Question 47

[kristinadi7224]

I am trying to figure out the solution of the problem, but i do not understand the math. Why are they multiplying 1.8x 10^-5 and X to get 16x10^-6? I understand the solution up until we plug in Ka from Table one to calculate the concentration of protons.

[mcat_premed3832]

I created line spaces in that part of the explanation that should make the calculation more clear for you.

[dnpgr16513]

Why can't we use pKa=-log (Ka)? And then use pKa=pH? Am I confusing these concepts?

thanks.

[jellywing_2058]

YES, you are confusing weak acid solutions with buffer solution calculations!

The explanation shown in the exam is called the ICE method (or IRF Table). It is used to calculate the [H+] of a weak acid, which is then used to calculate the pH using pH = -log [H+].

We cannot use pKa = -log Ka because this will NOT give the pH. Moreover pKa does NOT equal pH. This only happens in buffers when the conjugate base concentration equals the acid concentration.

In this case, what is calculated is the pH value of a weak acid, and not of a buffer solution, which would be a solution made up of a weak acid and its salt (or conjugate base). You cannot say for weak acids alone that pKa = pH