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OCR 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

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

OCR A-level Chemistry pH calculations If the concentration of hydrogen ions of a solution is 1 x 10⁻⁸ mol l⁻¹, what is the hydroxide concentration?

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2

✅

If a solution has hydroxide ion concentration 1.2 x 10⁻⁵ mol l⁻¹, what will the pH of the solution be?

OCR A-level Chemistry pH calculations If a solution has hydroxide ion concentration 1.2 x 10⁻⁵ mol l⁻¹, what will the pH of the solution be?

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3

✅

What does the term amphoteric describe?

OCR A-level Chemistry pH calculations What does the term amphoteric describe?

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4

✅

What is the pH when Kw is 5.5 x 10⁻¹⁶?

OCR A-level Chemistry pH calculations What is the pH when Kw is 5.5 x 10⁻¹⁶?

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5

✅

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

OCR A-level Chemistry pH calculations At 303 K, Kw has a value of 1.471 x 10⁻¹⁴. What is the pH of water?

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6

✅

If an acid has a high pKa value, is there a high or low degree of dissociation?

OCR A-level Chemistry pH calculations If an acid has a high pKa value, is there a high or low degree of dissociation?

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7

✅

Glycolic acid has Ka 1.3 x 10⁻⁴. Calculate it's pKa.

OCR A-level Chemistry pH calculations Glycolic acid has Ka 1.3 x 10⁻⁴. Calculate it's pKa.

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8

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What is the pH of a 1.2 x 10⁻⁴ mol l⁻¹ solution with Ka 2.3 x 10⁻³?

OCR A-level Chemistry pH calculations What is the pH of a 1.2 x 10⁻⁴ mol l⁻¹ solution with Ka 2.3 x 10⁻³?

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9

✅

If the concentration of acid and base in a buffer solution is the same, what will the pH be?

OCR A-level Chemistry pH calculations If the concentration of acid and base in a buffer solution is the same, what will the pH be?

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10

✅

A pH 4.7 buffer solution is composed of 2.3 x 10⁻⁴ mol l⁻¹ acid and 2.8 x 10⁻⁴ mol l⁻¹ acid. Calculate pKa.

OCR A-level Chemistry pH calculations A pH 4.7 buffer solution is composed of 2.3 x 10⁻⁴ mol l⁻¹ acid and 2.8 x 10⁻⁴ mol l⁻¹ acid. Calculate pKa.

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