Mole Concept And Its Value

       We have already learnt that entities of substances like atoms, ions, molecules, electrons etc are extremely very small in size and their numbers (atoms,molecules) is extremely very large even in a small substance. 

The word Mole is derived from a Latin word “moles” which means “heap” or “pile” which means heap of atoms or molecules. The word mole was used for the first time in 1967 to provide a simple way to calculate large number of atoms in a small substance.

Now, Mole is used as unit of measurement in the International System of Units (SI) for a substance. As we use pair, dozen, bunch etc to measure the quantity of the substance, mole is also used to measure the specific number of particles.

One mole of the substance is an amount of substance containing Avogadro’s number of particles. Avogadro number or Avogadro constant (N_A) is equal to 602,214,199,000,000,000,000,000. It can be simply taken as 6.022 x 10²³. 

A mole or Avogadro number is the number of particles that are exactly equal to number of atoms present in 12 grams of Carbon-12 isotope. 

Use Of Mole Concept In Calculations:

1.Molarity= Number of moles of solute/Volume of solution (litres)

2.Molality= Number of moles of solute/Weight of solvent (kg)

3.Mole Fraction(X) = Fraction of substance in a mixture expressed in mol

             X_A= n_A/(n_A+n_B); X_B= n_B/(n_A+n_B)

             X_A+X_B=1

        (n= number of moles)

Molar Mass And Molecular Mass:

Molar mass is defined as the mass of 1 mole of substance expressed in grams.

Molecular mass or molecular weight is defined as the sum of the atomic masses of atoms in the molecule. Molecular mass has unified mass units.

Example- H₂O

    Molar mass of water (H₂O)= 18 grams.

       18 grams water has 1 mole molecules of water which is 6.022 x 1023 molecules.

    Molecular mass of water (H₂O)= 18 u.

     18 u of water has only one molecule of water.

Atomic Mass And How It Is Measured

                John Dalton was the first scientist who postulated in his Dalton’s Atomic Theory that each element had a characteristic atomic mass. Atoms are extremely tiny particles which are even difficult to see through microscopes. It became difficult for scientists to measure their individual masses.

We have already seen about the formation of ions in the previous post in this blog. Similar to how ion is formed where atoms gain and loose electrons; even though the atoms are of less electrons it is still the same element.

In the same way, if an atom is losing a neutron or gaining a neutron that type of atom is called as ISOTOPE. They are still the same element.

Finally scientists thought of finding the atomic mass by comparing with standard atomic mass of some other element. Before 1961, the physicists and chemists considered two sets of atomic mass of Oxygen (atomic mass 16).

Chemists used naturally occurring oxygen which is the mixture of isotope of Oxygen-16, Oxygen-17 and Oxygen-18. Physicists used the isotope of Oxygen-16, because it given their measurement according to their mass spectrometry.

So it confused the scientists which isotope of oxygen should be taken for the mass comparison. In 1961, their confusion came to an end by instead taking the isotopes of oxygen or hydrogen, the isotope of Carbon was chosen as it contains 6 protons and 6 neutrons giving a mass of exactly 12. Carbon- 12 atomic mass is measured accurately compared with other elements in the periodic table.

So, now it is used as a standard reference to measure the atomic mass of other elements. Carbon-12 is divided into 12 equal parts where each part represents 1/12 of atomic mass of crabon-12.

Atomic Mass:

              The number of times one atom of given element is heavier than 1/12^th part of atomic mass of Carbon-12 is known as Atomic mass. Atomic mass of an element is defined as the average mass of all the isotopes of the element as compared to 1/12^th of the mass of one carbon-12 atom.

Atomic mass is measured in “amu” (atomic mass unit). But through latest recommendations of IUPAC, the amu has been replaced with “u” which is known as unified mass.

                        Atomic mass of some elements 

Compounds And Way To Write Formula For Compounds

         All the matter or substance in the world is made extremely tiny particles called ATOMS. Group of Atoms of same substance or different substance are combinedly called as “COMPOUNDS”.

Chemical formula for molecules or compounds is made by using criss-cross method by using its valency.

With example of compound Sodium Carbonate. The following steps are taken while writing the formula for every compound as of Sodium Carbonate.

1. First see the elements and the number of atoms present to that elements in the given compound or molecule.

2. Write the symbol of each atom in the given compound side by side usually cation first followed by anion. Ex- Na CO₃

3. Write the valency of each atom on the top of its element. Ex- Na¹ (CO₃)²

4. Divide the valency numbers with highest common factor to get simple ratios (like in given example among (1,2)- 1 is the highest common factor)Ex- Na¹ (CO₃)²

5. Interchange the valency of two elements and write the numbers to the lower right of the constituents as subscripts. Ex- Na₂(CO₃)₁

6. If the constituents receive number 1 then ignore it while writing the formula of compound.

7. If the constituents receive 2 or more than 2 atoms enclose it with in brackets and write in as subscripts. Ex- Na₂CO₃

Example-

Formula for some common compounds