A student determines the chromium(III) content of a solution by first precipitating it as chromium(III) hydroxide, and then decomposing the hydroxide to chromium(III) oxide by heating. How many grams of chromium(III) oxide should the student obtain if her solution contains 56.0 mL of 0.600 M chromium(III) nitrate?

Explanation:

Chromium nitrate Cr( NO₃ )₂

Chromium hydroxide Cr( OH)₃

Chromium oxide ( Cr₂O₃ )

1 mole of chromium nitrate will form 1 mole of chromium hydroxide

1 mole of chromium hydroxide will form one mole of chromium oxide .

Moles of chromium nitrate in 56 mL of .6 M solution

= .056 x .6 = .336 moles

.336 moles of chromium nitrate will produce .336 moles of chromium hydroxide and .336 moles of chromium hydroxide will produce .336 moles of chromium oxide .

So chromium oxide produced = .336 moles

Molecular weight of chromium oxide = 152

.336 moles of chromium oxide is equal to

.336 x 152 grams of chromium oxide

= 51.07 grams .

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