# Stoichiometry Practice Problems

Aluminum oxide is formed from the reaction of metallic aluminum with oxygen gas. How many moles of Aluminum are needed to form 3.4 moles of Aluminum oxide?

I don't necessarily need help solving the problem itself (I want to try to do it on my own) but I don't understand how to set the equation up. Aluminum oxide? --> AlO? Metallic aluminum? --> Al? Oxygen gas? --> O?

Al + O --> AlO ?

That seems too easy...and wrong... :rofl:

Borek
Mentor
Look at the periodic table - Na2O, MgO, SiO2 - what should be the formula of aliminum oxide?

Sorry for the wait, computer problems. :)

Anyways, is it...

Al + O2 --> Al2O3

Borek
Mentor
Exactly.

Now it should be a breeze...

Well, we're talking about me doing this so we'll see...:tongue:

Is this correctly balanced?

4Al + 3O2 --> 2Al2O3

Ah, I know it was supposed to be easy (And I'm sure it is as my teacher said so as well on the lesson page.) but I don't know what to do. I think the problem lies in what it is I'm doing exactly...what is stoichiometry? I read the definitions and took the notes but it's still a bit fuzzy.

Borek
Mentor
Reaction is balanced. Now - think in terms of proportions. You need 4 moles of Al to obtain 2 moles of oxide. If so, how many moles of Al are needed to prepare 3.4 mole of oxide?

I can do that but I don't understand why I'm doing it if I need to know how many moles of Aluminum are needed to make 3.4 moles of Aluminum Oxide...

Gokul43201
Staff Emeritus
Gold Member
AngelShare said:
I can do that but I don't understand why I'm doing it if I need to know how many moles of Aluminum are needed to make 3.4 moles of Aluminum Oxide...
Perhaps you haven't thought about what the balanced equation is telling you. How would you put the equation into words ?

I'd probably say, "Four atoms of Aluminum and six atoms of Oxygen make Aluminum Oxide"...which seems to be quite wrong.

Gokul43201
Staff Emeritus
Gold Member
AngelShare said:
I'd probably say, "Four atoms of Aluminum and six atoms of Oxygen make Aluminum Oxide"...which seems to be quite wrong.
Try "4 atoms of Aluminum react with 3 molecules of Oxygen, making two molecules of Aluminum Oxide".

The lesson to learn from this (that's relevant to the question asked) is that 4 atoms of Al give rise to 2 molecules of Al2O3.

There ! Piece of cake, eh ?

So you aren't counting the two after the O? If not, why? I was which is how I got confused.

Determine the number of grams of NH3 produced by the reaction of 3.5g of hydrogen gas with sufficient nitrogen gas. (Check for diatomic elements).

I wrote down the diatomic elements but I'm not sure if this is right:

H2 + N2 --> NH3

Math Is Hard
Staff Emeritus
Gold Member
So you aren't counting the two after the O? If not, why? I was which is how I got confused.
note that he said 3 molecules of Oxygen, not atoms.

Math Is Hard said:
note that he said 3 molecules of Oxygen, not atoms.

Ah, okay. And that's done with Oxygen and not Aluminum because Oxygen is a diatomic element?

Math Is Hard
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Gold Member
AngelShare said:
Ah, okay. And that's done with Oxygen and not Aluminum because Oxygen is a diatomic element?
yes, indeedy. Don't forget those diatomic elements --
Nobody Has Fear Of Ice Cold Beer.
And you can see there on the left side of the equation that your Al doesn't have a subscript like the O2.

So, when creating an equation like that one, the diatomic element involved (If there is one.) always has a subscript of two, right? The other element needs no such thing? So, if I'm right about that, my other equation is correct?

H2 + N2 --> NH3

Hydrogen and Nitrogen are both diatomic elements so they both require the subscripted two? However, I'm not sure why I ended up with what I did in the first equation...

Al + O2 --> Al2O3

The product switched subscripts...Al was three and Oxygen was two at the beginning. Why is that exactly? I remembered doing that with my tutor last year which is why I did it here but why I did it isn't clear to me.

Math Is Hard
Staff Emeritus
Gold Member
AngelShare said:
So, when creating an equation like that one, the diatomic element involved (If there is one.) always has a subscript of two, right? The other element needs no such thing? So, if I'm right about that, my other equation is correct?

H2 + N2 --> NH3
yes, but you will need to balance it.
Hydrogen and Nitrogen are both diatomic elements so they both require the subscripted two? However, I'm not sure why I ended up with what I did in the first equation...

Al + O2 --> Al2O3

The product switched subscripts...Al was three and Oxygen was two at the beginning. Why is that exactly? I remembered doing that with my tutor last year which is why I did it here but why I did it isn't clear to me.
well, you started with this
Al + O2 --> Al2O3
but then you used coefficients to balance it, so your subscripts did not change, just the coefficients.
4Al + 3O2 --> 2Al2O3

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Math Is Hard
Staff Emeritus
Gold Member
maybe it was the step where you balanced the charges to get Al2O3 that is confusing you?

I was hoping I had gotten the second one correct (Balancing the equation usually isn't too hard for me but setting it up is a different story.:rofl: ).

What confused me about the first one is that I started out with this:

Al + O2 --> Al2O3

On the left side of the equation, Al has no number near it. Oxygen has a two after it. The final product is Al2O3 which means Al now has a number after it. Where did it come from? I vaguely remember my tutor saying something about, "Switch the numbers..." which is kind of confusing being that it is a vague memory.

Now, I know I was given the product in this particular equation but, let's say I was given this:

Al + O2 --> ?

...and I had to write the final product. I'd have to get Al2O3, right? If so, how would I get that all on my own? Am I hitting on something I should know or am I unnecessarily confusing myself?

As for balancing the second equation, would it be:

3H2 + N2 --> 2NH3

How did you get the numbers subscripted in your other post?

Math Is Hard
Staff Emeritus
Gold Member
To get the Al2O3 :
You know that you are going to have some aluminum ions and some oxygen ions bonding together for your product. But how many Al ions and how many O ions are going to hook up with each other? What you have to do is look at what the charge is on your Al ion and the charge on the O ion. The Al ion has a charge of +3 (the same as its group number) and the O ion has a charge of -2 (the group number minus 8). But you need to make the charges balance (cancel each other out). You can figure that the least common multiple of 2 and 3 is 6. If you have two Al ions, the overall charge is +6 and if you have three Oxygen ions the overall charge is -6. Now you know what the subscripts need to be:
Al2O3

It's much easier if you have something like Na and Cl, where the Na ion charge is +1 and the Cl ion charge is -1. It's just NaCl.

To do the subscripts, you can just put tags around your numbers for sub and /sub, each enclosed with []. If you hit quote to reply, you should be able to see what I did.

p.s. this might be helpful in understanding what's going on when you are writing the formulas for ionic compounds:
http://www.iun.edu/~cpanhd/C101webnotes/composition/formioncmpds.html

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Math Is Hard
Staff Emeritus
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AngelShare said:
As for balancing the second equation, would it be:

3H2 + N2 --> 2NH3
yes, that's it.

1. 1.2 moles
2. 2.3 grams

Is that right?

Math Is Hard
Staff Emeritus
Gold Member
AngelShare said:
1. 1.2 moles
Is that right?
4Al + 3O2 --> 2Al2O3
Borek said:
Reaction is balanced. Now - think in terms of proportions. You need 4 moles of Al to obtain 2 moles of aluminum oxide. If so, how many moles of Al are needed to prepare 3.4 mole of oxide?

This is what I was told: do not over think this one. Just use the mole ratio from the balanced equation. Grams do not enter into this one. It is as simple as it seems.

Okay, following the example on the lesson page (Step by step... )...

Al/Al2O3
4/2

He said that grams don't enter into this one so I simply divided 4 by 2...wait, no I didn't....what in the hell did I do? (My method was clearly a dumb one.)

4 to get 2

I need to get 3.4...

4/2 = 3.4/x
4x = 6.8
x = 1.7?

So, I got the second one right or is that one so far off you can't figure out how I even got that answer?:rofl: