Question

Show calculation for the assigned buffer. Recall: [acid] + [base] = 0.20 M. Calculate the individual concentrations needed (mass or volume) for HA and A- based on your assigned buffer and pH. The condition [acid] + [base] = 0.20 M must be satisfied. The final volume of solution is 100.0 mL.

Assigned Buffer: sodium dihydrogen phosphate and disodium hydrogen phosphate

pH = 7.69

Supplemental Data (I believe Ka is 6.2 x10^-8)

Acid	Species	Ka	Molar Mass g/mol	Compound
Phosphoric density = 1.685 g/mL	H3PO4	7.5X10^-3	98.00	H3PO4 (conc. acid)
	H2PO4-	6.2 X 10^-3	137.99	NaH2PO4 . H2O
	HPO4 2-	4.0 X 10^-13	268.07	Na2HPO4 . 7H2O
	PO4 3-		380.12	Na3PO4 .12H2O

Answer 1

According to the question we have to prepare a buffer of sodium dihydrogen phosphate and disodium hydrogen phosphate with pH = 7.69

H₂PO₄^- <===> HPO₄^-2 + H⁺

Using Henderson-Hasselbalch equation

pKa of H2PO4- = -log Ka = -log(6.2 x 10^-8) = 7.2

Plug in the vallues to find concentration of base and acid

[base] = 3.09 and [acid] = 1

but total [acid] + [base]= 2.0

we got total = 1+3.09

x = 0.49 M

[H₂PO₄^-] = x = 0.49 M

concentration of HPO₄^2-= 2 - 0.49 = 1.51 M