Question 16 of 32 Map General Chemistr sity Science Books presented by Sapling L

Question

Question 16 of 32 Map General Chemistr sity Science Books presented by Sapling Lea Donald McQuarrie. Peter A Rock .Ethan Gallogly Phosphoric acid is a triprotic acid (Kat 6.9x 10-3, Ka2 6.2x10 and Ka3 4.8x 10 3). H2PO4 (aq) and HPO4 (aq), which pKa value would you use in To fin d the pH of a buffer composed of the Henderson-Hasselbalch equation? O p 2.16 pKa2 7.21 O pKa3 2.32 Calculate the of buffer solution obtained by dissolving 30.0 g of KH2PO4(s) and 48.0 g of Na2HPO4(s) n water and then diluting to 1.00 L Number pH

Explanation / Answer

your first answer is correct

no of moles of KH2PO4 = 30.0 g / 136.08 g/mol = 0.2204 mol

concentration of KH2PO4 = 0.2204 M since V = 1L

no of moles of Na2HPO4 = 48.0 g / 141.96 g/mol = 0.3381 mol

concetration of Na2HPO4 = 0.3381 M

use the handerson equation

pH = pKa + log(Na2HPO4 / KH2PO4)

here we have to use the pKa2 as a pKa value which is 7.21

pH = 7.21 + log(0.3381 / 0.2204)

pH = 7.21 + 0.18

pH = 7.4

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