Textbook 6E.3

[Jessy Ji 2J]

In this question, it says "ignoring second deportations only when the approximation is justified." I was wondering what does it mean when it says approximation is justified?

[Vy Le 1G]

Hello,

Correct me if I’m misinterpreting your question but Dr. Lavelle mentioned we can justify our approximation if the percentage deprotonation is less than 5%. In the context of your quote, since K2 is really weak compared to K1, we usually have a good enough (<5%) calculation with our approximation. There is an exception with H2SO4 though!

Hopefully this helped.

[Michelle_Duong_3H]

Hello,

When we have an equilibrium constant that is less than 10^-3, then we can approximate the change in reactants. This is because the decrease in reactants is extremely small compared to the initial amount of reactant. Approximation makes these problems a lot simpler, especially when it comes to cubic equations. However, in order for this approximation to be valid (or justified, as you mentioned), the percent ionization must be less than 5%.

Hope this helps!

[Hannah_Pon_1F]

For polyprotic acids:
Estimate the pH of a polyprotic acid for which all deprotonations are weak by using only the first deprotonation equilibrium and assuming that further deprotonation is insignificant (assuming the percent ionization is less than 5%). An exception is sulfuric acid, the only common polyprotic acid that is a strong acid in its first deprotonation.