If 72.5 grams of calcium metal (Ca) react with 65.0 grams of oxygen gas (O2) in a synthesis reaction, how many grams of the excess reactant will be left over when the reaction is complete? Be sure to write out the balanced equation for this reaction and to show all of your work.

2Ca + O2 = 2CaO

First, determine which is the excess reactant

72.5 g Ca (1 mol) = 1.8089725036

(40.078 g)

65 g O2 (1 mol) = 2.0313769611

(15.999g * 2)

Since the ratio of to O2 is 2:1 in the balanced reaction, divide Ca's molar mass by 2 to get 0.9044862518. this isn't necessary because Ca is already obviously the limiting reactant. therefore, O2 is the excess reactant.

Now do the stoichiometry

72.5 g Ca (1 mol Ca) (1 mol O2)

(40.078 g Ca) (2 mol Ca) (31.998g O2)

=0.0282669621 g of O2 left over