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Weak Acids and Bases, Ka for weak acids 



Acids & Bases

AQA Content

Construct an expression for Ka
Perform calculations relating the pH of a weak acid to the concentration of the acid and the dissociation constant, Ka
Convert Ka into pKa and vice versa.

Specification Notes

Weak acids and weak bases dissociate only slightly in aqueous solution.
Ka is the dissociation constant for a weak acid.
pKa = –log10 Ka


Calculating the pH of strong acids is straightforward - the acid dissociates completely, and the H+ ion concentration is equal to the acid concentration. Calculating the pH of weak acids is not as straightforward. Weak acids only dissociate partially and the H+ ion concentration is much smaller than the acid concentration. Here, [HA] is the concentration of the undissociated acid and the ‘products’ are the ions formed from dissociation:
To calculate the H+ ion concentration of a weak acid, we use the acid dissociation constant (Ka) and the equation:
Two assumptions are made when using this equation:
1. The concentration of hydrogen ions is equal to the concentration of the anions
2. The concentration of undissociated acid, HA, at equilibrium is unchanged

Assumption 2 implies:

Using the concentration values of acids is a little unwieldy – it is sometimes better to use pKa. pKa is another measure of the strength of an acid - the lower the value, the stronger the acid and the greater its ability to donate its protons. The relationship between pKa and Ka is:


Write an expression for the acid dissociation constant Ka for ethanoic acid.

For this dissociation, we have:

The expression is, therefore:

We are given the pH so can find the hydrogen ion concentration:


Using the acid dissociation constant:
rearranging gives:


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