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Ionic Product of Water (Kw) and pH

2017 Paper One

Question Three
Part of a series looking at past AQA A-level Chemistry questions with a focus on better performance in examinations.
If you haven't gone through the question/paper, click the direct link below, and have a go...

AQA A-level CHEMISTRY Paper 1 Inorganic and Physical Chemistry
Tuesday 13 June 2017

A 9-mark, 5-part question

The answer is C.

The collision of water molecules can lead to a transfer of a hydrogen ion from one molecule to the other. The resulting products are a positively charged hydronium ion and a negatively charged hydroxide ion.

We often use the simplified form of the reaction seen for C.

We can consider there to be equal concentrations of H+ and OH- ions. We can rewrite the expression as:

Note: pH is always to 2 decimal places

The concentration of hydrogen ions is equal to the concentration of OH ions
[H+] = [OH-]

Trends in Group 2: The hydroxides become more soluble down the group. Therefore magnesium hydroxide is less soluble than calcium hydroxide. Magnesium hydroxide will have a lower pH (more acidic).
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