Question

JUST NEED PART C, A AND B ARE FOR REFERENCE

A)Suppose you wanted to make a buffer of exactly pH 7.00 using KH2PO4 and Na2HPO4. If the final solution was 0.17 M in KH2PO4, what concentration of Na2HPO4 would you need? (pKa for H3PO4, H2PO4−, and HPO2−4 are 2.14, 6.86, and 12.40, respectively.) (answer 0.234)

B)Now assume you wish to make a buffer at the same pH, using the same substances, but want the total phosphate molarity ([HPO42−]+[H2PO4−]) to equal 0.35 M. What concentration of the KH2PO4 would be required? (answer 0.147)

C) Now assume you wish to make a buffer at the same pH, using the same substances, but want the total phosphate molarity ([HPO42−]+[H2PO−4]) to equal 0.35 M. What concentration of the Na2HPO4 would be required?

Answer 1

pH = pKa + log [salt / acid]

7.00 = 6.86 + log [HPO42-/ H2PO4-]

[HPO42-/ H2PO4-] = 1.38

[HPO42- + H2PO4-] = 0.35

1.38 H2PO4- + H2PO4- = 0.35

[H2PO4-] = 0.147

[Na2HPO4] = 0.203 M