Homework Help: Chemistry+Empirical formula

Posted by Chopsticks on Thursday, January 15, 2009 at 11:48pm.

This is kind of tricky for me.

What os the empirical formula (lowest whole number ratio) of the compounds below?

A.

25.3% Cu, 12.9% S, 25.7% O, 36.1% water

How would i set this up? The water at the end is confusing me. I think I need to find the percentage of water inside the whole thing first, but I would be wrong. Can you help?

Chemistry+Empirical formula - DrBob222, Thursday, January 15, 2009 at 11:58pm
Do it just as you did the MgBr2.
25.3/atomic mass Cu =
12.9/atomc mass S =
25.7/atomc mass O =
36.1/molar mass H2O =
You already have the percent water in the entire molecule. The formula will come out
as Cu_xS_yO_z*WH₂O
I don't know that this is the formula but it would look somthing like this.
CuSO4*5H2O
Chemistry+Empirical formula - Chopsticks, Friday, January 16, 2009 at 12:03am
I'm still confused. Okay I solved everything but H20

25.3g Cu 1 mole/63.54 = .40
12.9g S 1 mole/32.064 = .40
25.7g O 1 mole/15.999 = 1.61

But what do i do with the H20? Can you work this part for me? I want to see how you did it
Chemistry+Empirical formula - DrBob222, Friday, January 16, 2009 at 12:23am
Cu = 25.3/63.54 = 0.398 (You have 3 places in 25.3 so don't throw anything away, at least not yet).
S = 12.9/32.064 = 0.402
O = 25.7/15.999 = 1.606
H2O = 36.1/18.015 = 2.00
Now we divide everything by the smallest, which is 0.398

Cu = 0.398/0.398 = 1.000
S = 0.402/0.398 = 1.01
O = 1.606/0.398 = 4.035
H2O = 2.00/0.398 = 5.025

So we round off to whole numbers now.
Cu 1.000 becomes 1.00
S 1.01 becomes 1.00
O 4.035 becomes 4.00
H2O 5.025 becomes 5.00
The formula is
Cu1S1O4*5H2O or since the 1s are not needed, the formula is CuSO4*5H2O
Chemistry+Empirical formula - Chopsticks, Friday, January 16, 2009 at 12:28am
My teacher always told us to round to the hundreths place. How did you get 18.015 ??
Chemistry+Empirical formula - Chopsticks, Friday, January 16, 2009 at 1:08am
aah wait i see how now.
Chemistry+Empirical formula - DrBob222, Friday, January 16, 2009 at 11:45am
I use a web site to do all my molar mass stuff. Here is a link if you want to use it. http://environmentalchemistry.com/yogi/reference/molar.html

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