Question

Can someone help me with Stoichiometry?

Answer 1

How many moles of aluminum metal would be needed to produce 35.6 grams of aluminum oxide in the following synthesis reaction?

Al + O2 → Al2O3

there is an example of a problem.

Answer 2

Sure. First we need to balance the reaction. What goes in must come out. So we need 4Al + 3O2 -> 2AlO3. Following me so far?

Answer 3

I mean 2Al2O3.

Answer 4

Yes :)

Answer 5

Great! So now we need to take what we have been given and use ratios to get what we want. We have 35.6 g of Al2O3. We can find out how many moles that is by using the molar mass.

Answer 6

okay. gotcha. So keep going and explain molar mass to me..

Answer 7

Al is 26.9815 g/mol and O is 15.9994 g/mol (from the periodic table). 2 Als and 3 Os add up to 101.9612 g/mol.

Answer 8

The molar mass of Al2O3.

Answer 9

you lost me on the second part.

Answer 10

the 2 Als and Os. how did you get that

Answer 11

Because there are 2 Aluminums and 3 Oxygens in one mol of Al2O3.

Answer 12

OH OKAY. that makes sense. okay. So how am i going to write out my equation.

Answer 13

Ok. Start with 35.6 g of Al2O3 * (1 mol Al2O3/101.9612 g Al2O3) = 0.349 mol Al2O3.

Answer 14

okay and you got the 101.9612 from adding together the 26.9815 and the 15.999. right?

Answer 15

Well, adding 2(26.9815) + 3(15.9994).

Answer 16

oh okay. wow this is confusing.

Answer 17

It should get easier with practice. :)

Answer 18

okay. so was that all to it?

Answer 19

of*

Answer 20

No, not yet.

Answer 21

Did you see how I converted from grams of Al2O3 to moles of Al2O3?

Answer 22

uhm.. Sortof.. not really..>__<

Answer 23

Ok. This would be much easier to explain on a whiteboard. I can give you a free lesson if you want.

Answer 24

lol okay

Answer 25

Just use the draw button or something or draw it out and attach it or something. D:

Answer 26

Something.

Answer 27

I am on an iPad, so I don't have a draw button. :/

Answer 28

darn. okay, uhm... lets do this. I have another practice problem. Ill post that. and you don't have to tell me the answers, just guide me through it. and if i'm wrong, explain why.

I'm homeschooled so I don't really have a teacher. and I don't have a test for this or anything yet, but I still want to understand it.

Answer 29

That is cool. I'm glad you want to understand it. I will try to break it down.

Answer 30

How many grams of sodium metal are needed to react completely with 45.6 grams of water?

okay. there's the problem.
I need to make an equation first. right?

Answer 31

Ok, this problem has a little easier calculations. So, basically, the idea is that we can't convert from grams of one component to grams of another component. But we can figure out their mole ratios.

Answer 32

So, we need to convert from grams of water to moles of water to moles of sodium to grams of sodium.

Answer 33

Do you see the overall idea?

Answer 34

I think so. I think i already took notes on how to do that awhile back. one second.

Answer 35

To convert grams of water to moles of water we need the molar mass of water.

Answer 36

okay. so.. this is just me guessing.

We need to put the molar mass of Hydrogen and Oxygen together. Right?

Answer 37

Right, but there are 2 Hydrogens for every 1 Oxygen in water (H2O), so we need to do 2(molar mass of Hydrogen) + 1(molar mass of Oxygen).

Answer 38

Try to calculate it and tell me what you get.

Answer 39

So
2 (1.0079) + 1 (15.999)

Answer 40

Yes, perfect.

Answer 41

= 18.0148

Answer 42

Great. So now we can calculate the moles of water by multiplying 45.6 g water by (1 mol water/18.0148 grams water) to cancel out grams of water and leave us with moles of water.

Answer 43

So, we have 2.53 moles of water.

Answer 44

Right. thats what I got.

Answer 45

And thats all...?

Answer 46

Now we need to look at the reaction of sodium and water to find their mole ratios.

Answer 47

okay

Answer 48

so sodium is 22.990

Answer 49

True but first we need to look at the reaction. Na + H2O -> H2 + Na2O

Answer 50

okay. so that is balanced.

Answer 51

No, we need 2 Sodiums in the reactants to balance it.

Answer 52

2Na + H2O -> H2 + Na2O.

Answer 53

but doesnt the 2 on the other side just go with the O?

Answer 54

there isnt 2 Na..

Answer 55

This means for every 2 moles of Na and 1 mole of H2O, we will make 1 mole of H2 and 1 mole of Na2O.

Answer 56

you lost me. I'm so sorry. D:

Answer 57

No, the 2 behind the Na means 2 Nas bonded to 1 O.

Answer 58

okay

Answer 59

This is very difficult to explain since I don't know any of your background and I can only type. :(

Answer 60

well. pretty much. i'm a junior in high school and I'm horrible at math :D really anything helps. Explain it for the stupid. It helps.

Answer 61

I was just assuming that you understand the notation of chemical formulas.

Answer 62

Uh well. okay. lets go with i understand the basics. I had some problems balancing equations.

Answer 63

Ok, so do you understand why I need to balance the formula as 2Na + H20 -> H2 + Na2O?

Answer 64

No. Because i see it as. you have 1 Na on one side and now you have 2 Na on the other side.

Answer 65

Thats how i see it. Feel free to explain why i'm wrong. haha

Answer 66

Na2O means 2 Nas and 1 O.

Answer 67

Okay so the 2 goes with the Na. not the O.

Answer 68

This is an ionic compound. Na has a charge of +1 and O has a charge of -2. You need 2 Nas to balance out the O.

Answer 69

okay. I sortof get it.

Answer 70

So pretty much all you did was you balanced the equation

Answer 71

Right. So now, using the coefficients of the reaction, I can say that for every 2 moles of Na and 1 mole of H2O I can make 1 mole of H2 and 1 mole of Na2O.

Answer 72

okay. i get that.

Answer 73

So we made it to 2.53 moles of water last time. We can convert to moles of Na by multiplying by (2 moles of Na/1 mole of H2O).

Answer 74

So we have 5.06 moles of Na.

Answer 75

wait so my equation is 2.53 --> 2 mol Na / 1 h2O

Answer 76

right?

Answer 77

No, 2.53 moles of H2O * (2 mol Na/1 mol H2O) = 5.06 moles Na

Answer 78

okay thats what i got,

Answer 79

Great. Now we can use the molar mass of Na you mentioned earlier of 22.9898 g / mol. 5.06 moles * (22.9898 grams of Na/1 mol Na) = 116 g Na.

Answer 80

okay. uhm. well. The steps it shows. Obviously do not make sense to me. That's why i'm asking. lol but. the lesson says

45.6g H2O x 1 mol H2O / 18.01g H2O x 2 mol Na / 2 mol H2O x 22.989 g Na / 1 mol Na = 58.2g Na

Answer 81

Sorry, I had to shut down. That is rright. I am sorry. I accidentally looked at the wrong equation. The actual reaction is 2Na + 2H2O -> H2 + 2NaOH.

Answer 82

So, if you change the molar ratio, you get what your book got. Sorry again...

Answer 83

no big deal :)