Ionic EquilibriumHard

Question

Equilibrium constant for the acid ionization of Fe3+ to Fe(OH)2+ and H+ is 9.0 × 10–3. What is the maximum pH, which could be used so that at least 90% of the total Fe3+ in a dilute solution exists as Fe3+? (log 3 = 0.48)

Options

A.3.0
B.1.08
C.1.92
D.2.04

Solution

$Fe^{3 +} + H_{2}O \rightleftharpoons Fe(OH)^{2 +} + H^{+} $$$0.9x\text{ M0.1}x\text{ M}?$$

$9 \times 10^{- 3} = \frac{0.1x \times \left\lbrack H^{+} \right\rbrack}{0.9x} \Rightarrow \left\lbrack H^{+} \right\rbrack = 0.081 \Rightarrow P^{H} = 1.08$

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