Question

Magnesium reacts with Oxygen gas forming Magnesium oxide as shown;
2Mg(s)+O2(g)--->2MgO(s)
What mass of O2(g) is needed to completely react with 5.00 g of Mg?

Answer 1

Ugh this site is buggy just lost the answer to this thus going to provide you with summery of how to solve

Basically what you need to do is this
1) Find moles of Mg that are present in 5g of Mg using formula:


or in your case
5g/(24.3050g/mol) = moles of Mg

2) take the moles and divide them by 2 because you have a 2:1 ratio of Mg to O2 this will provide you with the moles needed to react 5grams of Mg.

3) take the moles of O2 and sub them back into the formula:
mole = grams/molecular weight
to solve for grams of MG all you have to do is rearrange the formula to look like this:
mole * molecular weight = grams

This will give you your answer enjoy, man this question makes me pine for chemistry class again

Answer 2

2Mg + O2 → 2MgO
5g / (24.305g/mol) (x/32) moles of reactants
-2*[5/ 24.305] - (x/32) +2* [5/ 24.305] moles consumed and
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　produced
----------------------------------------------------------------------------
0 0 + [5/ 24.305] "completely reaction"

from above, we can know that we need x = 32\[\times\][5/ 24.305] (g) of O2

Answer 3

Just an aside,
Remember to multiply the molecular weight of oxygen by 2 (because it is O2) so you should have a formula that looks like this:
mole * (15.9994g/mol)(2) = grams

Answer 4

Malibu: I just worked out a similar problem in detail. These are called limiting reagent problems. See http://openstudy.com/#/updates/4ed81c7de4b0baf52978ace0
Both the approaches above are correct.