Finding pH of salts

Question

Assume a pH of 25°C

1. What is the pH of a 2.4E-2M barium cyanide?
2. Is a solution of 0.73M potassium dihydrogen arsenide acidic, basic, or neutral? Show your work.
3. Is a solution of 4.3E-3M solution pyridinium iodate (HC₅H₅NIO₃) acidic, basic, or neutral? Show your work.

Answer 1

(2). potassium dihydrogen arsenide = KH₂AsO₄

This is the salt of a strong base (KOH) and a weak acid H₃AsO₄. The pH will be basic

(3). pyridinium iodate = HC₅H₅NIO₃

This is the salt of a weak base (pyridine) and a strong acid (HI) The pH will be acidic

(1). 2.4x10^-2 M Ba(CN)₂

This is the salt of strong base, Ba(OH)₂ and a weak acid (HCN). The pH will be basic. To calculate the actual pH, we must look at the hydrolysis of CN^-

Ba(CN)₂ + H₂O ==> Ba(OH)₂ + 2HCN

CN^- ==> HCN + OH^-

Kb CN^- = 1.62x10^-5 (my source)

Kb = 1.62x10^-5 = [HCN][OH^-]/[CN^-]

1.62x10^-5 = (x)(x) / 8.6x10^-3 - x and assuming x is small relative to 8.6x10^-3 M, we can neglect it...

1.62x10^-5 = (x)(x) / 8.6x10^-3

x² = 1.39x10^-7

x = 3.7x10^-4 and this is about 4% of the value 8.6x10^-3 so meets the 5% criterion so our assumption is valid

[OH^-] = 3.7x10^-4 M

pOH = 3.43

pH = 14 - 3.43

pH = 10.6