MOLE CONCEPT

 

THE MOLE CONCEPT

INTRODUCTION 

In ordinary life items are counted using a named collection of them or counting units. For example, farmers eggs in dozens (12), shop keepers sell sheets of papers in reams (500 papers), Elementary chemical particles (atoms, ions, molecules, electrons, etc)are very small.  Chemist therefore, for convenient reasons count them by weighing a collection of them. The counting unit in chemistry is called THE MOLE (6.02X1023particles).

The number of atoms in exactly 12 grams of carbon-12 isotope is chosen as the standard. 

What is a mole?

The mole of a substance is the amount of substance that contains the same number of elementary particles which are (atoms, ions, molecules) as there are in exactly 12 grams of carbon-12 isotope. 

Mole=mass/molar mass

In the definition of mole above,  we see that:

1 mole of hydrogen atoms, H, contains 6.02x1023 hydrogen atoms, H.

1 mole of oxygen molecule, O2 , contains 6.02x1023 oxygen molecules, O2.

1 mole oxygen atom, O, contains 6.02x1023 oxygen atoms, O.

1 mole of electrons, e- contains 6.02x1023 electrons.

NOTE: It is always important to refer to what the particle is. For example, “1mole of oxygen”. This could refer to either oxygen atom (O) or oxygen molecule (O2). It is always safest to quote the formula to which the amount of substance refers. 

RELATIVE ATOMIC MASS (Ar)

Reasons why carbon-12 isotope is chosen as the standard for defining relative atomic mass

-Carbon is a very common element.

-The carbon-12 isotope is very stable.

-Carbon is a solid and easily handled and transported.

On the carbon-12 scale, atoms of the carbon-12 isotopes are assigned a relative atomic mass of 12. The relative atomic masses of all other elements are obtained by  comparing with the mass of a carbon-12 atom. 

Definition of relative atomic mass

The relative atomic mass of an element is the number of times the average mass of one atom of it is heavier than one-twelfth the mass of one atom of carbon-12 isotope.

     Ar=mass of one atom of an element /(1/12 the mass of one atom of carbon-12)

NB: Relative atomic mass has no unit since it is a ratio.

RELATIVE MOLECULAR MASS (Mr)

The relative molecular mass of a compound id the number of times the average mass of one molecule of it is heavier than one-twelfth the mass of one atom of carbon-12.

Relative molecular mass=(mass of one molecule of a compound)/(1/12 the mass of one atome of carbon-12)

A vast number of compound consist of ions and not molecules. The compound sodium chloride, for example, consist of sodium and chlorine ions. One cannot correctly refer to a “molecule of sodium chloride”. For example ionic compounds, the term formula unit is used to describe the ions which make up the compound. The formula unit sodium chloride is NaCl and the formula mass is 58.5. however, the term relative molecular mass is often used because most compounds are neither entirely ionic nor covalent.

Relative formula mass=(mass of one formula unit)/(1/12 the mass of one atom of carbon-12)

MOLAR MASS

The molar mass of any substance is the mass of one mole of the substance. The symbol is M. The term molar mass is applied to both compound and elements. The molar mass of any element is the relative atomic mass expressed in grams.  The unit of molar mass is gram per mole (gmol-1). For example the molar mass of copper is 63.5gmol-1 .The molar mass of sodium hydroxide is 40gmol-1.

AVOGADRO CONSTANT (L)

The molar mass of all elements contains the same number of atoms. The mass of a single carbon-12 atom from mass spectrometry is 1,993x 10-23 g. the number (L) of carbon atoms in one mole of carbon is given by

L=(molar mass)/(mass of one atom of C-12 )  =(12g/mol)/█(1.993x10 23g)  

L = 6.02x 1023mol-1

This value 6.02x 1023mol-1 is known as the Avogadro constant. 

The Avogadro constant is the number of atoms in exactly 12g of the carbon 12 isotope.

We can count out 6.02x 1023mol-1 atoms of each element by weighing out the relative atomic mass.

 

CALSULATIONS OF RELATIVE MOLECULAR MASS OF COMPOUNDS

The relative molecular mass of a compound is the sum of the relative atomic masses of all atoms in one molecule of the compound. 

Example