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PRINCIPLE OF ATOM CONSERVATION (PAOC):

We know that, in a chemical reaction, the atoms of each element remain conserved. If atoms are conserved, moles of atoms will also be conserved. This is known as the principle of atom conservation.
ILLUSTRATION OF POAC:  Take a chemical reaction as example.
(A) According to the principle of atom conservation (POAC) for K atoms:
    Total moles of K atoms in reactant = total mole of K atoms in products
         Or, moles of K atoms in KClO3 = moles of K atoms in KCl.
Now, since 1 molecule of KClO3 contains 1 atom of K, 1mole of KClO3 contains 1 mole of K similarly, 1mole of KCl contains 1mole of K.
     Thus, mole of K atoms in KClO3  = 1 × moles of KClO3
             And mole of K atoms in KCl = 1 × moles of KCl
Hence            1 × moles of KClO3 = 1 × moles of KCl
 (B) Similarly applying the principle of atom conservation for O atoms:
                 Mole of O in KClO3 = 3 ×moles of KClO3
             And moles of O in O2 = 2 × moles of O2 
                 3 × moles of KClO3 = 2 × moles of O2
As O2 is gas, its mole may also be directly used to calculate the volume of gas.
ILLUSTRATIVE EXAMPLE (1) : Write the POAC equations for all the atoms in the following reaction.
ILLUSTRATIVE EXAMPLE (2) : 27.6 gm  K2CO3  (138) was treated by a series of reagents so as to convert all its carbon into K2Zn3[Fe(CN)6]2(698) Calculate mass of K2Zn3[Fe(CN)6]2.
ILLUSTRATIVE EXAMPLE (3) :  0.32 gm mole of LiAlH4 in ether solution was placed in a flask and 7.4 gm (1 mole) of t- butyl alcohol was added . The product is LiAlHC12H27O3. Calculate mass of product if all lithium atom are convert.
ILLUSTRATIVE EXAMPLE (4) :  25.4 gm I2 and 14.2 gm Cl2 react completely to produced a mixture  of ICl and ICl3 . calculate moles of ICl and ICl. 
ILLUSTRATIVE EXAMPLE (5) : All the carbon atom present in KH3(C2O4)2.2H2O weighting 254 gm is converted to CO2. How many gram of CO2 were obtained. 
ILLUSTRATIVE EXAMPLE (6) :  For BaCl2.XH2O , 2.1 gm of Compound gives 2 gm of anhydrous BaSO4 upon treatment with H2SO4 then calculate value of X.
ILLUSTRATIVE EXAMPLE (7) :calculate In a determination of P an aqueous solution of NaH2PO4 is treated with a mixture of Ammonium and magnesium ions to precipitate magnesium Ammonium phosphate (Mg(NH4)PO4.6H2O. This is heated  and decomposed to magnesium pyrophosphate. Which is weighed . A solution of NaH2PO4 yield 3.33 gm of Mg2P2O7 . What is weighed of NaH2PO4 was present Originally.
ILLUSTRATIVE EXAMPLE (8) :Two Substance  A2 and B2 on  reacting simultaneously produce B3B4 and B2B4 . Initially 5/4 moles of A2 and 2 moles of B2 are taken in container then what will be the amount of product ? 

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