Question

when 31g of copper(2)carbonate is used how many grams of co2 form ?

Answer 1

Hard to say, since you don't specify the reaction.
However, you could have something like
CuCO3 + ... -> Cu?? + CO2 + H2O
That would release 1 mole of CO2 for every mole of CuCO3.
Now just convert 31g CuCO3 to moles, and convert that back to grams of CO2

If that's not your reaction, then write the balanced equation and compare moles of CuCO3 and CO2

Answer 2

I assume this is a decomposition reaction.
CuCO3 ==> CuO + CO2

mols CuCO3 = grams/molar mass = ?
1 mol CuCO3 produces 1 mols CO2.
grams CO2 = mols CO2 x molr mass CO2 = ?

Answer 3

12