Law Of Chemical Combination, Law Of Conservation Of Mass, Important Topics JEE 2025

Law of Chemical Combination : Welcome, young scientists, to the captivating realm of chemistry! As we delve into the mysteries of chemical reactions and transformations, we encounter foundational principles that shape our understanding of matter. One such principle, the Law of Conservation of Mass, stands as a cornerstone in the world of chemistry. In this article, we'll embark on a journey to unravel the secrets of this law, exploring its significance and implications in simple terms that resonate with students like you.

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Law of Chemical combination

Law of Chemical combination  : Atoine Lavoisier, John Dalton and other scientists formulated certain laws concerning the composition of matter and chemical reactions.  These laws are known as the law of chemical combination.

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Law of indestructibility of matter or conservation of Mass

• This law was proposed by Lavoisier in 1789.

• According to this law, in all physical or chemical changes, the total mass of the system remains constant or in a physical or chemical change, mass is neither created nor destroyed.  Thus, in a physical or chemical change.

Total mass of reactant reacted = Total mass of products formed.

• If reactants are not completely consumed then the relationship will be:

Total mass of reactants = Total mass of products + Mass of unreacted reactants.

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Example Of Law of Chemical combination

Ex.1

2 71 2(1 + 35.5)

2 + 71 = 73 g 73 g

Ex.2

Above reaction shows the physical change and the wt. of H ₂ O _(s) = Wt. of (H ₂ O) _{( l )} . In case the reacting materials are not completely consumed, the relationship will be Total masses of reactants = Total masses of product + masses of unreacted reactants

Important Points To Remember

• Nuclear reactions are exception of law of conservation of mass.  In nuclear reaction mass + energy is conserved.

• According to the modern views, the law of conservation of mass is not always valid.  The law hold good only in case of such chemical reactions where there is no evolution of heat or light.

• During chemical processes, the loss of mass is negligible.  But in nuclear reactions, tremendous amount of energy is evolved.  Consequently, the change in mass is quite significant.  Thus, it is clear that the law of conservation of mass and law of conservation of energy are two ways of looking at the same law.

• Thus, combining the two we get general law known as law of conservation of mass energy.  It states that, Mass and energy are inter convertible.  But the total sum of mass and energy of the system remains constant.

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Solved Example Of Law of Chemical combination

Q.1 : When 4.2 g NaHCO ₃ is added to a solution of CH ₃ COOH weighing 10.0 g, it is observed that 2.2 g CO ₂ is released into atmosphere.  The residue is found to weigh 12.0 g.  Show that these observations are in agreement with the law of conservation of mass.

Sol.

Initial mass = 4.2 + 10 = 14.2

Final mass = 12 + 2.2 = 14.2

Thus, during the course of reaction law of conservation of mass is obeyed.

Q2. 10 g of CaCO ₃ on heating gives 4.4 g of CO ₂ then determine weight of produced CaO in quintal.

Ans. Total mass of reactant = 10 g

Mass of CO ₂ = 4.4 g

Mass of produced CaO = x

According to law of conservation of mass

10 = 4.4 + x

10 –4.4 =  x

x = 5.6 g

∴ 1 quintal = 100 kg

∴ 1 kg = 1000 g

quintal

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