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?

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?

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

Explanation / Answer

Cr(NO3)3 + 3NaOH --> Cr(OH)3 +3NO3- + 3Na+

the solid is Cr(OH)3 (chromium hydroxide)

then, after extra OH-in solution

Cr(OH)3 + OH- = Cr(OH)4-(aq)

which is aquous

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

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