MOLE

The mole is the amount of a substance that contains the same number of particles as the number of atoms in exactly 12g of carbon-12. The elementary particles can be atoms, molecules, ions, electrons or any other. The number of atoms in a mole of any element is 6.02 x 1023. It is called Avogadro’s number. Hence, one mole of any substance contains 6.02 x 1023 elementary particles of that substance. Thus,

1 mole of an element contains 6.02 x 1023 atoms of the element

1 mole of a compound or molecule contains 6.02 x 1023 molecules

1 mole of an ion contains 6.02 x 1023 ions

The relative atomic mass of an element = mass of 1 mole of the element

The relative molecular mass of a compound or molecule = mass of 1 mole of the compound or molecule

The number of ions present in an ionic compound is obtained by summing up the amount of individual ions present in the compound.

By definition:

Mole = Reacting mass

Molar mass

Mole = Number of particles

6.02 x 1023

:- Reacting mass = Number of particles

       Molar mass X 6.02 x 1023

Examples

1. What is the mass of 3 moles of oxygen gas? (O=16)
2. How many atoms are present in 5g sample of sodium (Na=23, N=6.02 x 1023.)
3. How many ions are there in 1 mole of calcium chloride?

Solution

1. Mass of 1 mole of O2 = (2×16) = 32g

Mass of 3 mole of O2 = (3×32) = 96g

Mole = Reacting mass = 5 = 0.217

     Molar mass        23

But the number of moles = the number of particles (atoms)

Avogadro’s number

:. 0.217 = number of atoms

6.02 x 1023.

Number of atoms = 0.127 x 6.02 x 1023 =1.3 x 1023 atoms

1 mole of CaCl2 contains: 1 mole of Ca2+ and 2 moles of Cl–

1 mole of an ion contains 6.02 x 1023 ions

:- 1 mole of CaCl2 contains (1 x 6.02 x 1023) + (2 x 6.02 x 1023) ions = 18.06 x 1023 ions

Molar mass: The molar mass of any substance is the mass of one mole of the substance expressed in grams. Its unit is g/mol. The molar mass of an element is its atomic mass expressed in grams and the molar mass of a compound is its relative molecular mass expressed in grams.

FORMULA MASS AND PERCENTAGE COMPOSITION

Example: calculate the percentage by mass of each element in calcium hydroxide, Ca (OH)2    

[Ca=40, O=16, H=1]

Solution

Rmm or formula mass of Ca (OH) 2 = 40 + (16 x2) + (1 x2) = 40 + 32 + 2 = 74

%by mass of Ca in Ca (OH) 2 = 100 x 40/74 = 54.1%

% by mass of O in Ca (OH) 2 = 100 x 32/74 = 43.2%

%by mass of H in Ca (OH) 2 = 100 x 2/74 = 2.7%

Assignment

1. What is the percentage by mass of sodium in sodium hydroxide? (Na = 23, O = 16, H=1)
2. How many atoms are there in one mole of hydrogen gas? [Avogadro’s constant = 6.02 x 1023mol-1]

Tutorial questions

1. How many elementary particles are there in 4g of CO2?
2. What is the relative molecular mass of MgSO4? [Mg = 24, S = 32, O = 16]
3. A sample of chlorine contains 75% of 3517Cl and 25% of 37 17Cl. Calculate the relative molecular mass of Chlorine.
4. What is the percentage by mass of carbon in Ca (HCO3)2? [Ca = 40, H = 1, O = 16]