A more rigorous treatment of worked problem

This replaces the statement that [CeHsCOOH]actual = [OH ]actuai by the electrical neutrality statement  

Although the salt of a weak acid/strong base behaves as a solution of a weak base it must not be overlooked that the solution is that of a salt, and not that of a weak base on its own. The electrical neutrality statement must always contain a contribution from the cation of the salt, here Na (aq). 

This can be approximated by assuming very slight conversion of the benzoate ion to benzoic acid, justified by the results of the first part of Worked Problem 5.1 above, so that [CeHsCOO-Jactuai = [CfiHsCOO ltotai, giving  

This new value, [OH ]actuai = 3.97 x 10 moldm using the more rigorous method can be contrasted with the fully approximate treatment which gave [OH ]actuai = 3.85 x 10 moldm . The difference between the two values is 0.12 x 10 mol dm showing that the fully approximate value is 3% low. This indicates it is just possible to avoid having to consider the self ionisation of water for this salt. 

If the concentration is reduced to 5 x 10 mol dm it is likely that the self ionisation of water will have to be included in the calculation. Using the rigorous expression derived above  

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