Question

Mg + H2O --> Mg(OH)2 + H2

If you mix 40 grams of Mg and 60 grams of H2O
What is the theoretical yield of H2

Answer 1

First convert from grams to moles like you did in the previous question

Answer 2

The only thing I am confused on is the fact that H2O isn't the limiting reactant and Mg is. I get 1.65 M of Mg and 3.33 M of H2O. the required H2O is 3.29 M but we have more than enough so Mg is the limiting.

Answer 3

Would I flip around the ratio now??? So instead of 2:1 is it 1:2? So I would multiply the 1.65 * 2 because there's 2H2O?

Answer 4

1.65 moles of magnesium and 3.33 moles of water

So you are right that Mg will be our limiting reagent

Answer 5

So we're going to use up all of our magnesium which means all 1.65 moles react.

How many moles of water will react with 1.65 moles of Mg?

Answer 6

Would it be the 3.3? because the ratio is 1:2 and 1.65*2 = 3.3? I'm not 100% sure

Answer 7

Yup!

Answer 8

So basically our x is 1.65 and 2x is 3.3

How many moles of the products will we have?

Answer 9

.825 apiece?

Answer 10

Because 1.65/2 = .825 and the products are 1 to 1

Answer 11

nonono

Remember it's moles to moles

For 1 mole of Mg and 2 moles of H2O, we make 1 mole of Mg(OH)2 and 1 mole of H2

If we're using 1.65 moles of Mg, how many moles of Mg(OH)2 are we forming?
Remember it's 1 mole of Mg to 1 mole of Mg(OH)2

And similarly,
How many moles of H2 would we make?
Remember, it's 1 to 1

Answer 12

So it's 1.65?

Answer 13

Spot on!

Answer 14

Just because we have two products doesn't mean we have to divide by 2 or anything.

You just have to keep that moles to mole ratio.

Answer 15

So then the H2 would be 3.33 because we used up all the 1.65 already?

Answer 16

Why would it be 3.33?

Answer 17

How many moles of H2 are there?

Answer 18

1 mole, but didn't we already use up the 1.65 towards the Mg(OH)2?

Answer 19

Sooooo that's not how chemical reactions work haha

Answer 20

There is the law of conservation and all but it doesn't work with the numbers. If you look at the elements then you'll see it's balanced because every atom bonds with another atom and blah blah and so you're not losing anything

Just like how we used 40 grams of Mg and 60 grams of water, but the products are not going to add up to 100 grams

Answer 21

It's just moles to mole

1 mole of Mg and 2 moles of water will give us
1 mole of Mg(OH)2 AND

the keyword here is AND
and 1 mole of of H2

Answer 22

Does that clear it up a little more or are you just as confused?

Answer 23

Ohh so the limiting reagent accounts for both of the products?

Answer 24

yes

Answer 25

OK that clears it up a lot better thanks

Answer 26

Of course!

Answer 27

Now that we have 1.65 H2, all that I have to do is convert that to grams for the theoretical yield

Answer 28

You got it!

Answer 29

And that gets back to 3.33 lol because the molar mass of H2 is 2 and 1.65*2 = 3.33

Answer 30

and that's your theoretical yield :)

Answer 31

I have a question: So I'm not sure if you remember the last problem we did a lil ago, but since the limiting reactant was H2O and it was 2 moles, we had to divide it by 2 to get each of the products, but if it was just 1 mole of H2O, the products would be .277 both?

Answer 32

And thanks for your help again

Answer 33

No problem!