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How to calculate pH of the weak acid.

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When we are asked to find pH, the first thing we need to recognize is whether we are dealing with acid or base? Also we need to determine if it is strong or weak?

Strong acids are the following: HCl - hydrochloric acid, HNO3 - nitric acid, H2SO4 - sulfuric acid, HBr - hydrobromic acid,HI - hydroiodic acid, HClO4 - perchloric acid, HClO3 - chloric acid.

If an acid is not one of these, it is a weak acid. Moreover, if ka value is given for your acid, then there is a 99% chance that it is weak as well, unless the professor wants to trick you by giving you a very high ka value.

Weak acids dissociate very little and that is why we can not use regular stoichiometry for their calculations, but rather have to use the ICE chart.

Steps to do a pH calculation for a weak acid (example: Find pH of 0.1M acetic acid whose ka = 3*10^-5):

1. Write out the generic weak acid equation: HA --> H+ + A -

2. Write out the generic ka expression based on the equation above: Ka= [H+][A-]/ [HA]

3. Write out the ICE chart based on the weak acid equation

HA --> H+ + A -

I(initial concentration) 0.1 0 0

C(change) -x x x

E (equilibrium) 0.1-x x x

4. Plug in the equilibrium values into the Ka expression: Ka=x*x/ (0.1-x). The x in the bottom is negligible in most cases giving us Ka=x*x/ (0.1)= 3*10^-5

Now solving for x, we get x=0.00173205

5. X is our H+ concentration, therefore to find pH, we need to use formula pH=-log[H+]

pH = -log (0.00173205)= 2.76

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