Percentage Deprotonated

[Rachel Lipman]

When calculating the percentage, it is the concentration of A- divided by the initial concentration of HA times 100%. In the textbook it says it is also H3O+ divided by initial concentration. I am confused because when making the ice block chart, H3O+ is different than A-. Clarification please...

[Anne Cam 3A]

The textbook does this assuming that [H3O+]=[A-], because according to the standard weak acid dissociation equation, HA(aq) + H2O(l) <--> H3O+(aq) + A-(aq), the mole ratio of H3O+ and A- is 1:1. Thus H3O+ and A- would be interchangeable in the deprotonation equation.

[Rachel Lipman]

Great thank you. But in other cases the ratio is not always one-to-one? And sometimes A- is not equivalent to H3O+?

[Anne Cam 3A]

You would need to use [A-] for the deprotonation equation in that case, because the definition of deprotonation is the conversion of the acid HA to its conjugate base A-.