(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Calculate the volume of 0.10M phosphate solution to mix to prepare 100mL of a buffer with pH 6.0 starting with 0.10M stock solutions of NaH2PO4 and Na2HPO4. The pKa for this reaction is 7.21.

2. Relevant equations

pH=pHa+log( [A-]/[HA])

3. The attempt at a solution

When I did it by myself, I let x+y=1 and x=Na2HPO4 and y=NaH2PO4, solved for x using the HH equation but got weird numbers that didn't make sense (like .984). Then I tried x=Na2hPO4 and .100L-x=NaH2PO4:

A^{-}=.1x/.1L and HA=.1x/(.1-x), plugged that into the HH equation (.1L cancel) so

6=7.21+log([.1x]/[.1(.1-x)] and solved for x and once again got a very small number that doesn't make sense (.0058).

Now I'm lost and flustered. Thank you before hand for any help offered!

*after staring at this for a while, my final answer of .0058 is L, which is 5.8mL, therefore 5.8mL of Na2HPO4 and then 94.2mL of NaH2PO4 would be needed to achieve a pH of 6?

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# Homework Help: Volumes of a Buffer using Henderson-Hasselbalch?

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