• Law of conservation of mass states that Mass is conserved regardless of chemical / physical changes. 
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• Meaning the total mass of reactant must equal the total mass of products.
Thus we can say that law of conservation of mass affect a system in equilibrium state as follows : 
A +B ↔ C + D 
• if we take out any of the reactants , say we take out A and B , the concentration at equilibrium will move backward , and similarly, if we take out the products ( C and D ) the concentration at equilibrium will shift forward.
K = [C][D]/ [A][B) 
• this shows that if if you increase the mass of the products , the concentration at equilibrium increases for the products , meaning the equlibrium state is compromised/ cannot be reached. 
 opposite effect will happen if we decrease mass of products .
We can conclude and say mass of reactant must be equal to mass of products in order to reach equilibrium state .