A4.86-gram sample of calcium reacted completely with oxygen to form 6.80 grams of calcium oxide. this reaction is represented by the balanced equation below.
2ca(s) + o2(g) ==> 2cao(s)
54 determine the total mass of oxygen that reacted.

The number of mole of Ca reacted is:
4.86 g Ca/ (40.08 g/mol Ca)= 0.121 mol Ca

Because Ca reacted completely with oxygen and there is 2 mol Ca, there is 1 mol O2 reacted.

Total mass of oxygen that reacted is:
0.121 mol Ca* (1mol O2/ 2 mol Ca)* (32 g O2/ 1 mol O2)= 1.94 g O2 reacted.

Hope this would help~