I would REALLY appreciate any assistance or advice on how to solve this question!!!

Show how you would make 500 ml of 20 mM HEPES buffer (pH 7.0) with 100 mM NaCl. You cannot use a pH meter, but pretend you were given the following:

1.) HEPES dry powder (CAS Number 7365-45-9, Empirical Formula C8H18N2O4S, Molecular Weight 238.30; pKa 7.5) (Hint: What form of the compound is this?)

2.) 1.0 M solutions of NaOH and HCl (Hint: You will have to decide to use one of these or both of these, then calculate how much to use.)

3.) 5 M NaCl

------------------------------------------------------------------

Here’s where I started:

I looked up the CAS number for HEPES and it appears to be in the acid (fully-protonated) form, from what I can tell. So I figured I would use only NaOH, not HCl, to make the buffer. Then I plugged the target pH (7) and the pKa of HEPES (7.5) into the Henderson-Hasselbach equation and solved for the ratio of conjugate base to acid (A-/HA), for which I got 0.316. Then I used algebraic substitution with the following two equations:

A-/HA = 0.316 and A- + HA = 20 mM

and then solved for HA, for which I got 15.198 mM. Then I solved for A-, for which I got 4.802 mM. At this point, I am stumped…I don’t know what quantities of HEPES, NaOH, and NaCl I need, nor how to even find them.