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###### pH Calculations

Chemistry (Year 12) - Acids and Bases

Michael Swift

# pH and Hydronium Concentration

Strong and weak acids will ionise/dissociate in solution to produce hydronium ions. This will cause the pH of the solution to change. The pH of a solution is calculated using...

# pH Calculation for Strong Monoprotic Acids

Strong monoprotic acids will fully ionise/dissociate in solution, meaning that they will react completely to form hydronium ions. Therefore...

For example, if a solution was initially 1 M hydrochloric acid, then once this strong monoprotic acid fully dissociated, the concentration of hydronium ions in the solution would be 1 M.

# pH Calculation for Weak Monoprotic Acids

Weak acids will only partially ionise/dissociate in solution, meaning that not all of their molecules will form hydronium ions. As a result, it cannot be assumed that the concentration of hydronium ions will be equal to the initial concentration of acid in the solution - the concentration of hydronium ions will be less than the initial concentration of the weak acid.

At a Year 12 Chemistry ATAR level, to calculate the pH of a weak acid, the concentration of hydronium ions in solution must be given to us.

# pH Calculation for Strong Polyprotic Acids

Strong polyprotic acids have multiple ionisation/dissociation steps.

The first ionisation/dissociation step for a strong polyprotic acid will proceed to completion - all of the molecules will form hydronium ions.

The second ionisation/dissociation step for a strong polyprotic acid will proceed to a lesser extent than the first - only some of the molecules will form hydronium ions.

If any additional ionisation/dissociation steps exist, then they will each proceed to a lesser extent than the last.

The first ionisation/dissociation step for a strong polyprotic acid proceeds to completion. Further ionisation/dissociation steps proceed to lesser extents, however they still produce additional hydronium ions. As a result, the concentration of hydronium ions will be greater than the initial concentration of the strong polyprotic acid.

At a Year 12 Chemistry ATAR level, to calculate the pH of a strong polyprotic acid, the concentration of hydronium ions in solution must be given to us.

# pH Calculation for Weak Polyprotic Acids

Weak polyprotic acids have multiple ionisation/dissociation steps.

The first ionisation/dissociation step for a weak polyprotic acid will not proceed to completion - only some of the molecules will form hydronium ions.

The second ionisation/dissociation step for a weak polyprotic acid will proceed to a lesser extent than the first - an even smaller proportion of molecules will react to form hydronium ions.

If any additional ionisation/dissociation steps exist, then they will each proceed to a lesser extent than the last.

Whether the concentration of hydronium ions is greater or less than the initial concentration of the acid is not known in the general case for weak polyprotic acids.

For this reason, at a Year 12 Chemistry ATAR level, to calculate the pH of a weak polyprotic acid, the concentration of hydronium ions in solution must be given to us.

# pH Calculation for Bases

Calculating the pH of a base is different to acids. pH is a measure of the concentration of hydronium ions, but bases produce hydroxide ions in solution. As a result, an additional step must be taken to calculate the concentration of hydronium ions.

To do this, we can use the fact that the product of the concentration of hydronium and the concentration of hydroxide will equal a temperature-specific self-ionisation constant of water.

For example, at 25 degrees celsius...

This can be re-arranged to give the concentration of hydronium...

From here, the concentration of hydronium can be substituted into the pH formula to calculate the pH of the basic solution.

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