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reacting masses

could someone start me off with this simple q.

what mass of oxygen reacts with 192g of Magnesium?

I'm not sure whether to divide the mass by Magnesium or oxygen to find the number of moles???

thanks!

Reply 1

billet-doux

2Mg + 02= 2Mg0

Find the moles of Magnesium= mass/ AR= 192/24

Whatever the answer to this is, divide it by 2 to find the moles of oxygen because the ratio of Mg to 02 is 2:1

To find the mass of 02= moles x Mr

Reply 2

Malsy

isn't is 192/48 because it#s 2Mg

and thanks!

Reply 3

modini

Malsi101

isn't is 192/48 because it#s 2Mg

and thanks!

Strictly speaking its 192/24 but then you x by 2 because theres 2 moles of Mg for every 1 mole of O2

Reply 4

billet-doux

Nope it's not. You account for the ratio of 2:1 further on.

Reply 5

Malsy

ok now i'm confused because it changes the oxygen as well if i divide it by 24.

could someone just take me through each step please :o:

Reply 6

billet-doux

1. You form the equation
2Mg +02= 2MgO

2. You need to find the moles of magnesium:
Moles= Mass/Ar
Moles= 192/24
(We account for the ratio of 2Mg: 02 later on)

3. You use your answer to 2. to find the moles of oxygen:
The ration of 2Mg:02 is 2:1 so you divide the previous answer by 2 to get the moles of oxygen

4. You use your answer to 3. to find the mass of oxygen:
Mass= Moles x Mr
Mass= (answer to 3) x 32 (02 is a diatomic molecule so it's 16 x 2)

Is that any clearer?

Reply 7

Malsy

OK, I got 8 x 32 for the mass of O2 ..

Reply 8

billet-doux

The moles of 02 is 4, because the moles of Mg is 192/24= 8. Then you divide 8 by 2 because the ratio of Mg to 02 is 2:1

Reply 9

Malsy

oh ok. What I did was, times the 8 by 2 due to it being 2 moles of Mg. then divide again to get one mole of O2 but clearly that aint right...

Reply 10

billet-doux

If you don't get it you can always post another question and we can go through that too?

Reply 11

Malsy

what I don't get is why we've not done something about the 2Mg and just carried on like it's only Mg because, as you, and someone else said you do something further with it..but we clearly haven't :p:

Reply 12

billet-doux

We have though. We accounted for the ratio of 2Mg:O2 later on when we divided the number of Mg moles by 2 to get the number of 02 moles.

Reply 13

v1oXx-

Malsi, do you know what stochiometric ratio is?

It is the ratio at which things react.

When Mg reacts with 02, you need 2 Mg as the equation is:

2Mg + 02 ----> 2Mg0.

Therefore the moles of 02 is half that of 2Mg.

Reply 14

tomster

Yeah, pretty much what -V1OxX said.

I would divide the 192 (mass)/ 24 (Mr) and then divide by two (2 Mgs to one oxygen) and times by 32 (02) to get the answer.

Here is my advice for any style of question regarding reacting masses

x (mass in g)/ y of either product/reactant and then multiply by the other.

Reply 15

leopardsnowy

moles of Mg = 192/24 = 8 molmoles of O2 = 8/2 ( ratio of Mg to O is 2:1) = 4 molmass of O2 = 4 x 16 x 2 ( Mr of O2 is 16x2 since it is a diatomic molecule) = 128ghope it was easy to undersatand! :smile:

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