Question

In: Chemistry

4. Calculate the pH of the following salt solutions (5 pts each). a. 0.237 M sodium...

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)

Solutions

Expert Solution

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


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