When carbon is burned in air, it reacts with oxygen to form carbon dioxide. When 24.0 of carbon were burned in the presence of 76.8 of oxygen, 12.8 of oxygen remained unreacted. What mass of carbon dioxide was produced?

Answer 1

I will assume you mean 24 grams of carbon burned

C + O2 --> CO2
C is about 12 grams/mol
O2 = 2*16 = 32 grams/mol
CO2 = 12+32 = 44 grams/mol therefore

24/2 = 2 mols carbon
that means 2 mols O2 and 2 mols CO2
2 mols CO2 = 88 grams --- answer

they also gave you redundant information again, check it
76.8-12.8 = 64 grams of O2 they say we used, check that
2 mols O2 = 2*32 = 64 grams, sure enough