Chemistry: Hydrates

Alright, I have a simple problem here. I took a quiz. I used a method different from what the method taught in class was. I got 1 point out of 4 after arguing. (I had none). I belive my process is perfectly valid and will work every time.

The problem:

A hyrdrate of copper (II) sulfate contains 1.77 grams of water and 5.24 grams of anhydrous copper (II) sulfate. Calculate the empirical formula of the Hydrate.

My work:

Cu: 63.5g/mole
S: 32.1g/mole
O: 16.0g/mole*4 moles of oxygen per mole of CuSO4 so that's 64g/mole

Add it all up to: 159.6 g/mole of CuSO4

Now, I figure since the ratio of grams carries over to the ratio of moles, I can use simple ratios to figure out the problem.

1.77/5.25=.338
18*3/159.6=.338

Since the ratio of 3 moles of water per mole of CuSO4 equals the ratio of 1.77g of water to 5.25g CuSO4, the formula is CuSO4-3H2O

Now, for those of you who know, is there anything wrong with this?

Doesn't seem to be a problem to me.

The "correct" way to do this problem would be to find out how many moles of CuSO4 there are, which is 5.24/159.6 = .0328 moles.

Then the "MW" of the hydrate can be found by doing grams divided by moles, or 1.77/.0328, which becomes 53.91.

This is some number of water moles, so you divide that by 18 and you get 3.

What you did works perfectly fine. Your teacher was probably just being an a-hole because you didn't learn the "standard" method. :p At least you fought for one point back.

But you should probably use the "standard" method from now on regardless, as there may be times where you won't have enough to do ratios anyways.

in general if your method achieves the correct answer 100% of the time in which it is able to be applied, then it should be accepted. Being able to devise your own methods of solving problems is in and of itself a sign of mastery of a concept.

furthermore it seems that your method is algebraically equivillent to the "standard method"

D(A + B +C) = E = DA +DB +DC
if that makes any sense

Quite right quite right. Well, my teacher gave me the points back (took her quite a while) so I'm satisfied. Most of my teachers aren't very bright, so oh well.

let us be honest, Method is irrelevant it is the answer that matters.

Originally Posted by AzaISaZELDALORD

let us be honest, Method is irrelevant it is the answer that matters.

Don't show work don't get credit. Teacher thinks it's cheating. Yeah, teachers are morons.

Not at all.

If you don't show the work then you don't prove that you know anything at all about the process. You could be lucky (I can't count the number of times I've made two mistakes that canceled each other out.) or you could have cheated.


You're 16, so you're probably in grade 10, right? Give it a couple of years, and you'll probably have an epiphany about how important showing your work is.

Originally Posted by GDwarf

Not at all.

If you don't show the work then you don't prove that you know anything at all about the process. You could be lucky (I can't count the number of times I've made two mistakes that canceled each other out.) or you could have cheated.

I am in lower VI at school, I don't noramally bother with working, It has not harmed my academic career.


You're 16, so you're probably in grade 10, right? Give it a couple of years, and you'll probably have an epiphany about how important showing your work is.

Actually, in any science you need to show your work or not bother working the problem at all.

The reason is that other scientists need to be able to reproduce the same results given the same data or experiment.

If you don't show your work, this is not possible.

Also, on a test it will help you get partial credit, so you need to show your work all of the time.

I am in lower VI at school, I don't noramally bother with working, It has not harmed my academic career.

Doesn't make any difference that your teachers let you off easy, the fact of the matter is that in a science if you don't show your work your results are useless everyone, including yourself once you inevitably forget what you were doing at the time.

well, I just write from calculator, did it in my exams, got top results...what does that tell me...

You still should write down your procedure.

If you make a mistake, and have no written step-by-step process, then all your teacher knows is that you got the wrong answer. They don't know why, or how, or if you were almost write but wrote a 3 instead of an 8 (or whatever).

Whereas if you do show your work, they can give you part marks.


In addition, in university and in any career at all that involves any kind of math, you must show your work so that people can check it for errors and see where you're coming from.

Originally Posted by GDwarf

Not at all.

If you don't show the work then you don't prove that you know anything at all about the process. You could be lucky (I can't count the number of times I've made two mistakes that canceled each other out.) or you could have cheated.


You're 16, so you're probably in grade 10, right? Give it a couple of years, and you'll probably have an epiphany about how important showing your work is.

Unless I had written out a paragraph on exactly what I did, my work couldn't be much clearer than what it was.

If you arrived at the correct answer, you should have received more than 1/4 points.

If you are failing the class, you need to bring it up with the head of the department/principal/ whoever is that teacher's boss.

If you are not in danger, you are better off just accepting the method taught in class and using that and letting it go.

After you learn the method taught by your instructor, prove to him that you arrive at the same answer every time using your own method and step by step evidence that you are correct.

Or at least prove it to yourself anyway, most teachers I have known don't like it when students are smarter than they are and in high school it is usually the case, since teaching colleges are some of the most relaxed and easy there are, at least in my area. (College is a totally different matter, so don't get used to it.)

I don't see anything wrong with it either. Your teacher is just stubborn. I agree with CagedCrado, you should bring this up to the principal if you're not doing very well.