of water, and give the units of Kw.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).