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Summary Chemistry Alevel Acid Dissociation Constant Notes
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Chemistry Alevel Acid Dissociation Constant Notes
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Acid Dissociation Constant - KaKa is the acid dissociation constant.
For a weak acid to simplify the formulae you get [H+]2/[HA].
This is because when dealing with weak acids you assume that all the H+ ions come from the
acid not the base - so A- is removed.
[H+] + [A-] are equal because they have disassociated in a 1:1 ratio.
As the amount of disassociation is small assume that the initial concentration of the
undissociated acid remains constant.
1. Work out Ka from pKa using Ka = 10-pKa
2. Expression for Ka = [H]2/[HA]
3. Rearrange to give [H+] = square root of Ka x [HA]
4. Work out pH by -log10[H+]
Having a higher Ka value means that the acid is stronger. They are directly proportional.
Having a higher pKa value means the acid is weaker. They are inversely proportional.
Finding pH of weak acids:
1. Write an expression of Ka for the weak acid.
2. Rearrange the equation and substitute values to work out [H+].
3. [H+] squared divided by the [HA] concentration given.
4. Substitute H+ into the pH equation: pH = -log10[H+].
Finding concentration of weak acids:
1. pH should be substituted into the equation to work out [H+].
2. Write the Ka expression and substitute.
Finding Ka of weak acids:
1. Use equation.
pH values for weak acids should be around 2-5 if the answer is bigger or smaller check the
working out.
pKa
The values of Ka can be difficult to manage so pKa is used instead.
pKa + pKb = 14.
p refers to the -log10
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