Question

Determine the pH of a 0.0520 M solution of the following salts. A table of pertinent K values is found to the right. a) NaHCO3 b) NaH2PO4

acid	formula	K1	k2	k3
arsenic acid	H3AsO4	5.8e-3	1.1e-7	3.2e-12
carbonic acid	H2CO3	4.45e-7	4.69e-11
hydrogen sulfide	H2S	9.6e-8	1.3e-14
phosphoric acid	H3PO4	7.11e-3	6.32e-8	4.5e-13

Answer 1

a)

NaHCO3 --> Na+ + HCO3-

note that for HCO3-

we could assume this is half point of the titration of HCO3- and CO3-2

then

pH = 1/2*(pKa1 + pKa")

pKa1= -log(4.45*10^-7) = 6.35

pKa2= -log(4.69*10^-11) = 10.328

pH = 1/2*(6.35+10.328) = 8.339

pH = 8.339

b)

For NaH2PO4

similarly,

the pKa values are between H2PO4- and HPO4-

then

pH = 1/2*(pKa2 + pKa3)

pKa2 = -log(Ka2) = -log(6.32*10^-8) = 7.21

pKa3 = -log(Ka3) = -log(4.5*10^-13) = 12.35

pH = 1/2*(7.21+12.35)

pH = 9.78