Question Video: Identifying a Metal Cation That Precipitates When a Few Drops of Aqueous Ammonia Are Added to Its Salt or Solution Chemistry

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⁺ [E] Cu²⁺

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Video Transcript

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.

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