[insomaniacal]

Basically, I have a problem, and want to know the best way to do it. Thanks a ton to anyone who can help me out.

Basically, I have 5g of Aluminum, and want to create Aluminum Oxide, using all of the Aluminum. I calculated the amount of oxygen I would need by using ratios. .185 moles of Aluminum * (3mol O/4 mol Al), and got .13875 as an answer. Now, converting this to Liters gives me 3.108. This is the same answer as on the answer sheet (3.1L), so I'm assuming I did everything correctly. If not, please feel free to tell me what I did wrong.

Now, the next question is asking how many grams of aluminum oxide will be created, and where the confusion starts. The balanced chemical equation is 4Al + 302 --> 2Al2O3 . So, the ratio looks like it would be 4:6:4:6, however, doing this, I get a really unrealistic answer (over 102 grams, which would be equal to one mole.). Just for laughs, I did it with the unbalanced equation (1:2:2:3), and got the correct numbers for the answer (94.35). However,the real answer is 9.435. I have no idea how to get there using only this ratio method.

Basically, I had taken the .185 moles, multiplied it it's ratio (4), then did the same for oxygen (3), added them together, and multipled by the molar mass of Al2O3, which is 102. I did the same method when using 1:2:2:3 .

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Now, I did finally find a way to get the correct answer, using limiting reactants, but I'm almost 100% sure we never learned about those. I'll admit I haven't payed attention 100% of the time in this class, and I've been absent for at least 3 periods due to sickness.

Basically, since there is enough of each so that they both run out at the same time, I took the .185, divided it by 2, then multiplied that by the molar mass (102), and got the correct answer. This also works if I took the .13875, multiplied it by two (since the formula says o2), divided by three, and multiplied by the molar mass (102).

Is that the only way to do it, or am I missing something with the ratios? I've tried asking this around other corners of the internet, and have gotten mostly flame, along with some unrealistic answers (like over 30g, which just by using logic seems impossible).

Thanks a lot to anyone who answers...

[Borek]

The balanced chemical equation is 4Al + 302 --> 2Al2O3 . So, the ratio looks like it would be 4:6:4:6

No, ratio is 4:3:2...

[insomaniacal]

Okay, I was thinking 4:6:4:6 because you have 3 moles of O2, or 6 moles of O, since it is diatomic.

Would it be 4:3:2:3, in other words, using the balanced equation in the first half, but using the unbalanced one in the second? Otherwise, why would there be a two?

Thanks for your reply.

[Borek]

For some reason you assume there are FOUR numbers in the stoichometric ratio, when there are THREE - it is all about number of moles of SUBSTANCES, not moles of atoms inside of some molecules.

[insomaniacal]

Yes, I realized that, I understand that it's my bad there.

However, I've figured this out on my own.

Basically, you take a number of moles of something that you know, then with the ratio do this:

#mol * (what you want to know/ratio of something to cancel out the other element)

It's worked for this problem, as well as several others I've found online to practice on. Thanks for the help though, it's appreciated.