Question

Ukesources LX Give Up? V Hint Check Answer tion 4 of 5 > Phosphoric acid is a triprotic acid (K,1 -6.9 x 10,K2 = 6.2 x 10-6, and K, -4.8 x 10-1). To find the pH of a buffer composed of H,PO, (aq) and HPO- (aq), which pK, value should be used in the Henderson-Hasselbalch equation? OpK 1 = 2.16 OpK2 = 7.21 OPK 3 = 12.32 Calculate the pH of a buffer solution obtained by dissolving 21.0 g of KH,PO,(s) and 38.0 g of Na, HPO (s) in water and then diluting to 1.00 L. Question Source: MRG General Chemistry Publisher: University

Answer 1

0 The buffer so uffer solution contains Hq pozi (aq) and 14 Pog- (aa). here H₂PO4 is weak acid and HP OG is the lonjugate base of H₂PO4 so we need the pka value of Hapo we have Ka(H2Pon) = 6.2x10-8 : pka -107 6:2 x 1087.21 10 . Answer is pkar 27.21 Molar mass of k H₂PO4 = 136 g/mol Molar mass of Naz Hpoza 1429/mal 21.0 g k H₂PO4 = 21.094 189 m = 0.154 mot 38.og Na2HP = 38./142 g/mw = 0+268 mot. These two salts are dissolved in th ho. : [ Nazpoq] = 0.268M [KIH PO4) = 0.154M. pH = pka + log [ conjugate base] [wear acid] [Na₂HPO4 = p + log T HP OUT - 7.21 +logº266 = 7.45 o 159 pH = 7.45 (Auswer).