Question

what mass of CaCo3 has to be decomposed to get 28 gram of CaO ?

Answer 1

First, write reaction equation: CaCO3 ------> CaO + CO2
It is already balanced.
Second, you have to calculate how many moles of CaCO3 do you have.
It's easy: n(CaCO3)=m(CaCO3)/M(CaCO3)     where M is the molar mass in g/mol

n= 50g / 100g/mol = 0.5 mol

Third, find how many moles of product do you get, in this case also 0.5 moles.
n(CaCO3)=n(CaO)
m(CaO) = n(CaO)*M(CaO) = 0.5mol*56g/mol = 28g.