Mole concept: Elements and Compounds

Key Concepts

• Matter

• Dalton’s atomic theory

• Mole

• Relative atomic mass

• Atomic mass

Introduction

In 1808, John Dalton proposed and gave a few explanations that: 

• Matter consists of indivisible atoms.  

• Atoms of a given element have different properties, including different masses.  
• Compounds are formed when atoms of other elements combine in a fixed ratio.  
• Atoms can neither be created nor destroyed. 

Law of Conservation of mass: 

The law of conservation of mass was first given by Lavoisier, and he stated that “Matter can neither be created nor destroyed.” This defines that the mass of the reactants before the reaction equals the mass of products after the reaction. 

Law of definite proportion: 

The law of constant or definite proportion was given by “Proust.” He states “All the chemical compounds are found to be in a fixed or constant composition irrespective of the method of preparation.” 

Law of Multiple proportions: 

The law of multiple proportions was given by “Dalton”. When one element combines with another to generate two or more distinct compounds, the mass of one element bears a simple ratio to the mass of the other. 

Law of Reciprocal Proportion:  

The law of Reciprocal proportion was “Richter.” When elements combine individually with a definite mass of a third element, the ratio of their masses at the time of the combination is either the same or a whole number multi of the ratio at the time of the combination. 

Explanation:

Mole Concept: 

The mole concept is a convenient way to express a substance’s quantity. Any measurement is made up of two parts: the numerical magnitude and the units in which the magnitude is expressed. When the mass of a ball is 2 kilogram, for example, the magnitude is ‘2’ and the unit is ‘kilogram.’ 

The mole concept is commonly applied in the context; that is, it focuses mostly on the mole, which really is a count of a huge number of particles. 

Mole Concept: One mole is nothing but the collection of many species as there are in a number of atoms exactly in 12 gms of C-12 isotope. 

1 mole = collection of 6.02 × 10²³ species = N_A= Avogadro’s number 

1 mole of an atom is also named as 1gm-atom, 1 mole of an ion is also named as 1gm-ion.  

1 mole of the molecule is also named as 1 gram molecule.  

The number 6.023 × 10²³ which is usually called Avogadro’s number, which is denoted with N_A. 

Atoms, molecules, monoatomic/polyatomic ions, and other particles are examples of fundamental particles that are represented in moles (such as electrons). 

For example, one mole of pure carbon-12(¹²C) will have a mass exactly equal to 12 grams and that which contain 6.023 × 10²³ number of atoms 

1 mole = 6.023 × 10²³ molecules = 6.023 × 10²³ atoms = 6.023 × 10²³ electrons 

The relation of the mole is similar to the use of 1 dozen, which represents 12 quantities as we take 12 as the reference for finding out the quantity. 

The number of moles in a pure sample can be represented as

where n = no of moles,  
N = no of elementary species and N_A = Avogadro’s Number 

Example:  

1. How much is one mole of iron? 

One mole of iron is 6.023 × 10²³ iron atoms as it has a mass of  grams/mole 

2. How much is one mole of water? 

One mole of water = 6.023 × 10²³ water molecules 

Mass is 18 grams of water, and volume is 18 milliliters of water. 

How Big Is A Mole? 

The volume occupied by one mole of softballs would be about the size of the earth. One mole of Olympic shot-put balls has about the same mass as the earth. 

Relative atomic mass: 

Relative atomic mass or relative atomic weight is one of the most important concepts that has come out from Dalton’s atomic theory. Dalton first used hydrogen(H) as the reference atom. Later, oxygen(O) is taken as a reference in place of hydrogen. So, therefore, the one carbon atom 

The number of moles in a pure sample can be represented as

where n = no of moles, N = no of elementary species and N_A= Avogadro’s number 

From 1961, internationally, the standard unit is based on the mass of the carbon-12 atom. 

Relative atomic mass (RAM) =

Mass of one atom of an element × 1/12 X mass of one carbon atom

Mass of one atom of an element112 × mass of one carbon atomMass of one atom of an element112 × mass of one carbon atom

Relative atomic mass is also related to isotopes. The relative atomic mass of an element is the average value for the isotopes of the element. It takes into account the percentage abundance of isotopes. 

Isotopes are atoms that have the same atomic number but a different mass number. 

For example: Isotope of chlorine. It has two isotopes they are ³⁵Cl and ³⁷ Cl with 75% abundance and 25% abundance, respectively. 

³⁵Cl has 18 neutrons, 17 electrons and 17 protons 

³⁷Cl has 20 neutrons, 17 electrons and 17 protons 

 Relative atomic mass= (75% × 35) + (25% × 37) = 35.5   

Atomic mass unit: 

Atomic mass of an element is defined as the number which indicates how many times the mass of one atom of element is in comparison to

112112

^th part of the mass of one carbon C-12 atom. 

AMU =
Mass of atom of element / 1/12 mass of atom of c−12 atom

Atomic mass unit =
1/12 mass of atom of c−12 atom isotope

1amu =1.6605 × 10^-24 kg 

112×126.023×1023112×126.023×1023

1 amu = 1.6605 × 10^-24 kg 

The masses of proton and neutron ≌ 1 amu 

AMU = RAM × 1 amu 

1 Amu is defined as the mass which is exactly equal to 

1/12^th part of the carbon atom. 

Example:  

1. What is the atomic mass unit of hydrogen? 

1. Hydrogen atom is 8.400% of carbon-12 

2. Hydrogen atom mass unit =0.08400 × 12=1.008amu 

2. How many amu are there in 27g of Hg? 

27gm ×1 amu1.66×10−24gm=1.62×1025gm of Hg27gm ×1 amu1.66×10−24gm=1.62×1025gm of Hg

Methods for calculation of mole: 

1. Number of moles= Given  no / NA

2. Number of moles= Given mass / Atomic weight

3. If the volume of gas is given along with temperature(T) and pressure(P) use n = PV/RT

Where R = 0.0821 lit-atm/mol-K (when P is in atmosphere and V is in liter.) 1 mole of any gas at STP occupies 22.7 liter. 

Gram atomic mass:  

The atomic mass of any substance which is expressed in grams is known as Gram atomic mass. 

Gram atomic mass = Mass of 6.02 × 10²³ atoms = mass of one mole atoms. 

For example: Oxygen atom 

AS we know that atomic mass of ‘O’ atom= Mass of ‘O’ atom 

Gram atomic mass = mass of 6.02 × 10²³ of ‘O’ atom 

                                    = 16amu × 6.02 × 10²³ (mass of ‘O’ atom= 16amu) 

                                    = 16 × 1.66 × 10^-24 × 6.023 × 10²³(1 amu = 1.66 × 10^-24 Kg) 

                                    = 16gm (1.66 × 10^-24 and 6.023 × 10²³ = 1) 

Comparison of RAM, AMU and GAM 

Relationship between GRAM and AMU: 

1 amu = 1/12weight of one C-12 atom 

For ‘Carbon(C)’ 1mole ‘C’ =12 g = 6.023 × 10²³ atoms 

Weight of 6.023 × 10²³ atoms = 12 grams 

Weight of 1 ‘C’ atom=12/Na gm ( N_A =Avogadro’s number = 6.23 × 10²³

1amu = 1/12 Weight of C-12 atom

Weight of C-12 atom 

1amu =

1/12×12/N_Ag

1amu =

1/N_Ag

Did You Know? 

We must celebrate! Mole Day (October 23^rd) is upon us! On that day, chemistry students around the world celebrate the mole from 6:02 am to 6:02 pm.  

Mole Day is 10-23 because of Avogadro’s Number, 6.022 × 10²³ (named in honor of Italian scientist Amedeo Avogadro), which represents the number of particles contained in one mole of any substance. 

Summary

• Law of Conservation of mass:This defines that the mass of the reactants before the reaction is equal to the mass of products after the reaction.

• Law of defmite proportion: All the chemical compounds are found to be in a fixed or constant composition irrespective of the method of preparation.

• Law of Multiple proportions: When one element combines with another to generate two or more distinct compounds, the mass of one element bears a simple ratio to the mass of the other.

• Law of Reciprocal Proportion: When elements combine individually with a definite mass of a third element, the ratio of their masses at the time of the combination is either the same or a whole number multi of the ratio at the time of the combination.

• One mole is nothing but the collection of many species as there are in a number of atoms exactly in 12gms of C-12 isotope.

• The number 6.023 x 1023 which is mostly called Avogadro’s number, which is denoted with NA.

• The number of moles in a pure sample can be represented as n =  where n = no of NA moles, N = no of elementary species and NA= Avogadro’s Number.

 • The atomic mass of any substance which is expressed in grams is known as Gram atomic mass.

• Grain atomic mass = Mass of 6.02 x 1023 atoms = mass of one-mole atoms. Mass of one atom of an element

• Relative atomic mass (RAM)— Mass of one atom of an element / 1/12 x mass of one carbon atom

• Atomic mass of an element is defined as the number which indicates how many times the mass of one an atom of element is in comparison to 12th part of the mass of one carbon C-12 atom.

• Isotopes are atoms that have the same atomic number but a different mass number.