Law of Conservation of Mass

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Dalton’s Atomic Theory:

Assumptions of Dalton’s Atomic Theory:

  • Every element is made up of extremely small particles called an atom.
  • The atoms are indivisible and they can neither be created nor be destroyed. Atoms of the same element resemble each other in all respects but differ from the atoms of other elements.
  • When chemical compounds are formed they do so by the combination of atoms of different elements in a simple proportion of whole numbers.
  • Atoms of different elements may combine in more than one proportion to form different compounds.
  • The combining weights of the elements represent the combining weights of the atoms.


Limitations of Dalton’s Atomic Theory:

  • Now it is proved that the atom consists of particles like electrons, protons, and neutrons. Thus the assumption of Dalton’s theory that the atom is not divisible is false.
  • Discovery of isotopes revealed that the atoms of the same element possess different weights. Thus the assumption of Dalton’s theory that the atoms of the same element are identical in all respect is false.

The Review of Dalton’s Atomic Theory:

  • Every element is made up of extremely small particles called an atom. The atoms are indivisible.
  • Modern Concept: Atoms are not the smallest particles. They can be further divided into subatomic particles like protons, neutrons, and electrons.
  • Atoms can neither be created nor be destroyed.
  • Modern Concept: With the advent of nuclear chemistry, atoms of one element can be converted into atom of another element by transmutation.
  • Atoms of the same element resemble each other in all respects;
  • Modern Concept: With the knowledge of isotopes we can have atoms of same elements having the same atomic number but different mass number. Thus atoms of the same element may not resemble each other in all respects;
  • Atoms of the different element differ from the atoms of other elements.
  • Modern Concept: There are certain elements; with the difference in their atomic number have the same mass number. Thus atoms of different elements may resemble in some properties
  • Dalton didn’t consider the mass-energy relation. According to Einstein’s theory in a chemical reaction, some mass is getting converted into energy.

Law of Conservation of Mass Lomonosov

  • Mikhail Vasilyevich Lomonosov (19 November 1711 – 15 April 1765) was a Russian polymath, scientist, and writer, who made important contributions to literature, education, and science.
  • A lunar crater bears his name, as does a crater on Mars. In 1948, the underwater Lomonosov Ridge in the Arctic Ocean was named in his honor. Moscow State University was renamed ‘’M. V. Lomonosov Moscow State University’’ in his honor in 1940.
  • The Lomonosov Gold Medal was established in 1959 and is awarded annually by the Russian Academy of Sciences to Russian and foreign scientists.

Law of Conservation of Mass:

  • The law of conservation of mass was given by Russian scientist Lomonosov in 1765 and French Scientist Antoine Lavoisier in 1783 independently.
  • This law is also called as the law of indestructibility of matter.

Statement of Law of Conservation of Mass:

  • In a chemical reaction, the total mass of the reactants before the reaction is the same as the total mass of the products after reaction. i.e. in a chemical reaction, the total mass is conserved.

Explanation Law of Conservation of Mass:

Consider following chemical reaction

A  +  B   →   C   +   D

Let MA = Mass of reactant A



MB = Mass of reactant B

MC = Mass of product C

MD = Mass of product D

By law of conservation of mass



Total mass of reactants = Total mass of products

MA + MB =  MC + MD

lllustration of Law of Conservation of Mass:

Consider following chemical reaction

NaCl  + AgNO3     →  NaNo3  + AgCl ↓

The total mass of reactants   = ( 23×1 + 35.5×1)  +  (108×1 + 14×1 + 16×3)=  228.5



The total mass of products    = (23×1 + 14×1 + 16×3) + (108×1 + 35.5) = 228.5

Thus, The total mass of reactants is equal to the total mass of products. Thus, the law of conservation of mass is illustrated.

Landolt’s Experiment:

Law of Conservation of Mass Landolts Apparatus

  • Landolt used this experiment to verify the law of conservation of mass. He used H – Shaped tube for his experiment. One arm (limb) of the apparatus is filled with sodium chloride solution and the other arm is filled with silver nitrate solution.
  • The tube is then sealed and weighed accurately and carefully so that the two solutions do not mix with each other.
  • Then the apparatus is well shaken and the two solutions are allowed to mix and react with each other white precipitate of AgCl is obtained.

NaCl  + AgNO3     →  NaNo3  + AgCl ↓

  • The apparatus is weighed accurately again. Within experimental limit, it is found that the weight of the apparatus before the reaction is equal to the weight of the apparatus after the reaction. Thus the total mass of reactants is equal to the total mass of the products. Thus the total mass is conserved. Thus the law of conservation of mass is verified.


Limitations of Landolt’s Experiment:

  • The reaction between sodium chloride and silver nitrate is exothermic. Due to which evaporation of the moisture on the outer surface of the apparatus takes place and this takes a long time to recondense back on the apparatus.
  • There is an increase in the volume of the apparatus, due to the evolution of heat. It takes a long time to get original volume. Due to increase in the volume the up thrust of air on the apparatus increases and hence it shows less weight.
  • These errors can be minimized by weighing the apparatus after very long time allowing the apparatus to cool thoroughly.

Limitations of the Law of Conservation of Mass:

  • The law of conservation fails for chemical reactions in which a large amount of heat is evolved or absorbed during the reaction.
  • During a chemical reaction, energy is evolved or absorbed in the form of heat, light etc. During the evolution of heat, some mass of reactants is converted into energy. But this loss of mass is so small that it cannot be recorded on very sensitive balance.
  • According to Einstein’s theory, mass and energy are interconvertible. The relation between the two quantities is given by E = mc² Where, E = energy liberated, m = mass lost in the reaction. c = speed of light in vacuum Thus the law of conservation of mass is valid if both the mass and the energy are conserved.
  • Combined Law of Conservation of Mass and Energy: In a chemical reaction, the total amount of mass and energy of reactants is equal to the total amount of mass and energy of products i.e. In a chemical reaction the total amount of mass and energy are conserved.
  • Law of Conservation of Energy: Energy can neither be created nor destroyed, but it can be converted from one form to another. i.e.  the total amount of energy of the universe is conserved.

Explanation of  the Law of Conservation of Mass on the Basis of Dalton’s Atomic Theory:

  • According to atomic theory, atoms can neither be created nor be destroyed.
  • In chemical reactions, the atoms rearrange, but they do not themselves break apart. Since each atom has a definite mass, the total mass after chemical change remains constant and this explains the law of conservation of mass.

Applications of Law of Conservation of Mass:

Example -1:

  • 10.0 g of CaCO3 on heating gave 4.4 g of CO2 and 5.6 g of CaO. Show that these observations are in agreement with the law of conservation of mass.

    Solution:

CaCO3 → CaO +     CO2



Mass of reactants = 10.0 g

Mass of products = 5.6 g + 4.4 g  =  10.0 g

Thus,  Mass of reactants = Mass of products.

Hence the observations are in agreement with the law of conservation of mass.

Example -2:

  • x g of potassium chlorate on decomposing produced 1.9 g of oxygen and 2.96 g of potassium chloride. What is the value of x?

    Solution:

KClO3 →  KCl +     O2



Mass of reactants = x g

Mass of products =  4.86 g

Now, by the law of conservation of mass. Mass of reactants = Mass of products.

x = 4.86



The value of x is 4.86

Example – 3:

  • When 4.2 g of NaHCO3 is added to a solution of CH3COOH weighing 10 g; it is observed that 2.2 g of CO2 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.
  • Solution:

NaHCO3  + CH3COOH → Residue  + CO2

Mass of reactants = 4.2 g + 10 g = 14.2 g

Mass of products = 12.0 g + 2.2 g = 14.2 g

Thus,  Mass of reactants = Mass of products.

Hence the observations are in agreement with the law of conservation of mass. Now, by the law of conservation of mass.



Example – 4:

  • When 6.3 g of NaHCO3 is added to 15.0 g  solution of CH3COOH; the residue is found to weigh 18.0 g. What is the mass of CO2 released in the reaction.
  • Solution:

NaHCO3  + CH3COOH → Residue  + CO2

Mass of reactants =  6.3 g + 15.0 g = 21.3 g

Mass of products =  18.0 g + Mass of   CO2

Now, by the law of conservation of mass. Mass of reactants = Mass of products.



∴  21.3 g  =   18.0 g + Mass of   CO2

∴   Mass of   CO2 = 21.3 g – 18.0 g = 3.3 g

Thus mass of CO2 released is 3.3 g.

Science > Chemistry > Basic Concepts of ChemistryYou are Here
Physics Chemistry  Biology  Mathematics

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