If a 5.0*g mass of calcium carbonate is strongly heated to give a 2.8*g mass of calcium oxide, and a 2.2*g mass of carbon dioxide, how does this demonstrate the conservation of mass? How is this represented as an equation?

$C a C {O}_{3} \left(s\right) + \Delta \rightarrow C a O \left(s\right) + C {O}_{2} \left(g\right) \uparrow$
Is mass balanced in the given equation? We started with a mass of $5.0 \cdot g$ of calcium carbonate; from the reaction we get a mass of $2.8 \cdot g$ of oxide, and $2.2 \cdot g$ of gas. $\left(2.8 + 2.2\right) \cdot g = 5.0 \cdot g$ as required, and mass is manifestly conserved.
We can take this further, and investigate the particle nature of calcium carbonate, and we have a molar quantity of $\frac{5.0 \cdot g}{100.09 \cdot g \cdot m o {l}^{-} 1} = 0.05 \cdot m o l$ of calcium carbonate, and we get a $0.05 \cdot m o l$ of calcium oxide, and a $0.05 \cdot m o l$ quantity of carbon dioxide gas.