Which of the following metal
cations does not produce a precipitate when a few drops of aqueous ammonia are added
to a salt or solution of that metal cation? (A) Zn2+, (B) Al3+, (C) Ca2+, (D)
Cr3+, or (E) Cu2+.
The question describes these five
options as metal cations. We’re dealing with cations of zinc,
aluminum, calcium, chromium, and copper. And we’re being asked which of
these metal cations as a salt or in solution would not respond to aqueous ammonia
and produce a precipitate. The formula for ammonia is NH3. Ammonia can react with water to
produce ammonium hydroxide, a good source of the hydroxide ion. The question doesn’t tell us
directly if the ammonium hydroxide is dilute, to an excess, or concentrated. But since it’s only a few drops, we
can safely assume it’s dilute.
The question is, what is going to
happen when we treat each of these ions with dilute aqueous ammonia? Zn2+ ions will react to produce a
white precipitate of zinc hydroxide, and we get a similar reaction with Al3+ ions,
producing aluminum hydroxide. However, calcium two plus ions do
not react with aqueous ammonium hydroxide no matter the concentration. There isn’t an easy way to
understand why; you’ll just need to remember it. Meanwhile, chromium three ions do
react, forming a gray-green precipitate of chromium three hydroxide, while the
precipitate we get from copper two plus is pale blue copper two hydroxide.
Now, there is one possible point of
confusion with this question if we’re not quite sure if we’re dealing with dilute or
excess ammonium hydroxide. In excess ammonium hydroxide, the
precipitates formed from Zn2+ and Cu2+ ions dissolve. But since we are only dealing with
a few drops, we can be fairly confident that we aren’t dealing with dilute
conditions. Therefore, out of the five options,
the only metal cation that does not produce precipitate when treated with a few
drops of aqueous ammonia is calcium two plus.