PrintCalculator Periodic Table Question 5 of ct Map General Chemistr Donald McQu

Question

PrintCalculator Periodic Table Question 5 of ct Map General Chemistr Donald McQuarrie Peter A. Rock.Ethan Gallogly University Sdlence Books presented by Sapling Learning Phosphoric acid is a triprotic acid (Kt-6.9× 10-3 Ka-6.2× 10-8, and Ka,-4.8x 113). (aq), which pka value would you use in H"(aq) + H,PC), (aq)] H.(aq) + HPC), (aq)| To find the pH of a buffer composed of H2PO (aq) and HPO the Henderson-Hasselbalch equation? Incorrect. Phosphoric acid ionizes in three steps: pka.-2.16 | Ka. | H,P0,(aq) pKg1-2.16 _ Which equation O pK12-7.21 OpKs,-12.32 | Ka2|H2PC), (ag) the buffer? sent in contains both of the species present in Calculate the pH of a buffer solution obtained by dissolving 12.0 g of KH2PO4(s) and 25.0 g of NazHPO4(S) in water and then diluting to 1.00 L Number pH- ^O Give Up & View Solution # Try Again O NextExit Previous

Explanation / Answer

H2PO4- ---> HPO4-2 and H+ this is the second ionization, that is Ka2

you need to use pKa2 = 7.21

Apply henderson hasselbalch equations for this buffer

pH = pKa2 + log(A-/HA)

find moles of each

MW K2H2PO4 = 136.086

moles of K2H2PO4 = mass/mol = 12/136.086 = 0.08817

MW NaHPO4 = 141.96

moles of NaHPO4 = 25/141.96 =0.176

Substitute

pH = pKa2 + log(A-/HA) = 7.21+ log(0.176/0.08817) = 7.51

pH = 7.51

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