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April 24, 2014

Homework Help: Chemistry

Posted by L.Bianchessi on Sunday, February 26, 2012 at 10:02pm.

Find the pH of mixture of acids. 0.185 M in HCHO2 and 0.225 M in HC2H3O2

Im using an ice chart of weak acid and putting in strong acid in H+ initiAL concentration. I've done the problems many different ways but cannot seem to get the right answer help please?Answer is pH of 2.19.



You said...
You must recognize that this is a mixture of two weak acids; i.e., formic acid and acetic acid. I looked up Ka for both and used 1.77E-4 for Ka HCOOH and 1.8E-5 for Ka CH3COOH.

Calculate the H^+ from the strong acid, then add the H^+ from the weak acid. Formic acid first since it is the stronger. .........HCOOH ==> H^+ + HCOO^-initial..0.185.....0......0 change...-x........x......x equil..0.185-x.....x......x

Ka = (H^+)(HCOO^-)/(HCOOH) Solve for H^+. This is what acts as the common ion (remember Le Chatelier's Principle). This causes the acetic acid to ionize less than it would if were just acetic acid solution.

........CH3COOH ==> H^+ + CH3COO^-initial..0.225.......0......0 change....-x........x........x equil...0.225-x......x........x

Ka acet acid = (H^+)(CH3COO^-)/(CH3COOH) Substitute TOTAL H^+ into the Ka expression for CH3COOH. That will be about 0.00572 from HCOOH from the above calculation plus xfrom this ionization. Solve for x,add this (H^+) to the 0.00572 from HCOOH, then convert to pH. I obtained pH = 2.19


I STILL CAN'T GET 2.19???

I got concentration of HCOOH rxn to be 5.77X10^-3. And the CH3COOH rxn to have concentration of 2.01X10^-3. I added those together to get 7.78X10-3. What do you mean add to ka expression? I keep getting 1.35 now.

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