Excess and Limiting Reactants
The chemical name for chalk is calcium carbonate. The reaction between sodium carbonate and calcium chloride produces calcium carbonate.
Na2CO3(aq) + CaCl2(aq) → CaCO3(s) + 2 NaCl(aq)
Stoichiometry allows us to compare the amounts of various species involved in a reaction. In order to determine which of the reactants is the limiting reactant, we must take into account both the amounts present and how they relate stoichiometrically in the balanced equation. Why do chemists use limiting reactants? The reason lies in the fact that not all reactions go to 100% completion; in fact the majority of the really interesting ones do not. However, scientists can use an equilibrium “trick” to get the stubborn reactions to go to completion. They start with an excess of one of the reactants to “push” the reaction to make more product. This essentially makes the other reactant the limiting reactant.