Answer:
1. A double replacement chemical equation is a reaction in which the cations and anions of the compounds that react exchange their positions forming new compounds. Double replacement reaction usually leads to the formation of water, a precipitate or a gas as one of the products.
Example of a double replacement reaction:
CaCl_{2}(aq)+Na_{2}SO_{4}(aq)-->CaSO_{4}(s)+2NaCl(aq)
2. The above equation obeys the Law of conservation of mass as the number of atoms of each element on the reactant side is equal to that of the product side. So the overall mass on reactant side will be equal to that on the product side.
CaCl_{2}(aq)+Na_{2}SO_{4}(aq)-->CaSO_{4}(s)+2NaCl(aq)
Mass of each reactant per mole:
(1 * Ca + 2 * Cl)=(1*40.08g/mol) + (2*35.45g/mol)=110.98g/mol
(2*Na)+(1*S)+(4*O)=(2*22.99g/mol)+(1*32.07g/mol)+(4*16.00g/mol)=142.04g/mol
Total mass on reactant side = (110.98+142.04) g/mol= 253.02g/mol
Mass of each product per mole:
(1*Ca)+(1*S)+(4*O)=(1*40.08g/mol)+(1*32.06g/mol)+(4*16.00g/mol)=136.14g/mol
(2*Na)+(2*Cl)=(2*22.99g/mol)+(2*35.45g/mol)=116.88g/mol
Total mass on product side = (136.14+116.88)g/mol=253.02g/mol
Total mass on reactant side = Total mass on product side =253.02g/mol
So, it obeys the law of conservation of mass as the total mass remains same before and after the reaction.
3. In a chemical reaction, mass is conserved that means the mass of reacting species is equal to the mass of the products formed. Similarly the total number of atoms of each element is also conserved in a chemical reaction.
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