A solution contains 0.10 M each of Fe^3+ and Mg^2+. Dilute NaOH is added slowly (no volume change). K_sp(Fe(OH)3) = 1.0 x 10^-38, K_sp(Mg(OH)2) = 1.0 x 10^-11, K_w = 1.0 x 10^-14. Select the correct statement(s).

Fe(OH)3 starts to precipitate at pH 1.67

Fe^3+ is reduced to 10^-5 M at pH 3.00

Holding the pH at 4.0 removes Fe^3+ almost completely while Mg^2+ stays in solution

Fe(OH)3 starts to precipitate at pH 1.67

Mg(OH)2 starts to precipitate at pH 9.50

Fe^3+ is reduced to 10^-5 M at pH 3.00

Holding the pH at 4.0 removes Fe^3+ almost completely while Mg^2+ stays in solution

JEE Main levelOne or more correct optionsDifficulty 2 of 3ia066

A solution contains $0.10 M$ each of $Fe X^{3 +}$ and $Mg X^{2 +}$ . Dilute $NaOH$ is added slowly (no volume change). $K_{s p} (Fe (OH) X_{3}) = 1.0 \times 1 0^{- 38}$ , $K_{s p} (Mg (OH) X_{2}) = 1.0 \times 1 0^{- 11}$ , $K_{w} = 1.0 \times 1 0^{- 14}$ . Select the correct statement(s).

A $Fe (OH) X_{3}$ starts to precipitate at pH 1.67
B $Mg (OH) X_{2}$ starts to precipitate at pH 9.50
C $Fe X^{3 +}$ is reduced to $1 0^{- 5} M$ at pH 3.00
DHolding the pH at 4.0 removes $Fe X^{3 +}$ almost completely while $Mg X^{2 +}$ stays in solution

For

Fe (OH) X_{3}

[OH X^{-}] = (1 0^{- 38} /0.10)^{1/3} = 4.6 \times 1 0^{- 13} M

, pH = 1.67. For

1 0^{- 5} M

Fe X^{3 +}

[OH X^{-}] = 1 0^{- 11} M

, pH = 3.00. For

Mg (OH) X_{2}

[OH X^{-}] = 1 0^{- 11} /0.10 = 1 0^{- 5} M

, pH = 9.00 (not 9.50). At pH 4.0 the

Fe X^{3 +}

is already below

1 0^{- 5} M

and the

Mg (OH) X_{2}

threshold (pH 9) is not reached.