What are the respective concentration of each of these

Question

1. Biologists frequently use phosphate-buffered solutions in experiments. You need to prepare a sodium phosphate solution at pH 6.7 by mixing NaH₂PO₄ and Na₂HPO₄ (pK_a =7.2). What are the respective concentration of each of these substances if you want the final concentration of phosphate ([HPO₄-2]+[H₂PO₄-] to be 150 mM?

2. Activity of the bacterial enzyme acetoacetate decarboxylase sharply declines below pH 6, suggesting there is an ionizable catalytic residue with pK_a ~ 6. Using mutagenesis, this critical catalytic residue was determined to be Lys-115.

a) For catalytic Lys-115, what is the ratio of deprotonated to protonated side chain at pH 7.5?

b) The neighboring residue at position 116 is also lysine. Explain how this perturbs the pK_a of the catalytic lysine residue.