[Rutherford]

The synthesis of chromium(3+) ammine complexes usually starts from a freshly prepared in situ solution of a chromium(2+) salt. To the solution of a chromium(2+) salt, the solution of ammonia and a solid ammonium chloride are added. Then a stream of air is passed through the solution. The red precipitate is formed that contains 28.75 wt.% of N. Determine the composition of the precipitate and give the reaction equation.

I wrote the possible formula: Cr(NH₂)_xCl_y(OH)_z. There is the proportion: 14x:M=28.75:100, M=48.696x g/mol. x_max=6. If x=1, the molar mass is to small, for x=2 its the same. For 3 or 4 no compound fits. For 5 and 6 the mass gets too big. I am stuck here now.

[Borek]

Ignoring everything else for a moment - NH₂? 14?

[Rutherford]

It is the weight percent of nitrogen only, so it should be 14x.

[Rutherford]

I even assumed that it is an anionic complex, with NH₄⁺, but still couldn't get a reasonable solution  .

[AWK]

Chromium complex contains NH₃ not NH₂

[Borek]

For 5 and 6 the mass gets too big.

Too big for what?

[Rutherford]

Thanks for the replies.
It is Cr(NH₃)_xCl_y(OH)_z
Again, from the proportion 14x:M=28.75:100, M=48.696x g/mol.
Now, NH₃ makes a difference. If x=5, M=243.5g/mol, this would mean 1 or 3 chlorine atoms. As Cr has the charge 3+, I need 3 negative ions, so y+z=3. The mass without chromine and amino groups is 106.5 which corresponds to 3 chlorine atoms, so the formula is [Cr(NH₃)₅Cl]Cl₂. Is this good?

[AWK]

http://www.webqc.org/molecular-weight-of-Cr(NH3)5Cl3.html

[Rutherford]

Good. Now I have another question: How can one prepare a solution containing Cr²⁺ ions using metallic chrome? Specify the conditions.

When it gets dissolved in acid it will be Cr³⁺, but what to use to reduce it to Cr²⁺? Or is there a better way?

[AWK]

read
http://en.wikipedia.org/wiki/Chromium(II)_chloride

or buy (relatively inexpensive)

[Rutherford]

Got this, thanks.

[Rutherford]

Arrange the hexammine complexes of iron(2+), chromium(3+) and ruthenium(2+) in a row of increasing stability towards the acidic water solutions. Explain your choice.

How can this be determined? What property do I need to know for this?

[AWK]

Hint - stability constants

[Rutherford]

No stability constant is given, so I have to deduce the answer logically  .

Maybe, the bigger the cation, the repulsion of donated electrons from NH₃ will be smaller as the distance between them increases, so it would be: iron(2+)<chromium(3+)<ruthenium(2+).
Is my reasoning correct?

[XGen]

3. What oxidizer can be used instead of oxygen to obtain the same product?

4. What product will be formed if the experiment described above is performed under inert atmosphere without oxygen?

These are another couple questions that I've been having trouble with.