What mass of propanoic acid, CH3CH2COOH, pKa= 4.87, should be dissolved in water to 2.00L volume in order to produce a pH of 2.50?

Question

THANKS!

-J

Answer 1

Let's call propionic acid HP, then it will ionize as
HP ==> H^+ + P^-

First convert pH = 2.50 to (H^+) by
pH = -log(H^+).
Then set up the expression for Ka.
Ka = (H^+)(P^-)/(HP).
You know (H^+) now from the pH calculation. You know (P^-) because it equals the same as (H^+). (HP) = y-(H^+).
Solve for y which = (HP).
Then M x molar mass propanoic acid = grams propanoic acid.
Post your work if you get stuck.

Answer 2

When you do the -log of 2.50 you get a negative number, so I am not really sure what you do with that, do you not use the pKa for anything?