Question

# Show calculation for the assigned buffer. Recall: [acid] + [base] = 0.20 M. Calculate the individual...

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

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 HPO42-= 2 - 0.49 = 1.51 M