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# Principle of Atom Conservation with Examples for JEE

Last updated date: 28th May 2024
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## Principle of Atom Conservation

Atoms are conserved in chemical reaction, hence moles of atoms must be conserved as well. This is known as the atomic conservation principle. The Law of Conservation of Mass is used to generate the POAC: Principle Of Atom Conservation. The mass of an element's atoms in the reactant equals the mass of an element's atoms in the product. Moles of atoms of an element in a product = Moles of atoms of an element in the reactant.

This technique can be applied to practically any stoichiometric calculations to solve difficulties. It is based on the conservation of a mole of an atom or mass of an atom. If the POAC rule is obtained, the chemical equation does not need to be balanced.

## Example of POAC Method

$\mathrm{CaCO}_{3} \rightarrow \mathrm{CaO}+\mathrm{O}_{2}$

POAC formula for Ca atoms

Moles of Ca atoms in CaCO3 = Moles of Ca atoms in CaO

1 mole of CaCO3 contains 1 mole of Ca atoms and 1 mole of CaO contains 1 mole of Ca atoms

${ }^{\mathrm{n}_{\mathrm{CaO}}}={ }^{\mathrm{n}_{\mathrm{CaCO}_{3}}} \\$ $\dfrac{\mathrm{w}_{\mathrm{CaO}}}{\mathrm{m}_{\mathrm{CaO}}}=\dfrac{\mathrm{w}_{\mathrm{CaCO}_{3}}}{\mathrm{~m}_{\mathrm{CaCO}_{3}}}$

Furthermore, the number of reactions and the order in which they occur from reactants to products is not essential. It's important to note that POAC can be used for atoms that are preserved during a chemical reaction. Many POAC numerical problems can be solved by this method.

## Mole concept

The mole concept is a practical way of expressing a substance's quantity. A simple measurement value has two parts: the numerical magnitude and the units in which the numerical magnitude is expressed. The magnitude is '2' and the unit is 'kilogramme' when the mass of a ball is 2 kilograms. Even one gram of a pure element is known to contain a large number of atoms when dealing with particles at the atomic (or molecular) level. The mole concept is commonly employed in this context. It focuses mostly on the mole,which is a count of A mole is the amount of a material that includes exactly 6.02214076 $\times$ 1023 of the substance's 'elementary units.'

The Avogadro constant is the quantity 6.022$\times$1023 that is frequently denoted by the symbol 'NA'. Atoms, molecules, monatomic/polyatomic ions, and other particles are examples of entities that can be represented in moles (such as electrons).

One mole of pure carbon-12 (12C) has a mass of exactly 12 grams and contains 6.02214076$\times$1023 (NA) number of 12C atoms, for example. The following formula can be used to calculate the number of moles of a compound in a pure sample:

$\dfrac{N}{N_A}=n$

N is the total number of atoms of the substance (or elementary entity), and n is the number of moles of the substance (or elementary entity), NA is avogadro's number a very large number of particles.

## Concept of Limiting Reagent

When one of the reactants of a chemical reaction is low, the process abruptly comes to a stop. To calculate the amount of product produced, decide which reactant will limit the chemical reaction (the limiting reagent) and which reactant will be in excess (the excess reagent). Calculating the amount of product that each reactant can make is one technique to discover the limiting reagent; the one that gives the minimum product is the limiting reagent. The reactant present in less amount or is completely used up in reaction is the limiting reagent.

The compounds that are totally consumed in the completion of a chemical reaction are known as limiting reagents. They're also known as limiting reactants or limiting agents. The stoichiometry of chemical processes states that a specific number of reactants is required to complete the reaction. Take a look at the following ammonia formation reaction:

$\mathrm{N}_{2}+2 \mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2}$

2 moles of oxygen gas must react with 1 mole of nitrogen gas to produce 2 moles of NO2 in the above mentioned reaction. But what if there are only 1 mole of oxygen gas and 1 mole of nitrogen available during the reaction?

In that situation, the complete amount of nitrogen cannot be used (since 2 moles are required for the entire amount of nitrogen).The entire amount of nitrogen cannot be used in this case (because the entirety of nitrogen requires 2 moles of oxygen gas to react). As a result, the limiting reagent for this reaction is oxygen gas.

### Summary

In chemical reactions, atoms are conserved, so moles of atoms must be conserved as well. This principle states that the total amount of an element in an isolated system will remain the same over time. The law of conservation of mass is used to generate the POAC principle of atomic conservation.The mass of the atoms of an element in the reactant is equal to the mass of the atoms of an element in the product.The molar concept is a convenient way of expressing the amount of a substance. It focuses mainly on the mole, which is the amount of a substance. A mole is an amount of material that includes exactly 6.022$\times$1023 elementary units of matter.

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## FAQs on Principle of Atom Conservation with Examples for JEE

1. How to find a limiting reagent?

When only two reactants are present, write a balanced chemical equation and determine the amount of reactant B necessary to react with reactant A. Reactant A is the limiting reagent when the amount of reactant B is greater. The limiting reactant is the one that is present in less than the amount required by stoichiometry.

The amount of product formed by each reactant is determined in an alternative technique of determining the limiting agent. The limiting reactant is the one that produces the lowest amount of product.

2. Define law of conservation of mass.

The law of mass conservation states that

"Mass cannot be generated or destroyed in an isolated system, but it can be converted from one form to another." For a low-energy thermodynamic process, the mass of the reactants must equal the mass of the products, according to the law of conservation of mass. Mass conservation is thought to be defined by a few assumptions from classical mechanics. With the help of quantum mechanics and special relativity, the law of conservation of mass was later changed to say that energy and mass are one conserved quantity.