Achieve #5

[Chisomaga Azu]

To find the percentage of amine protonated, we divided the protonated amine (BH+) by the formal concentration of the amine (B). But, in this case, the formal concentration was B plus BH+. When do we know to divide by the sum versus dividing by B.

[Hao Tam Tran 3H]

Hi!

What you need to notice is the difference between this question and the other questions that ask for the protonated percentage. Notice that this question never gives you an initial concentration, meaning that all the values you calculate are at equilibrium concentration rather than initial concentrations (the percentage is of the initial concentration that was protonated). Thus, this is why you would need to add these equilibrium concentrations together in order to find what the initial was (realize that in the ice table, the equilibrium concentration of the reactant is usually initial - x = equilibrium, so all you are doing is reversing it into equilibrium + x = initial). Hope that helps!

[Jose-Joaquin Garcia 1K]

For this one I made an ICE table and assigned the value of BH to the variable x. You can find the concentrations for the addition to the value of x by subtracting the pH from 14.00 and taking the -log out of that value for the concentration of OH-. Just solve for x from the ICE table and then divide the [OH-] at equilibrium by the [B] initial. Kind of confusing but it makes sense when you draw out the ICE table.