[MichelleL]

Here is the Questions: Determine the empirical formula of each of the following compounds. The percentage compositon by mass is givin.
52.9% aluminum 47.1% oxygen
Work done...
m = 52.9g Al m=47.1g O
nAl = 52.9g divided by 26.98g/mol = 1.96 mol
nO = 47.1g divided by 16g/mol =2.94 mol
Al ratio = 2.94 divided by 1.96 = 1.5
O ratio = 1.96 divided by 1.96 = 1
But because it is not a whole number...
Al 1.5 X 2 =3
o  1 x 2 = 2   ... (what you do to one you do to the other 2X?)
Therefore Al₃O₂
But the answer in the back of the book is Al₂O₃

I'm confused on where i am going wrong.

I do have a few other questions in past sections but will make other post for. I am taking school from the independed learning center and it takes 5 bus days to get someone to call back. I have time today so I found this site. Looks really good and I am looking forward to some help.

[ARGOS++]

Dear MichelleL;

Please tell why you divide each by 1.96?

As both values are close to the next Integer, so you can only round to it.

Good Luck!
                    ARGOS⁺⁺

[MichelleL]

Thank you for the quick response
well, I only have one example in my book to follow and in step three it says divide both moles by the smallest number of moles...

um... if i was to round up... then the answer would still be Al₃o₂ ... I don't understand why the answer in the back of my book says Al2O3 . I feel like I'm just not understanding one part of this. 

[MichelleL]

oh yes and my book says for me to choose a total mass and it is easiest to consider 100g of a subsance since the percentages are out of 100%.

[ARGOS++]

Dear MichelleL;

The atomic ratio you have already given with the percentage.
So you tell that your formula must be build with 1.96 moles of Al and 2.94 moles of O,
   -  or in short:  Al_1.96O_2.94.
The last Step is left to you.

Good Luck!
                    ARGOS⁺⁺

[MichelleL]

oh wow. lol. ok I'm laughing now. thank you. I understand now.

[ARGOS++]

Dear MichelleL;

Do you see now where you did the mistake?
By your division by 1.96 you intermixed both Al & O ratios!!

Good Luck!
                    ARGOS⁺⁺

[MichelleL]

yes  . I was staring right at the answer and didn't even know it. lol.