Consider the following reaction.
CaCO3 + H2O + CO2 → Ca(HCO3)2
ElementNumber of atoms present
Reactant sideProduct sideCalcium11Carbon2YOxygenX6Hydrogen22
Recognise X and Y. The value of X+Y is
From the equation: CaCO3 + H2O + CO2 → Ca(HCO3)2
ElementNumber of atoms present
Reactant sideProduct sideCalcium11Carbon2(Y=2)Oxygen(X=6)6Hydrogen22
Thus, the value of X+Y=6+2=8