*PLEASE HELP! Have done this problem every single way I can think of and none of

Question

*PLEASE HELP! Have done this problem every single way I can think of and none of the answers are correct. Not sure what I'm doing incorrectly.

You are instructed to create 100. mL of a 0.47 M phosphate buffer with a pH of 7.6. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.) H3PO4(s) + H2O(l) equilibrium reaction arrow H3O+(aq) + H2PO4(aq) Ka1 = 6.9103 H2PO4(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + HPO42(aq) Ka2 = 6.2108 HPO42(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + PO43(aq) Ka3 = 4.81013 What is the molarity needed for the acid component of the buffer? What is the molarity needed for the base component of the buffer? How many moles of acid/base are needed for the buffer? How many grams of acid/base are needed for the buffer?

Explanation / Answer

Since the pH required is close to pKa2 (pKa2 = -log[Ka2) = 7.21) we would use this value for calculation,

Using Hendersen-Hasselbalck equation,

pH = pKa + log([base]/[acid])

Considering pKa2, we would have,

[acid] = H2PO4- (or NaH2PO4)

and,

[base] = HPO4^2- (or Na2HPO4)

Feed values,

7.6 = 7.2 + log([base]/[acid])

[base] = 2.51[acid]

[acid] + [base] = 0.47

[base] = 0.47 - [acid]

Feed in the above equation,

0.47 - [acid] = 2.51[acid]

So molarity needed for base and acid components are,

[acid] = 0.13 M

[base] = 0.47 - 0.13 = 0.34 M

Moles needed for acid and base components are,

moles = molarity x volume

acid = 0.13/0.1 = 1.3 moles

base = 0.34/0.1 = 3.4 moles

Grams needed for acid and base components are,

grams = moles x molar mass

acid = 1.3 x 119.98 = 155.97 g

base = 3.4 x 141.96 = 482.66 g

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