top of page # Weak Acids and Bases, Ka for weak acids

Physical

A2

Acids & Bases

AQA Content

###### Convert Ka into pKa and vice versa.

Specification Notes

Notes

###### 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: ###### 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: ###### Therefore: ###### Using the acid dissociation constant: ###### rearranging gives: 