Question

what mass of HCl is needed to decompose 50g of CaCO3

Answer 1

Assuming your reaction is

CaCO3 + 2HCl -> CaCl2 + H2O + CO2

then it will take two moles of HCl for each mole of CaCO3

So, how many moles of CaCO3 in 50g?
double that for HCl and figure the mass.