In: Chemistry
4. Calculate the pH of the following salt solutions (5 pts each). a. 0.237 M sodium cyanide, NaCN (aq) (HCN ka = 4.0 x 10-10) b. 0.166 M ammonium iodide, NH4I (aq) (NH4+ ka = 5.6 x 10-10)
a.(
(HCN ka = 4.0 x 10-10)
Kb = 10^-14 / Ka = 10^-14 / 4 x 10^-10 = 2.5 x 10^-5
CN- + H2O --------------> HCN + OH-
0.237 0 0
0.237 - x x x
Kb = x^2 / 0.237 - x
2.5 x 10^-5 = x^2 / 0.237 - x
x = 2.42 x 10^-3
[OH-] = 2.42 x 10^-3 M
pOH = -log [OH-] = -log (2.42 x 10^-3)
= 2.62
pH = 11.38
b.)
Kb = 1.8 x 10^-5
pH = 7 - 1/2 (pKb + log C)
= 7 - 1/2 (4.74 + log 0.166)
= 5.02
pH = 5.02