How do you ensure that all the Mg would react completely with O2?

Answer 1

First you must find out what mass of each would react perfectly, then, if you have more than is needed of one of the reactants (if it is in excess) all of the reactant will react.

Here is the calculation you need, for example, say you have 50g of each reactant.

Step 1-Write out formula of reactants

Mg + O2 = MgO2

1mole 1 mole

Step 2 - Find the gram formula mass of reactants

1 mole Mg= 24.3 g

1 mole O2= 32 g

24.3g Mg reacts with 32g O2

Step 3 - Find amount required to react

50g Mg -- 50/32x24.3

=37.9

=37.9g Mg

From that we can see that since there is 37.5g Mg and only 24.3g is needed to react completely with O2, the Mg is in excess.

Substitute your starting weights in there and use that calculation, and add more than the required amount, that way you can be sure.