The ionisation constants for phosphoric acid (H3PO4) in aqueous solution at 25 °C are: Ka1 = 7.2 x 10-3 pKa1 = 2.14 Ka2

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The ionisation constants for phosphoric acid (H3PO4) in aqueous
solution at 25 °C are: Ka1 = 7.2 x 10-3 pKa1 = 2.14 Ka2 = 6.3 x
10-8 pKa2 = 7.20 Ka3 = 4.2 x 10-13 pKa3 = 12.38 Which ONE of the
following mixtures would you use to make a buffer solution with pH
6.5?
(a) A solution which is 2.0 mol L-1 H3PO4 and 0.4 mol L-1
NaH2PO4
(b) A solution which is 2.0 mol L-1 NaH2PO4 and 0.4 mol L-1
Na2HPO4
(c) A solution which is 0.4 mol L-1 NaH2PO4 and 2.0 mol L-1
Na2HPO4
(d) A solution which is 2.0 mol L-1 NaH2PO4 and 0.4 mol L-1
Na3PO4
(e) A solution which is 0.4 mol L-1 NaH2PO4 and 2.0 mol L-1
Na3PO4