Anyone know Chemistry? Need some help!

[misle]

I'm taking General Chemistry II. I've never had Chemistry before, but for some reason my advisor thought it would be a good idea to jump into Chem II instead of taking Chem I first.

Anyway, we did an experiment in lab and in the post lab report I'm supposed to calculate a few things that I do not know how to calculate.

Basic Lab procedure:
I took 0.5496 grams of Aluminum Foil and dissolved it in 35 mL of 2M KOH.
I then filtered the liquid, keeping the liquid, and rinsing with 10mL of DI water.
Added 10 drops of methyl red.
Iced the liquid.
Then, I heated the liquid while stirring and adding 8-9 mL of 6M H2SO4 (the numbers should be subscripts)
Did this until it turned a clear red color.
Cooled the liquid and filtered it again
This time we kept the crystals - which are supposed to be Aluminum Crystals.
Final weight was 1.0678 grams of crystals.

Questions:
1. Calculate moles of Al used.
2. Theoretical yield of moles of Al.
3. Theoretical yield of grams of Al.

I think I know how to convert from grams to moles so, if I can get someone to explain how to do number 1 and 2 (or 3), it would help me a lot.

Any help would be appreciated.

I'm not asking that you work it for me, but if you can help me with equations or explanations, it would be greatly appreciated.

 

[minendo]

Not quite sure how you got more Al then you started with.

Unless, the actual crystals are aluminum oxide.

 

[arcenite]

To calculate the moles of Aluminum, you have to know how many grams are in 1 mole of aluminum. Aluminums atomic weight is ~27g. You are using .5496g of Aluminum. Therefore, to calculate how many moles of Aluminum were used, you use this equation

(.5486 gAl) (1 mole / 27g) = Your Answer for #1

Bill

 

[arcenite]

I believe Theoretical yield is how much Aluminum was yielded out of the equation... Actual yield is what you came up with... So you use math to figure out your yield, instead of the actual experiment..

I do not remember the equations, but you have to figure out what should have happened as opposed to what did happen... lemme see if i can find.

Bill

 

[misle]

Thanks for the help.

I'm still trying to figure out the whole "Mole" thing, so thanks for the help aRCeNITe.

I also confused me how we got more alum crystals out than alum we put in. Maybe the 'crystals' means that there is something else with it and it's not purely alum?

 

[arcenite]

Originally posted by: minendo
Not quite sure how you got more Al then you started with.

Unless, the actual crystals are aluminum oxide.

This is all Chem I stuff he's doing (last semester for me) I don't remember how to figure out theo yield

Why'd you edit your post? You were right. I was slow.

Bill

 

[arcenite]

Originally posted by: misle
Thanks for the help.

I'm still trying to figure out the whole "Mole" thing, so thanks for the help aRCeNITe.

I also confused me how we got more alum crystals out than alum we put in. Maybe the 'crystals' means that there is something else with it and it's not purely alum?

Aluminum crystal is an oxide.. Oxygen was gained, therefore increasing the mass.

If you're having trouble with moles, PM me. That's one part of chem I had no problems with.

Bill

 

[minendo]

Originally posted by: aRCeNiTe

Originally posted by: minendo
Not quite sure how you got more Al then you started with.

Unless, the actual crystals are aluminum oxide.

This is all Chem I stuff he's doing (last semester for me) I don't remember how to figure out theo yield

Why'd you edit your post? You were right. I was slow.

Bill

I editted because I used the Atomic number instead of mass in my calculation. Stupid mistake on my part.

In order to calculate theoretical yield you need to know the balanced chemical equation. This means, you must know what the final product is since it can't be pure aluminum. Unless, of course, the OP found some way to magically synthesize Aluminum out of non-aluminum containing base and acid.

 

[arcenite]

Originally posted by: minendo

Originally posted by: aRCeNiTe

Originally posted by: minendo
Not quite sure how you got more Al then you started with.

Unless, the actual crystals are aluminum oxide.

This is all Chem I stuff he's doing (last semester for me) I don't remember how to figure out theo yield

Why'd you edit your post? You were right. I was slow.

Bill

I editted because I used the Atomic number instead of mass in my calculation. Stupid mistake on my part.

In order to calculate theoretical yield you need to know the balanced chemical equation. This means, you must know what the final product is since it can't be pure aluminum. Unless, of course, the OP found some way to magically synthesize Aluminum out of non-aluminum containing base and acid.

So we don't have enough information, right? Unless we can take the Weight of the aluminum he started with, and subtract that from the final weight, to get the gained oxygen?

Bill

 

[minendo]

Originally posted by: aRCeNiTe

Originally posted by: minendo

Originally posted by: aRCeNiTe

Originally posted by: minendo
Not quite sure how you got more Al then you started with.

Unless, the actual crystals are aluminum oxide.

This is all Chem I stuff he's doing (last semester for me) I don't remember how to figure out theo yield

Why'd you edit your post? You were right. I was slow.

Bill

I editted because I used the Atomic number instead of mass in my calculation. Stupid mistake on my part.

In order to calculate theoretical yield you need to know the balanced chemical equation. This means, you must know what the final product is since it can't be pure aluminum. Unless, of course, the OP found some way to magically synthesize Aluminum out of non-aluminum containing base and acid.

So we don't have enough information, right? Unless we can take the Weight of the aluminum he started with, and subtract that from the final weight, to get the gained oxygen?

Bill

There is enough information.

I believe the balanced equation is :

2Al +3 KOH ---> Al2O3

But I need to double check and see what happens when H2SO4 disassociates.

 

[misle]

Got number 1, thanks aRCenITe.

#2/3) I think I'm supposed to use Al as the limiting reagent in the reactions to determine the theoretical yield, but since I had two different reactions (with KOH & H2SO4) and I filtered them (removing who knows what), I'm unsure on how to proceed.

I have emailed my Lab TA but he hasn't responded yet and it's due in ~ 2.5 hours.

 

[arcenite]

Originally posted by: minendo

Originally posted by: aRCeNiTe

Originally posted by: minendo

Originally posted by: aRCeNiTe

Originally posted by: minendo
Not quite sure how you got more Al then you started with.

Unless, the actual crystals are aluminum oxide.

This is all Chem I stuff he's doing (last semester for me) I don't remember how to figure out theo yield

Why'd you edit your post? You were right. I was slow.

Bill

I editted because I used the Atomic number instead of mass in my calculation. Stupid mistake on my part.

In order to calculate theoretical yield you need to know the balanced chemical equation. This means, you must know what the final product is since it can't be pure aluminum. Unless, of course, the OP found some way to magically synthesize Aluminum out of non-aluminum containing base and acid.

So we don't have enough information, right? Unless we can take the Weight of the aluminum he started with, and subtract that from the final weight, to get the gained oxygen?

Bill

There is enough information.

I believe the balanced equation is :

2Al +3 KOH ---> Al2O3

But I need to double check and see what happens when H2SO4 disassociates.

minendo owns me.

 

[arcenite]

Originally posted by: misle
Got number 1, thanks aRCenITe.

#2/3) I think I'm supposed to use Al as the limiting reagent in the reactions to determine the theoretical yield, but since I had two different reactions (with KOH & H2SO4) and I filtered them (removing who knows what), I'm unsure on how to proceed.

I have emailed my Lab TA but he hasn't responded yet and it's due in ~ 2.5 hours.

Glad I could help

Bill

 

[rgwalt]

A mole is simply a quantity of a substance. There are 6.02x10^23 representative particles in a mole. You can have a mole of anything... a mole of atoms or a mole of cookies. When dealing with atoms or molecules, a mole of a substance will have a mass equal to the atomic mass (or combined atomic masses) of the constituent particles.

So, a mole of aluminum will have a mass equal to the atomic mass of alumninum. You shouldn't have obtained more alumninum than you added to the system, so you probably have a compound of alumninum and something else, like oxygen. You should look at the underlying chemical reactions that are occuring. It sounds like you dissolved the aluminum in KOH to form alumninum hydroxide. What is the chemical formula for aluminum hydroxide? What is the valency of aluminum? These are all questions you should be asking yourself.

Did you wash your crystals and dry them before the final weighing?

Ryan

 

[samgau]

By the looks of it... its been close to 10 years since I had chemistry.... your final product is Aluminium sulphate....

 

[samgau]

Are your crystals [e]white[/e]?
I'm appaled at myself.... considering I score in the top 5th percentile in chemistry about 15 years ago...

Also the balanced equation should be something like this:
2(AL(OH)3) + 3(H2SO4) = 6H2O + AL2(SO4)3

Looks like I also got the equation wrong.... haha

 

[minendo]

Originally posted by: samgau
By the looks of it... its been close to 10 years since I had chemistry.... your final product is Aluminium sulphate....

Actually, I think you are right.

Complete equation would be

Al(OH)3 + H2SO4 => Al2 (SO4) 3.xH2O

 

[misle]

samgau is right.

I found the main equation in the text:
K + Al + 2SO4 + 12H2O -> KAl(SO4)2 ? 12H2O