Question

2.4 g magnesium burns to form 4 gram of magnesium oxide find out the amount of oxygen used ?
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Answer 1

Answer:

This problem is a limiting reactant type of problem. Basically you need to solve the problem twice. But first, you must have a balanced equation for the reaction. For this reaction the equation is: 

2 Mg(s) + O2(g) --> 2 MgO 

Now, starting with the magnesium, you must calculate the moles (and/or grams) of MgO which could be formed from that: 

2.4 g Mg / 24 g/mol = 0.10 mol Mg 

0.10 mol Mg ( 2 moles MgO / 2 moles Mg) = 0.10 mol MgO 

Now, beginning with 0.25 mol of O2: 

0.25 mol O2 X ( 2 mol MgO / 1 mol O2) = 0.50 mol MgO 

The amount of magnesium that you are giving limits the amount of MgO that you could possibly form. So, this reaction can only form 0.10 mol of MgO. The mass of 0.10 mol of MgO is: 

0.10 mol MgO ( 40.0 g/mol) = 4 grams MgO