Combustion of hydrocarbons produces CO2 gas.
C4H10+132O2→4CO2+5H2O
1 mol of butane produces 4 mol of CO2.
Now CO2 produced is allowed to pass through Ca(OH)2, the reaction is:
CO2+Ca(OH)2→CaCO3+H2O
4 mol 3 mol ?
Here the concept of limiting reagent comes into the picture and turns out that Ca(OH)2 is the limiting reagent here.
Since only 3 mol of Ca(OH)2 is present, hence 3 mol of CaCO3 will be formed.