Question

(III) precipitates from acidic solution by addition of ${\mathbf{OH}}^{\mathbf{-}}$to form $\mathbf{Fe}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{3}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}$. At what concentration of localid="1663570454087" ${\mathbf{OH}}^{\mathbf{-}}$will $\left[\mathrm{Fe}\left(\mathrm{III}\right)\right]$be reduced to $\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{}\mathbf{x}\mathbf{}{\mathbf{10}}^{\mathbf{-}\mathbf{10}}\mathbf{M}$? If $\mathbf{Fe}\mathbf{\left(}\mathbf{II}\mathbf{\right)}\mathbf{}$is used instead, what concentration of ${\mathbf{OH}}^{\mathbf{-}}$will reduce $\mathbf{Fe}\mathbf{\left(}\mathbf{II}\mathbf{\right)}\mathbf{}$ to $\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{}\mathbf{x}\mathbf{}{\mathbf{10}}^{\mathbf{-}\mathbf{10}}\mathbf{M}$?

Short answer

When using [Fe(III)], concentration of ${\mathrm{OH}}^{-}$reduce to $1.0\mathrm{x}{10}^{-10}\mathrm{M}$was found to be $2.5\mathrm{x}{10}^{-10}\mathrm{M}$and using [Fe(II)] concentration of ${\mathrm{OH}}^{-}$reduced to $1.0\mathrm{x}{10}^{-10}\mathrm{M}$was found to be $2.8\mathrm{x}{10}^{-3}\mathrm{M}$.

Step-by-step solution

concentration

Concentration of ${\mathbf{OH}}^{\mathbf{-}}$reduced to $\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{}\mathbf{x}\mathbf{}{\mathbf{10}}^{\mathbf{-}\mathbf{10}}$when using $\mathbf{\left[}\mathbf{Fe}\mathbf{\left(}\mathbf{III}\mathbf{\right)}\mathbf{\right]}$and using $\mathbf{\left[}\mathbf{Fe}\mathbf{\left(}\mathbf{II}\mathbf{\right)}\mathbf{\right]}$must be calculated.

reduction of the concentration of OH- When use [Fe(III)],

When use $\left[\mathrm{Fe}\left(\mathrm{III}\right)\right]$, the concentration of ${\mathrm{OH}}^{-}$reduced to $1.0\mathrm{x}{10}^{-10}\mathrm{M}$is,

role="math" localid="1663571809159" $$\begin{gathered} \left[{\mathrm{Fe}}^{3+}\right]{\left[{\mathrm{OH}}^{-}\right]}^3=\left({10}^{-10}\right){\left[{\mathrm{OH}}^{-}\right]}^3 \\ =1.6\mathrm{x}{10}^{-39} \\ \left[{\mathrm{OH}}^{-}\right]=2.8\mathrm{x}{10}^{-3}\mathrm{M} \end{gathered}$$

reduction of the concentration of OH- When use  [Fe(II)] ,

When use$\left[\mathrm{Fe}\left(\mathrm{II}\right)\right]$, the concentration of localid="1663572130523" ${\mathrm{OH}}^{-}$reduced to $1.0\mathrm{x}{10}^{-10}\mathrm{M}$is,

$$\begin{gathered} \left[{\mathrm{Fe}}^{2+}\right]{\left[{\mathrm{OH}}^{-}\right]}^2=\left({10}^{-10}\right){\left[{\mathrm{OH}}^{-}\right]}^2 \\ =7.9\mathrm{x}{10}^{-16} \\ \left[{\mathrm{OH}}^{-}\right]=2.8\mathrm{x}{10}^{-3}\mathrm{M} \end{gathered}$$

When using [Fe(III)], concentration of ${\mathrm{OH}}^{-}$reduce to $1.0\mathrm{x}{10}^{-10}\mathrm{M}$was found to be $2.5\mathrm{x}{10}^{-10}\mathrm{M}$and using [Fe(II)] concentration of ${\mathrm{OH}}^{-}$reduced to $1.0\mathrm{x}{10}^{-10}\mathrm{M}$was found to be $2.8\mathrm{x}{10}^{-3}\mathrm{M}$.