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How many grams of sodium succinate (140 amu) and disodium succinate (162 amu) must be added to 1L of water to produce a solution with pH 6.0 and a total solute concentration of 50mM? I use pH = pKa + log ([A-]/[HA]) and since pKa of succinic acid averages to 4.8, I get 6 = 4.8 + log ([A-]/[HA]) but then I get totally stuck...thank you!

Calculate the amount of glycine and HCl required to prepare a 1L solution of 0.1M buffer at a pH of 2.0. The pKa of the glycine carboxyl group is 2.4 and the pKa of the amine group is 9.6. The amu of glycine is 75. Using pH = pKa + log ([A-]/[HA]), I get 2.0 = 6.0 + log ([A-]/[HA]). I get 6.0 because that is the average of the pKa values of glycine. But we can't have the log of a negative number, so I'm totally stuck here... Thanks...