1. Give the pH of 0.1M NaH2PO4 to 3 sig figs: Ka’s are 7.5 x 10-3, 6.2 x 10-8, 4

Question

1. Give the pH of 0.1M NaH2PO4 to 3 sig figs:

Ka’s are 7.5 x 10-3, 6.2 x 10-8, 4.8 x 10-13

2. Consider the titration of H3PO4 with NaOH.

At which points in the titration would the species NaH2PO4 be present in the HIGHEST concentration (of all possible forms of this acid?). Note; the points below may not include the exact point where it is at its highest possible concentration, but the correct choice would still show it as higher than the others.

A)
After the first equivalence point

B)
After the second equivalence point

C)
After the third equivalence point

D)
After a very small amount of NaOH has been added

Explanation / Answer

1. Give the pH of 0.1M NaH2PO4

pKa1 = -log Ka1 = -log (7.5 x 10^-3)

pKa1 = 2.12

pKa2 = -log Ka2 = -log (6.2 x 10^-8)

pKa2 = 7.21

NaH2PO4 is highest concentration at first equivalence point.

here

pH = 1/2 (pKa1 + pKa2 )

pH = 1/2 (2.12 + 7.21)

pH = 4.66

2)

answer : A) After the first equivalence point

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