[noor123]

Hey!

So, im finding it difficult to understand when i am supposed to use Ka and when i should use Kb when given a problem.
I know Ka is for acid and Kb for bases.

But as an example question:  What is the pH of a solution containing 1,00*10^-4M o the medicine Propranololchloride (chloride salt of the base propranolol, Pr)? pK(HPr)=9,53


In this case, i thought, well ok the qustion has given that its a base, so i started doing the calculations based off that and got answer of pH=9,64
BUT the answer key says its pH=6,8  . so i tried doing it for Ka, and i got it right.
SOOO.... i dont understant why the Acid constant is used??? 

Anyone can explain? How do i know from the question stated, that i should use Ka and NOT Kb?

[Borek]

Protonated bases become acids.

[noor123]

But is it not like this? 

   Pr + H2O     HPr + OH^-

the base (Pr) gets protonated to HPr, and then its the [OH-] we need to find, and that gives pOH?  so need to use Kb... that was my reasoning. Where am i wrong?

[Borek]

You are told to find pH of a solution of a hydrochloride (or at least that's how I read what you wrote). That means your substance is already in the protonated (acidic) form even before being put in water.

[noor123]

oh ok,, can i tell this from the pK(HPr)  as well?  since it says the pKa, it must be acid constant to be used?

[Borek]

Actually the wording of the problem is ambiguous - but apparently it requires you to assume "whatever data is given, is the right data to use". HPr looks like an acid, so most likely pK given is a pKa, but IMHO it should be said explicitly, to avoid any doubts.

[noor123]

yea its really confusing. thanks for the help !