*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

Answer: we deal with a triprotic acid.of those the first and second one are very close to the pH we need, so we will prepare buffer using the H3PO4/NaHPO4. We will just put known values into the Henderson-Hasselbalch equation:

7.6 = 6.9 x10-3 + log [H2PO4] / [H3PO4] OR

[H2PO4] / [H3PO4] = 10 0.0007

Now it is a matter of stoichiometry. First, we need to convert all H3PO4 to H2PO4-:

H3PO4 + NaHPO4 + H2O NaH2PO4 + H3O+ + H2PO4-

as there are 0.100 L×0.0.45 M = 0.047 moles of NaH2PO4 we need for this step. Then, some of the H3PO4has to be converted to H2PO4-. We already know how to calculate additional amount of NaH2PO4, as we have 0.47moles of H3PO4-, and we need n moles of NaH2PO4, where n can be calculated from

[H2PO4]- / [H3PO4] = n / 0.47-n = 1.0016

so n = ..... moles. (please calculate)

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