Question 20 of 36 Map eneral Chemistr University Science Books 4th Edition prese

Question

Question 20 of 36 Map eneral Chemistr University Science Books 4th Edition presented by Sapling Learning Donald McQuarrie Peter A. Rock Ethan Gallogly Phosphoric acid is a triprotic acid (Kat 6.9x 10-3, Ka2 6.2x 10-a, and Ka 4.8x10 3). To find the pH of a buffer composed of H2PO4 (aq) and HPO4 (aq), which pKa value would you use in the Henderson-Hasselbalch equation? O pKa1 2.16 O pKaz 7.21 O pKa3 12.32 Calculate the pH of a buffer solution obtained by dissolving 14.0 g of KH2PO4(s) and 29.0 g of Na2HPO4(s) in water and then diluting to 1.00 L. Number pH

Explanation / Answer

H2PO4–   + H2O = HPO42-   + HO-

is the second dissociation step of H3PO4.

In this buffer, H2PO4– is the acid and HPO42- is the base, from the same pair of conjugated acid-base.

Then use pKa2 in the Henderson-Hasselbalch equation.

(b)

moles of KH2PO4 == 14 .0 g /136 g/mol ==0.103 moles

moles of Na2HPO4 == 29.0 g / 142 g/mol ==0.2042

since volume is 1 L, molarity will be equal to moles

we know,

pH =pKa + log (HPO4 2- / H2PO4 -)

=7.21 + log(0.2042/0.103)

=7.507

=7.51

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