pH Calculations for Polyprotic Acids

Calculating pH for polyprotic acids like phosphoric acid, H3PO4 and sulfurous acid, H2SO3 ... where all Ka values are small. This does NOT apply to calculation pH for sulfuric acid, H2SO4.

[@thephonecollector]

Great video! I've always found it faster to just set up a quadratic and solve for x and then you get an exact solution.

[@betul1860]

My professor told us if the ratio of concentration over Ka is greater than 100, then we can neglect the x on the denominator. But when i apply the rule in this video(0.02/7.5x10^-3) the result is smaller than 100. How do we decide that we can neglect the x ?