Stoichiometry

Black _roses · Post by **Black _roses** » Sun Mar 16, 2008 11:50 am

I have a bunch of questions that I dont understand.
The first one is
Calculate the mass of alumina (Al₂O₃) produced when 100g of Aluminum burns.
Can anyone help me?

sharan · Post by **sharan** » Sun Mar 16, 2008 1:29 pm

start with balanced equation of combustion of aluminuim

4 Al + 3 O₂ = 2 Al₂O₃

It means for every 4 Aluminium Moles, there are 2 Al₂O₃ moles.
Now, you have 100 gm of Al and you noe
Moles= Mass/Molar mass
Moles= 100gm/ 27gm/mol
=3.7mol

4 Moles of Al gives 2 Moles of Al₂O₃
3.7 Moles gives (2/4)*3.7 moles of Al₂O₃
= 1.85 moles of Al₂O₃

Moles= Mass/ Molecular mass
1.85 = Mass/ 102

Mass of Al₂O₃= 188.7 grams

-Sharan

Black _roses · Post by **Black _roses** » Sun Mar 16, 2008 3:43 pm

I understand how you got 3.7 moles of Al in the begining, but everything after that doesnt make sense. Wouldnt you have to factor in the 3 moles of Oxygen gas? And how would you set that up in a demensional analysis table? (Sorry if I spelt that wrong.)

sharan · Post by **sharan** » Mon Mar 17, 2008 2:40 pm

hey the reason why we ignoring the oxygen is because we always consider the limiting reagant (whichever reactant is in less amount) to determine how much product we are going to get. In this case obviosuly aluminium is limiting reagant since oxygen is pressence in abundance right. So we find moles of aluminuim and use the mole ratio to find the moles of alumina from the original reaction. Im not quite sure wut u meant by dimensional analysis setup.. if you would like to explain then i might be able to help you.. maybe..

-Sharan

Black _roses · Post by **Black _roses** » Mon Mar 17, 2008 6:36 pm

I dont know how to tell you what demensional analysis is. But I can try. Basically its taking one meassurement (like inches or grams or the number of something) and changing it into another measure (like meters form inches, froms grams to pounds and so on). I added a picture to try and help although I dont know if it will work.
[img]http://alainefrankland.com/CHEMNOTES/di ... nswers.jpg[/img]

sharan · Post by **sharan** » Mon Mar 17, 2008 9:07 pm

hey yeah i got the pic but i dunt understand why do u need to set up a dimensional analysis table.. we dunt hv anything we need to convert to umm maybe in the end we want to convert grams to kg :S then just
188.7 grams* 1kg/1000gm= 0.1887 Kg

or if you taking about the moles things...

we just went from
4 Moles of Al gives 2 Moles of Al₂O₃
3.7 Moles gives (2/4)*3.7 moles of Al₂O₃
= 1.85 moles of Al₂O₃

(2moles of Al₂O₃ * 3.7 moles of Al)/ (3.7 moles of Al)
=1.85 moles of Al₂O₃

Because moles of Al is in both numerator an denominator so it cancels out leving moles of Al₂O₃ which is wut we need.

but dunt noe if that is wut u wanted..

im sorry for the confusion but im learning a new thing lol interesting ..

-Sharan

sharan · Post by **sharan** » Mon Mar 17, 2008 9:09 pm

sorry for the typo i meant
(2moles of Al₂O₃ * 3.7 moles of Al)/ (4 moles of Al)
=1.85 moles of Al₂O₃

-sharan

Black _roses · Post by **Black _roses** » Sat Mar 29, 2008 9:52 am

We have to do a dimensional anaylisis table because my teacher wants us too. And it does help thankyou very much.