# biochemistry

if 4 volumes of 0.1M monobasic potassium phosphate, KH2PO4, are mixed with 2 volume of 0.1 M dibasic sodium phosphate, Na2HPO4, what will be the pH of the mixture?

What if 4 volumes of 0.1 M KH2PO4 are mixed with 8 volumes of 0.1 M ethanolamine base(pka=9.44) what will be pH of mixture?

1. 0
2. 3
1. Use pH = pKa + log[(base)/(acid)]
KH2PO4 is the acid. Na2HPO4 is the base.

For b part:
Write the reaction between KH2PO4 (the acid) and ethanolamine(the base) and see how much of each is left over and the amount of salt produced. Then plug into the HH equation.
Post your work if you get stuck.

posted by DrBob222
2. what's the pKa?

posted by dIrk
3. pH= pKa + log 0.1M/0.1M
what's the pKa value?

posted by dIrk
4. Won't that be pKa for k2 of H3PO4?
The 9.44 is pKa for ethanolamine.

posted by DrBob222
5. pH=7.20+log0.1M/0.1M
pH=7.20

posted by dIrk
6. base is not 0.1 and acid is not 0.1. Isn't the base 4 volumes x 0.1 = 0.4 moles; therefore, the concn is 0.4 moles/volume (whatever that is but it doesn't matter because the acid will be divided by the same V and V will cancel).

posted by DrBob222
7. pH=7.20+log 0.2M/ 0.4M
pH=7.20+(-0.30)
pH=6.69

I'm heading in the right direction?

posted by dIrk
8. Sorry the final pH of the mixture is 7.50, is that correct DrBob222?

posted by dIrk
9. the equation will look like this:

pH=7.20+log 0.4/0.2
= 7.50

Yes?

posted by dIrk
10. I believe you, correct if I'm not mistaken on part A. The answer is 7.50.

for part B: the Base is the amount of initial base minus the amount of base protonated.

Maybe Dr.Bob should show us Visually . DR.BOB!!!

posted by Jimmy

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