Phosphoric acid is a triprotic acid with the following pK_a values: pK_a1 = 2.14

Question

Phosphoric acid is a triprotic acid with the following pK_a values: pK_a1 = 2.148 pK_a2 = 7.198 pK_a3 = 12.375 You wish to prepare 1.000 L of a 0.0100 M phosphate buffer at pH 7.540. 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

NaH2PO4 and Na2HPO4   are needed for buffer

since pka of H2PO4- is 7.198   which is near 7.54

From Henderson eq

pH = pka + log [ conjgate base] /[acid]

acid is NaH2PO4 and its conjugate base is Na2HPO4

7.54 = 7.198 + log [Na2HPO4]/[NaH2PO4]

[Na2HPO4] = 2.2 [NaH2PO4] ...........(1)

Buffer conc = 0.01

[Na2HPO4]+[NaH2PO4] = 0.01 .......(2)

solving (1) ( 2) we get

[NaH2PO4] = 0.003125 and [Na2HPO4] = 0.006875

volume of solution = 1L

Moles = M x V

hence moles of NaH2PO4 = 0.003125 x 1 = 0.003125 , and moles of Na2HPO4 = 0.006875

Mass = moles x molar mass of compound

Mass of NaH2PO4 = 0.003125 x 119.98 g/mol = 0.375 g

Mass of Na2HPO4 = 0.006875 x 141.96 = 0.976 g

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