(1) What is the percentage oxygen in 4grams of caco3.
(2) What mass of Oxygen is require to react completely with 16grams of hydrogen.
(1) %O in CaCO3 = (3*atomic mass O/molar mass CaCO3)*100 = ?? and that is the same answer for 4 grams CaCO3 or 1000 g CaCO3.
(2) Follow the steps in this link.
http://www.jiskha.com/science/chemistry/stoichiometry.html