Use a balance to weigh reactants in grams. Calculate the moles of each reactant by dividing the mass that you have of each reactant by the molar masses of reactants, which you can obtain from a periodic table.

Determine the stoichiometric ratio of moles of any reactant to moles of CO2. For instance, if your equation is CaC03 + 2HCL => CaCl2 + CO2 + H2O, then the ratio of moles of CaCO3 to CO2 is 1:1. For every mole of CaCO3 that you are using, you have one mole of CO2. Alternatively, for every two moles of HCl, you have one mole of CO2.

Calculate moles of CO2 produced. If you are starting with one mole of CaCO3, then you might expect to produce one mole of CO2. But how many moles of CaCO3 did you determine in Step 1? That number is equal to the moles of CO2 produced. You could also use moles of HCl, divided by two, to calculate moles of CO2 produced.

Calculate volume of CO2 produced. The volume of one mole of CO2 produced is 24 dm^3 at room temperature and pressure. Alternatively, if your reaction took place at standard temperature and pressure (273 K, 1 atm), then the molar volume is 22.4 dm^3. Multiply the number of moles produced, calculated in Step 3, by the molar volume to calculate the volume of gas produced.