Calculations involving acids and bases

[sergey_le]

Summary:: finding ml of two solutions by the final pH

i have a NaOAc 0.1M and HOAc 0.1M , together the volume of the solutions is 20ml and the pH is 4. I need to find the volume of each solution.
I've tried to solve it for hours with no successes. i found the H+ concentration (-log(h)=4 ), it is 1*10^-4 M so in a 20 ml solution we got 2*10 ^-6 mol but I don't know what to do next. find the ka?

[Moderator's note: Moved from a technical forum and thus no template.]

 

[epenguin]

yes, you need the K_a, the acid dissociation constant of acetic acid. You have to look its numerical value up, there is no way to work it out. You may more likely find quoted not K_a but pK_a, and you have to know what that means. Also look up 'Henderson Hasselbalch equation'.

 

[sergey_le]

yes, you need the K_a, the acid dissociation constant of acetic acid. You have to look its numerical value up, there is no way to work it out. You may more likely find quoted not K_a but pK_a, and you have to know what that means. Also look up 'Henderson Hasselbalch equation'.

i found the pka=4.8 so the ka is 1.58*10^-5.
i guess the big issue i have is understanding what equation i should use. i attached what idid so far ,can you give it a look and tell me where i am wrong?

 

[epenguin]

I was unable to open your attachment. However please realize this is very elementary theory covered in thousands of books or sites (and the most frequent subject of problems in this forum section) though if you can work it out yourself that is still better.

 

[Borek]

Google Henderson-Hasselbalch equation (although it is just a rearranged Ka definition).