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Edexcel A-level Chemistry pH calculations

pH calculations

This page covers the following topics:

1. Determination of pH
2. The ionic product of water
3. The dissociation constant
4. Buffer calculations

pH and the hydrogen ion concentration are related by the equation pH = -log₁₀[H⁺].

Determination of pH

Water molecules dissociate into hydrogen and hydroxide ions in solution. The ionic product of water describes the relationship between the two at equilibirum.

The ionic product of water

In solution, strong acids/bases completely dissociate into ions, but weak acids/bases only partially dissociate. The acid dissociation constant is Ka and is related to pKa by the expression pKa = -log₁₀Ka. The pH of a weak acid can be calculated by pH = 1/2 pKa - 1/2 log₁₀c.

The dissociation constant

A buffer solution maintains a constant pH when small volumes of acid or base are added. The pH of a buffer solution can be calculated by the equation pH = pKa - log₁₀[acid]/[salt].

Buffer calculations

1

At 303 K, Kw has a value of 1.471 x 10⁻¹⁴. What is the pH of water?

At 303 K, Kw has a value of 1.471 x 10⁻¹⁴. What is the pH of water?

2

As temperature increases, Kw increases. Applying Le Chatelier's Principle, is the dissociation of water exothermic or endothermic?

As temperature increases, Kw increases. Applying Le Chatelier's Principle, is the dissociation of water exothermic or endothermic?

3

If the concentration of hydrogen ions of a solution is 1 x 10⁻⁸ mol l⁻¹, what is the hydroxide concentration?

If the concentration of hydrogen ions of a solution is 1 x 10⁻⁸ mol l⁻¹, what is the hydroxide concentration?

4

What are the Bronsted Lowry definitions of acids and bases?

What are the Bronsted Lowry definitions of acids and bases?

5

What acid is sodium ethanoate a salt of?

What acid is sodium ethanoate a salt of?

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