okay, here is the question: in the chemical reaction CaCO3=CaO+CO2, if 40.0g of CaCO3 is decomposed, how many grams of CaO are produced? Also, where did "the rest" of the mass go anyway? I'm kind of confused on this question any leads would help. Thanks.

Do you know about 'state symbols'? These are symbols such as (s), (l), (g), (aq) to denote whether a substance is solid, liquid, gas or aqueous respectively.

Well, I use them in equations like that:



As you can see, carbon dioxide is a gas, and as such, will escape into the atmosphere.

Now, the equation gives you the mole ratio of each reacting substance.

All we need now is the amount of CaCO3 and CaO.

1 mole of CaCO3 gives 1 mole of CaO from the equation.

So, find the number of moles of CaCO3.
You'll have that amount of CaO.
What is the mass of that amount of moles of CaO?