[chemistrykid]

OK, I have a bit of a problem that I am completely stumped on.

In our experiments, we made a nickel-1,2-diaminoethane complex, which we then had to identify by titration with a 0.0005M standard EDTA solution. I need to work out the % of Ni in the complex, and then work out the structure of the complex.

However, I have no idea how to find the %Ni. Any help would be much appreciated!

If you need more info just ask.

thank you.  

[AWK]

Mass of Ni(en)₂ complex is needed and volume of EDTA used.
Hint
EDTA reacts with Ni(2+) in 1:1 molar ratio

[chemistrykid]

Hmm, that's what I thought!

Mass: 2.25g
Average vol (I did 6 titrations): 17.30cm³

[AWK]

From volume (in dm3) and molarity of EDTA you can calculate moles of EDTA which are equal to moles of Ni (because of 1:1 stoichiometry of complex formation), then mass of Ni, and finally percentage of Ni (0.02 %)

[chemistrykid]

Wait, how did you know the %Ni?

I'm gonna gues, but if you would correct me..

You know the number of moles, therefore the mass of Ni = moles x Ar of Ni

Then %Ni = (mass of Ni/mass of total complex made) x 100

We then have to work out the relative molecular mass using the %Ni. Thing is I've reached a value for the Mr, but via a different method! I knew the no. of moles in 20cm³, so i know how many moles are in the 200cm³ standard solution me made, using 0.31g of the complex. Therefore I worked out Mr of the complex by  Mr = mass used/moles

How do I work out the Mr from the %Ni?

[AWK]

Mass of 1 mole nickel atoms is equal to relative molecular mass of Ni in grams.
For % Ni calculation you do not need the molecular mass of Ni complex, but assuming high purity of your sample and mononuclear complex (1 nickel atom in 1 molecule of complex) you can also calculate  the molecular mass of the complex.

But, you must use mass of complex in your sample used for titration, not for preparing solution, or calculate moles of EDTA nedeed for the whole sample, not for its part.

[chemistrykid]

OK, I've tried working out the %Ni using the method you suggested, and it came out to 16%.

I then worked out the Mr (using this 16%) to 366.8, which is very close to the 365.5, which I came to using the other method. What do you think?

We then have to deduce the structure from this Mr. I think its [Ni(en)_3]Cl2.3H₂O. The Mr of this complex is 363.5, which is as close as I can get it. What do you think?

[AWK]

I think, your result is reliable.