I lose many points because the answer is wrong please I want correct answer Phos

Question

I lose many points because the answer is wrong please I want correct answer

Phosphoric acid is a triprotic is a triprotic acid with the following p K_a values: p K_a1 = 2.148 pK_a2 = 7.198 pK_a3 = 12.375 You wish to prepare 1.000 L of a 0.0400 M phosphate buffer at pH 7.640. To do this, you choose to mix the two salt forms involved in the second ionization, NaH_2PO_4 and Na_2HPO_4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? (Check all that apply). H_3PO_4 and NaH_2PO_4 H_3PO_4 and Na_2HPO_4 H_3PO_4 and Na_3PO_4 NaH_2PO_4 and Na_3PO_4 Na_2HPO_4 and Na_3PO_4

Explanation / Answer

the pKa2 is H2PO4 <-----> H+ + HPO4

your desired pH is essentially that for Ka2 is pKa

pH = pKa + log [(HPO4)] / [(H2PO4)]

7.64 = 7.198 + log [(HPO4)] / [(H2PO4)]

7.64 - 7.198 = log [(HPO4)] / [(H2PO4)]

0.442 = log [(HPO4)] / [(H2PO4)]

taking out of logs
100.442 = [(HPO4)] / [(H2PO4)]

2.766 = [(HPO4)] / [(H2PO4)]

rearranging tells us that

[(HPO4)] = 2.766 of the strength of [(H2PO4)]

say "X" = the strength of [(H2PO4)]

we find that a 0.0400 M phosphate buffer = [X] + [2.766 X]

3.766 X = 0.0400 Molar

solving for "x" = 0.01 M

[X], the strength of [(H2PO4)], = 0.01 Molar NaH2PO4

solving for [2.766 X], the strength of [(HPO4)], = 0.029 Molar Na2HPO4

solving for moles in a 1.000 Litre buffer:

0.01 moles NaH2PO4

0.029 moles Na2HPO4

using molar masses, solve for mass:

(0.01 moles NaH2PO4) (119.98 grams per mole) = 1.1998 grams of NaH2PO4

(0.029 moles Na2HPO4) ( 141.96 grams per mole) = 4.17 grams of Na2HPO4

1.1998 grams of NaH2PO4

4.17 grams of Na2HPO4

2. Regarding the second question. You can prepare the buffer with : b, c and d.

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