Chemical Equations And Laws

Laws of conservation of mass: 

This law was proposed by Lavoiser. This law states that mass can neither be created nor destroye This law is applicable to all chemical reactions. During a chemical reaction no change in the mass of the substances take place i.e., the total mass of the reactants is always equal to the total mass of the products.     
Ex:     \( HCl + NaOH\xrightarrow{{}}NaCl + H_2 O \)
Molecular mass of HCl        =    1+35.5=36.5
Molecular mass of NaOH    =    23+16+1=40
Molecular mass of NaCl      =    23+35.5=58.5
Molecular mass of H₂O       =    1×2+16=18
Total mass before chemical change (reactants)
            = Molecular mass of HCl + Molecular mass of NaOH 
            = 36.5 + 40 = 76.5 u
Total mass after chemical reaction (products)
             = Molecular mass of NaCl + Molecular mass of H₂O
             = 58.5 + 18 = 76.5 u
Thus, in this chemical change, Total masses of reactants = Total masses of products.
    
Note: 
This relationship holds good when the reactants are completely converted into products. In case the reactants are not completely consumed, then 
total masses of reactants =  Total masses of products + masses of unreacted reactants.