Introduction To The Mole Concept

CONTENT

• Relative atomic mass
• Relative molecular mass
• Molar volume of gases
• Percentage of an element in a compound

THE MOLE

A mole is a number of particles of a substance which may be atoms, ions, molecules or electrons. This number of particles is approximately 6.02 x 10²³ in magnitude and is known as Avogadro’s number of particles.

The mole is defined as the amount of a substance which contains as many elementary units as there are atoms in 12g of Carbon-12.

RELATIVE ATOMIC MASS

The relative atomic mass of an element is the number of time the average mass of one atom of that element is heavier than one twelfth the mass of one atom of Carbon-12. It indicates the mass of an atom of an element. For e.g, the relative atomic mass of hydrogen, oxygen, carbon, sodium and calcium are 1, 16, 12, 23, and 40 respectively.

The atomic mass of an element contains the same number of atoms which is 6.02 x 10²³atoms; 1 mole of hydrogen having atomic mass of 2.0g contains 6.02 x 10²³ atoms.

EVALUATION

1. Define relative atomic mass of an element
2. State the relative atomic mass of the following elements: potassium, chlorine, silver, lead, phosphorus and nitrogen

RELATIVE MOLECULAR MASS

The relative molecular mass of an element or compound is 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

It is the sum of the relative atomic masses of all atoms in one molecule of that substance. It is also called the formula mass. The formula mass refers not only to the relative mass of a molecule but also that of an ion or radical.

CALCULATION

Calculate the relative molecular mass of:

1. Magnesium chloride
2. Sodium hydroxide
3. Calcium trioxocarbonate

[Mg=24, Cl=35.5, Na=23, O=16, H=1, Ca=40,C=12]

Solution:

1. MgCl ₂ = 24 + 35.5×2 = 24 + 71 = 95gmol^-1
2. NaOH = 23 + 16 + 1 = 40gmol^-1
3. CaCO₃ = 40 + 12 +16×3 = 100gmol^-1

EVALUATION

1. What is relative molecular mass of a compound?
2. Calculate the relative molecular mass of (a) NaNO₃ (b) CuSO₄.5H₂O

MOLAR VOLUME OF GASES

The volume occupied by 1 mole of a gas at standard conditions of temperature and pressure (s.t.p) is 22.4 dm³. Thus 1 mole of oxygen gas of molar mass 32.0gmol^-1 occupies a volume of 22.4dm³ at s.t.p and 1 mole of helium gas of molar mass 4.0gmol^-1 occupies a volume of 22.4 dm³ at s.t.p.

Note: When the conditions of temperature and pressure are altered, the molar volume will also change. Also, standard temperature = 273K and standard pressure = 760mmHg.

GENERAL EVALUATION

1. What is the number of molecules in 6.4g of SO₂ (N_A = 6.02 X 10²³)?
2. What is the volume in cm³ of 2.2g of CO₂ at s.t.p ( C=12, O=16)?
3. Determine the percentage by mass of oxygen in Al₂(SO₄).2H₂O.( Al=27, S=32, O=16, H=1)

READING ASSIGNMENT

New School Chemistry for Senior Secondary Schools by O. Y Ababio, Pg 28-31

WEEKEND ASSIGNMENT

1. What is the relative atomic mass of potassium A. 40 B. 39 C. 32 D. 24
2. An element with relative atomic mass 108 is A. Ca B. Cl C. Ag D. Al
3. Modern standard element with which chemist define relative atomic mass is A.¹²C B.C¹³ C.³H D.¹⁶O
4. Calculate the relative molecular mass of CH₃COOH.A. 60gmol^-1B. 70gmol^-1C. 80gmol^-1D. 90gmol^-1
5. How many moles are there in 12g of CO₂ (C=12, 0=16)?A. 0.27 B. 0.47 C. 0.16 D. 0.32

THEORY

1. Calculate the actual number of atoms contained in 2.8dm³ of chlorine (Molar volume of gas = 22.4dm³, N_A = 6.02 X 10²³)
2. How many moles are there in 10g of iron (II) tetraoxosulphate (VI)?