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### Topic: Determine the Empirical Formula of Cobalt Oxalate Hydrate from Oxide Combustion  (Read 6514 times)

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#### Habitat

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• Mole Snacks: +0/-0 ##### Determine the Empirical Formula of Cobalt Oxalate Hydrate from Oxide Combustion
« on: June 27, 2015, 01:09:33 PM »
Hey guys,

I am currently working on a lab problem and have been tasked with finding the empirical formula of Cobalt Oxalate Hydrate (Co(C2O4) * H2O from it's combustion into Cobalt Oxide (C3O4)

Heres what I've done so far (Copied from another post I made elsewhere):

I am doing a combustion equation for lab and the workup is asking me to find the empirical formula for Co(a)(C2O4)(b) * CH2O
where a, b, and c and are variables.

I started out with .3283g of Cobalt Oxalate Hydrate (Co(C2O4)) and ended with .1158g of Cobalt Oxide (C3O4).

The amount of cobalt in the product is the same as in the reactant, so I determined that
.1158g x (179.79g/mol Co / 240.79g/mol Co3O4) = .085g Co in the initial sample.

.085g Co / .3283g Cobalt Oxalate Hydrate x 100 = 25.89 (Mass percent of Cobalt in the oxalate hydrate)

But I am stuck here, How do I determine the mass percent of oxalate in the original sample so that I can use those two mass percentages to find the mass percent of H2O?

I am hoping yall can help me a little better than stack exchange chemistry. The guy told me the ratio to H2O was 4.5 which is clearly wrong as Cobalt Oxalate is a dihydrate which means the ratio is 2. I just don't know how to prove/show this in my calculations.

Thank you for your assistance.

#### Borek ##### Re: Determine the Empirical Formula of Cobalt Oxalate Hydrate from Oxide Combustion
« Reply #1 on: June 27, 2015, 01:43:38 PM »
I am assuming you mean Co3O4.

The only way I can think of is by trial and error. Neither of the three unknowns (a, b, c) is higher than 10 (most likely they are lower), they are all integers. See if any combination yields mass of the original sample that is close to 0.3283 g.

4.5 yields quite a good approximation. (While 4.5 is not an integer, case is covered by a=2, b=2, c=9).
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#### Habitat

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• Mole Snacks: +0/-0 ##### Re: Determine the Empirical Formula of Cobalt Oxalate Hydrate from Oxide Combustion
« Reply #2 on: June 27, 2015, 01:55:45 PM »
I am not sure what you mean by your first statement.

My lab sheet is specifically asking me for:

Cobalt Oxalate Analysis: Co(a)(C2O4)(b) * (C)H2O

It asks for the mass % of oxalate in the Sample

mass % of metal in the Sample

and mass % of water in the Sample

as well as subscripts a,b and coefficient C

Lastly the Formula of the Hydrate

It was told to me in class that the only reason for the measurement of the oxide is to determine the mass of cobalt that was present in the initial sample.

The only other information I have is the combustion formula I balanced:

3Co(C2O4)*H2O + 2O2 -> Co3O4 + 6CO2 + 3H2O

I apologize if I am not understanding, but how do I go about approximating like you are telling me to do?

#### Borek ##### Re: Determine the Empirical Formula of Cobalt Oxalate Hydrate from Oxide Combustion
« Reply #3 on: June 27, 2015, 06:27:55 PM »
I am not sure what you mean by your first statement.

Reread your original post - you named your oxide C3O4, I guess it was a typo.

Convert mass of cobalt to number of moles. Assuming any set of values for a, b, c (say a=2, b=3, c=4) you can easily calculate mass of the sample containing given number moles of cobalt and the formula. If the mass of the sample calculated this way equals (or is close to) the original mass, you can assume it is the correct formula. Or at least - you have no data to find a better one in any more systematic way.
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