Algebra of the Henderson-Hasselbach equation

[helpme12345]

I am weak on the algebra of the Henderson-Hasselbach equation.

I have a prelab question:

Using the H-H equation, calculate the volumes of 0.1 MM phosphate solutions to mix to prepare 100ml each of buffers with pH values 6.0, 6.5, 7.0,7.5, and 8.0 starting with 01.M stock solutions of NaH2HPO4 and Na2HPO. The relevant reaction is given below:

H2PO4- -->HPO4^2- + H+

The pKa is 7.21.

I know the pH is the values listed.

pH=pKa+log{[A-]/[HA]}

How do I start this problem?

 

[Mike H]

You know the pKa, and you know what the pH needs to be for each of the buffers. What you need to know is the ratio of the phosphate stock solutions, so you can then mix up the appropriate volumes. So, you get your known values on one side and your unknown values on another side. Once you have that, you need to get rid of the logarithm (base 10) using its inverse. Once you've done that, then it's just a matter of calculating what the ratio is equal to (which you should be able to do on a calculator, it'll be a little messy) and then figuring out the appropriate volumes, dilutions, and the rest.

I hope this helps without giving too much away...