How do you solve a gravimetric analysis? Ex. A student determines the chromium(II) content of a solution by first precipitating it as chromium(II) hydroxide, and then decomposing the hydroxide to chromium(II) oxide by heating. How many grams of chromium(II) oxide should the student obtain if his solution contains 48.0 mL of 0.561 M chromium(II)nitrate?

First, let's look at the chemical equations for these reactions.  We are starting with a solution of chromium (II) nitrate which disassociates into chromium and nitrate ions.

Cr(NO3)2 --> Cr2+ + 2NO3-

Next, the chromium content is determined by precipitating the chromium ions with hydroxide to make chromium (II) hydroxide.

Cr2+ + 2OH- --> Cr(OH)2

This is then immediately decomposed by heating to chromium (II) oxide.

Cr(OH)2 --> CrO + H2O

Since all of the chromium species in these equations is on a one to one molar ratio in the equations, we know that each mole of chromium from the original chromium (II) nitrate solution will produce a single mole of chromium (II) oxide.  So let's find the moles of chromium.

0.561 moles/L * 0.048 L = 0.027 moles Cr2+

This means that 0.027 moles of chromium ions will produce 0.027 moles of CrO after the sequence is finished.  Now convert moles of CrO to grams of CrO (the molecular weight of CrO is 68 g/mol).

0.027 moles CrO * (68 g/mole) = 1.84 g CrO

So 1.84 grams of CrO will be produced.

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