A few drops of sodium hydroxide are
added to a solution containing the cation X. X is known to be one of the
following cations: Al 3+, Zn 2+, NH4 1+, Ca 2+, and Cr 3+. With which of these potential
cations will both of the following observations be seen? Observation one: a precipitate is
formed. Observation two: the precipitate
does not dissolve in excess sodium hydroxide. (A) Al 3+, (B) Zn 2+, (C) NH4 1+,
(D) Ca 2+, or (E) Cr 3+.
In this question, we are asked to
find a cation that will react with sodium hydroxide to produce a precipitate that
will not dissolve in excess sodium hydroxide. Let’s first discuss the reaction of
a metal cation with hydroxide ions, which is related to observation one.
Many metal cations react with
hydroxide anions from the few drops of sodium hydroxide to form a metal hydroxide
precipitate. There are several cations that do
not form hydroxide precipitates, including alkali metal and ammonium cations. Since answer choice (C), the
ammonium ion, is one of these exceptions, we can eliminate it.
Let’s now have a closer look at
observation two. Some metal hydroxide precipitates
will dissolve when in excess sodium hydroxide. While some metal hydroxides will
dissolve in excess sodium hydroxide, others will not. Metal hydroxide precipitates
containing copper(II), iron(II) and (III), and calcium ions will not redissolve. Metal hydroxides containing
aluminum, zinc, and chromium(III) ions will dissolve when in excess sodium
hydroxide. Aluminum, zinc, and chromium(III)
ions are featured in our answer choices. Since cation X does not dissolve in
excess sodium hydroxide, we can eliminate answer choices (A), (B), and (E).
The calcium ion forms a hydroxide
precipitate when reacted with a few drops of sodium hydroxide, and this precipitate
does not dissolve in excess sodium hydroxide. The calcium ion would generate both
observations one and two and is therefore cation X. The correct answer is answer choice
(D), Ca 2+.