# Homework Help: Finding a stoichiometric coefficient

1. Sep 24, 2012

### rustynail

Hello, I am currently taking a college level chemistry class. I am struggling with this problem, any help would be greatly appreciated.

1. The problem statement, all variables and given/known data
We have a certain quantity of

$$CuSO_{4} \cdot \gamma H_{2}O$$

If our sample is formed of (25,5% Cu), (12% S), (57,7% O) and (4.04% H), what is the value of gamma ?

2. Relevant equations
None given.

3. The attempt at a solution
I have determined the mass of 1mol of each element, the mass of 1mol of CuSO_4 (111.611g) and really, I'm stuck there.

Please don't spoil the fun for me, just give me some hints.

(edit) : I have done this much more :

Mass of 1mol Cu = 63.546g

(63.546g/111.611g)*100 = 56.94% Cu in CuSO_4

56.94%/25.5% = 2.2328 (increment factor)

So the mass of the hydrated CuSO_4 should be

111.611g * 2,2328= 249.21g

249.21g - 111.611g = 137.6g H2O

and 137.6g/18(g/mol)= 7.6 mol H2O

but it does not give me an integer...

Last edited: Sep 24, 2012
2. Sep 24, 2012

### AGNuke

Try some Approximations in calculations. For example, Molar Mass of Cu is good as 63.5.

3. Sep 24, 2012

### rustynail

thanks AGNuke, you mean that some approximations could get me closer to an integer coefficient? Do you think my attempt at a solution made sense?

Last edited: Sep 24, 2012
4. Sep 24, 2012

### AGNuke

No. You can apply the percentage "after" you had taken the molar mass of all the compound (including the hydration) part in consideration.
Take molar mass of compound as M[CuSO4]+γ[M(H2O)]

Also, you took the molar mass of CuSO4 incorrect.

And don't panic if you don't get answer in integer. Just try rounding them up or down.