Mole Concept IIT JEE

Avagadro's Number and Mole Concept for JEE Chemistry

What is Mole?

Molecules and atoms are so small in size that they can’t be counted. In order to solve the issue, they are expressed in the form of Avogadro’s number NA = 6.022×1023. Mole is the number that is equivalent to Avogadro’s number. It is defined in the form of a unit that represents “6.023 x1023” particles of the same matter.

The symbol “mol” is defined in the form of the amount of a substance that comprises of as many molecules, atoms, electrons, ions, or any elementary entity as the number of carbon atoms present in 12 grams of the 12C isotope. So, it is the number of atoms in twelve grams of the 12C isotope. It is called Avogadro’s number.

What is the need for the invention of mole concept?

Sometimes it becomes difficult when we are dealing with particles that are very small in size just like the size of molecules and atoms. As these particles are present in large numbers, it becomes very tough to perform the calculations.

Even when seen in a tiny sample of a chemical compound or an element, the numbers of particles are present in the ascending order of “10”. In order to simplify the calculations, the concept of mole got invented. While solving issues, the numbers is expressed in the form of an Avogadro number. This has made it simpler to arrive at the solution of physical chemistry problems.

The number of moles present in a substance is calculated by several means that depends on the available data, as follows.

  • • Number of moles present in a molecule

  • • Number of moles present in an atom

  • • Number of moles present in gases (The standard value of “molar volume” at STP is 22.4 liters)

  • • Number of “moles” of particles such as molecules, ions, atoms, etc.

  • For any compound, y number of moles of A particle = x number of moles of B

    So, the entire concept of mole can be written as:
    Mole fraction = portion of the element present in the mixture statedwith respect of mol. It is known as mol fractionX.

    How is equivalent weight of a substance determined?

    Equivalent weight of any substance / element or a compound is defined in the form of a “The quantity of a substance by weight, that combines with or displaces indirectly directly 1.008 portions by mass of hydrogen, eight parts by mass of oxygen, 35.5 parts by mass of chlorine or the equivalent portions by mass of any other element”. This equivalent weight of a substance is based on the type of chemical reaction in which it is involved.

    As equivalent weight is represented as a relative quantity, there is no unit assigned to it. When equivalent mass of a specific substance is represented in the terms of grams, then it is referred to as called GWE or Gram Equivalent Weight.
    In order to calculate equivalent weight of a substance, we will be using the below formula:

    Equivalent weight = the ratio of Molar mass and Valence factor
    Here in the above equation,

    Valence factor for a base is equal to the acidity of that base. This means if the base is acid, then Valence factor for any acid is equal to the basicity of that acid. If the base is an element, then Valence factor for that element is equal to its valency.

    In what ways did Avogadro come up with the concept of mole?

    The concept of Avogadro's Number was proposed by Amadeo Avogadro. He first anticipated that the amount of any gas at a certain temperature and pressure is directly proportional to the number of molecules or atoms, irrespective of the kind of gas. 
    Though Avogadro didn’t come up with any exact proportion, he was credited for his idea. Avogadro's number is defined as 6.022×1023 molecules.

    How is the mole and Avogadro's number related to each other?

    Avogadro's number NA is a sort of proportion that shows a relation between physical mass present on a specific human scale and molar mass on a specific atomic scale. It is the number that definesthe number of particles such as molecules, compounds, atoms, etc. present in one mole of any substance. The value of Avagadro Number is 6.022×1023 mol-1. It is expressed in the form of a symbol NA.

    What do you mean by Mole Day?

    The mole day is celebrated internationally on October 23 of every year from 6:00 a.m. to 6:00 p.m. Mole Day honours the contribution of the scientist Amadeo Avogadro in bringing the concept of Avagadro Number (6.02 x 1023) and benefitting the entire science industry.

    A lot of inventions and theories are developed with the help of this number. It is also useful as a fundamental measuring unit in the area of chemistry. Apart from this, celebration of the mole day is one of the ways to foster the interest of people in chemistry.

    What is the significance of a mole concept is the field of chemistry?

    The mole is significant for people working in the chemical industry. It enables them to deal and work with the macro world units, its amounts and the entire subatomic world. Atoms, formula and molecules are very small units and also very tough to work with. By defining the mole, it helps the scientists to allow them to convert moles to its equivalent particles or grams to its equivalent moles.

    How did the concept of mole come into existence?

    One of the basic and the most important questions in chemistry is “what is implied by one mole of a specific substance”. To understand it, let us look closely at the concept of mole. The term “molar” is a Latin word that means “a large mass”. This term was introduced in chemistry by August Wilhelm Hofmann.

    He was a German chemist who in the period between 1818 and 1892 came up with this term. In the period around 1940, the usage of this term was increased. However, there is another related concept to molar mass, it is called as an equivalent mass. It has been in use since a century. The “mole” was regarded as the 7th SI base unit in the year 1971 by the fourteenth CGPM.

    What do you mean by a mole concept and molecular mass?

    Now let us talk about how mole concept is related to molecular mass. One mole is represented as the quantity of a substance that has as many entities or particles in it as the number of atoms present in precisely 12 grams or 0.012 kg of the “12C isotope”. The mass of a mole of any substance when specified in terms of grams is known as its molar mass.

    Mole is one of the commonly used terms to perform measurement in chemistry. This term is a standard means to express the quantity of any chemical substance. The total number of molecules contained in one single mole (1 gram of molar weight) is referred to as an Avogadro's number.

    This number is also called as Avogadro's constant in remembrance of the brilliant theories developed by Avogadro. The value of Avogadro's number is experimentally determined as 6.023x1023 molecules / gram-mole.

    Why is mole represented as 6.022 x10 23?

    Avogadro recommended that when equal volumes of gases are combined at the same pressure and temperature conditions then they possess the same number of molecules in them. 6.022 x 1023 atoms of oxygen weigh 16.00 grams. In this way, it can be stated that the mole is related to the atomic mass of an element.

    For what reason is “carbon 12 used for the definition the mole?

    Physicists selected the pure form of isotope Oxygen-16, as they wanted to perform their measurements solely on the foundation of mass spectrometry. In place of using Oxygen or Hydrogen, as the standard, they had chosen an “isotope of Carbon” that comes with six protons and six neutrons in the nucleus. Carbon-12 was assigned a molar mass of 12.

    Applications of Mole Concept:

    The use of defining mole is not just limited to only a number. It is used in several different calculations. Moles are defined for different conditions and states of matter. This concept is useful to perform several different types of calculations such as:

  • • To calculate the number of moles present in molecules. It is calculated as weight in grams / molecular mass

  • • To calculate the number of moles, present in atoms. It is calculated as Weight in grams / atomic mass

  • • To calculate the number of moles, present in gases. It is calculated as Volume of gas at STP / Std. molar volume

  • • To calculate the number of moles, present in particles. It is calculated as Number of particles / NA or Avogadro number

  • Moles are used to define the concentration of a solution with respect to its molality, molarity, and mass fraction.

  • • Molarity is defined as the number of moles present in a solute/Vol. of solution in liters

  • • Molality is defined as the number of moles present in a solute/Wt. of solvent present in kg

  • • Mole fraction is defined as a fraction of a substance present in the mixture when expressed with respect to mol is known as mol fraction (X)

  • Another application of the mole in chemistry is to determine the molecular and empirical formula used for balancing redox reactions. It is a good way to determine the stoichiometry of any chemical reaction.