Magnesium???

Question

When magnesium is burned in air the magnesium and oxygen combine to produce magnesium oxide. there are 15 grams of magnesium and 5 grams of oxygen. If all of magnesium and oxygen reacted. how many grams of magnesium oxide is produced?

Answer 1

I think in the last 2 weeks, this particular question or the same sort appeared numerous times. When 2 reagents are given with the amounts, it is called limiting reagent problem. When 2 reagents but only 1 amount is given, the other one assumed is excess.

2 Mg + O2 -> 2 MgO
So, we have to find which one is limiting first.
Mg: 15 g * 1 mol/24.31g = 0.617 mol
O2: 5 g * 1 mol/32.00 g = 0.156 mol
for every mole of O2, you need 2 mole of Mg.
0.156 mol O2 needs 0.312 mol Mg, Mg is too much, so O2 is the limiting reagent.

So, the amount of O2 is used for all calculation regarding the product.
0.156 mol O2 * 2 mol MgO / 1 mol O2 = 0.312 mol MgO
0.312 mol MgO * 40.31 g MgO / 1 mol MgO = 12.6 g MgO

Answer 2

Equation: 2 Mg + O2 -> 2 MgO

Oxygen available in moles = 5/32 or 0.17 appx
Mg available in moles= 15/24 or 0.62 appx
Reaction says 2 moles of Mg requires 1 mole of O2 and form 2 moles of MgO.
So oxygen is LIMITING reagent: 0.17 moles of O2 form 0.34 moles of MgO.
Weight of MgO = 0.34 * 40 = 14 g appx.