How do you calculate the pH of a weak acid?

Strong acids dissociate fully in solution. Examples of strong bases include HCl and H2SO4. Organic acids, such as methanoic acid or oxalic acid, as well as some inorganic acids such as hydrogen fluoride, are weak acids.

A weak acid differs to a strong acid in that it does not fully dissociate in solution (it does not release all of it's potential hydrogen ions). This means that you cannot use the usual formula (pH=-log10[H^+]) by simply letting [H^+] equal the concentration of the acid. Instead, you must use the acid dissociation constant, Ka. Where, at equilibrium, [H^+] is the hydrogen ion concentration, [A^-] is the dissociated acid concentration, and [HA] is the concentration of the undissociated acid:
 

If we assume that when the weak acid dissociates, [H^+]=[A^-], then:
 

If you know the equilibrium concentration of the acid and the Ka (which you will usually be given), you can rearrange this formula to give [H^+], which should be all you need to work out the pH.