You have an aqueous solution of chromium(III) nitrate that you titrate with an a

Question

You have an aqueous solution of chromium(III) nitrate that you titrate with an aqueous solution of sodium hydroxide. After a certain amount of titrant has been added, you observe a precipitate forming. You add more sodium hydroxide solution and the precipitate dissolves, leaving a solution again. What has happened?

1 The precipitate was chromium hydroxide, which then reacted with more hydroxide to produce a soluble complex ion, Cr(OH)4 . 2 The precipitate was chromium hydroxide, which dissolved once more solution was added, forming Cr3+(aq) . 3 The precipitate was sodium nitrate, which reacted with more nitrate to produce the soluble complex ion Na(NO3)2 . 4 The precipitate was sodium hydroxide, which re-dissolved in the larger volume.

Explanation / Answer

The precipitate was chromium hydroxide, which then reacted with more hydroxide to produce a soluble complex ion, Cr(OH)4

Cr(NO3)3(aq) + 3NaOH(aq) ---------------> Cr(OH)3 (s) + 3NaNO3(aq)

Cr(OH)3(s) + NaOH (aq) ---------------> Na[Cr(OH)4] (aq)

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