Ka and Kb

[Amanda Salgado 3D]

When a Ka and Kb are given, is it safe to always assume away the x when calculating it in the equation because they are a weak acid/weak base?

[edhuang]

No. My chemistry teacher told me that usually after ^10-5 it is safe to make the assumption, but you should always check that the concentration is less than 5% after you finish the calculation. Alternatively, you can plug the x back in the full equation and solve it again until the x does not change anymore. Make sure the Ka/Kb is very small before you make the assumption.