Spectrophotometric Determination of pH 2

Question

Salts of hypoiodite ion behave as a weak base, undergoing hydrolysis in water according to the equation:

IO^- + H₂O  HIO + OH^-

K_b for hypoiodite ion = 0.00044.

Calculate the [H⁺] and pH of a 0.713 M solution of NaIO

Now calculate the [H⁺] and pH of a 0.014 M solution of NaIO

Answer 1

IO^- + H₂O  HIO + OH^-

Kb = [HIO][OH-] / [ IO^-]

0.00044 = (x)(x) / 0.713 - x and assume x is small relative to 0.713

x² = 0.0003137

x = 0.0177 (which is only ~2.5% of 0.713 so above assumption was valid)

[OH-] = 0.0177 M

pOH = -log 0.0177 = 1.75

pH = 14 - 1.75

pH = 12.2

[H⁺] = 1x10^-12.2 = 6.31x10^-13 M (excluding the H⁺ from autolysis of H₂O)