Question #e8dce
1 Answer
Explanation:
You know that iron(II) oxide-hydroxide is insoluble in water, which implies that a dissociation equilibrium is established in aqueous solution between the undissolved solid and the solvated ions.
#"Fe"("OH")_ (3(s)) rightleftharpoons "Fe"_ ((aq))^(3+) + 3"OH"_( (aq))^(-)#
By definition, the solubility product constant,
#K_(sp) = ["Fe"^(3+)] * ["OH"^(-)]^3#
Now, in order for precipitation to occur, you need to have
#K_(sp) <= ["Fe"^(3+)] * ["OH"^(-)]^3" "" "color(darkorange)(("*"))#
As you know, an aqueous solution at room temperature has
#color(blue)(ul(color(black)("pOH" = 14 - "pH")))#
Since you know that
#color(blue)(ul(color(black)("pOH" = - log(["OH"^(-)]))))#
you can say that
#["OH"^(-)] = 10^(-"pOH")#
which is equivalent to
#["OH"^(-)] = 10^(-(14 - "pH"))#
Plug in the value you have for the
#["OH"^(-)] = 10^(-(14 - 3.48))#
#["OH"^(-)] = 10^(-10.52)#
Next, rearrange equation
#["Fe"^(3+)] >= K_(sp)/(["OH"^(-)]^3)#
Plug in your values to find
#["Fe"^(3+)] >= (2.79 * 10^(-39))/(10^(-10.52))^3#
#["Fe"^(3+)] >= (2.79 * 10^(-39))/( (10^(-10))^3 * (10^(-0.52))^3)#
#["Fe"^(3+)] >= 2.79 * 10^(1.56) * 10^(-39) * 10^(30) #
Therefore, you can say that you need
#color(darkgreen)(ul(color(black)(["Fe"^(3+)] >= 1.0 * 10^(-7)color(white)(.)"M")))#
The answer is rounded to two sig figs, the number of decimal places you have for the