Lesson Notes By Weeks and Term - Senior Secondary School 1

SUBJECT: CHEMISTRY

CLASS:  SS 1

DATE:

TERM: 2nd TERM

REFERENCE BOOKS

• New Chemistry for Senior Secondary School by Osei Yaw Ababio; U.T.M.E Past Questions and Answers.
• Practical Chemistry for Senior Secondary Schools by Godwin Ojokuku
• Outline Chemistry for Schools & Colleges by Ojiodu C.C.
• Chemistry Pass Questions for S.S.C.E and UTME.

 
WEEK ONE

TOPIC: 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 + 35.5x2 = 24 + 71 = 95gmol-1
2. NaOH = 23 + 16 + 1 = 40gmol-1
3. CaCO3 = 40 + 12 +16x3 = 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.

RELATIONSHIP BETWEEN QUANTITIES

Molar mass = mass(g)            i.e. M = m   gmol-1

             Amount (moles)            n

Note: Amount = Number of moles

Molar volume of gas = volume ( cm3 or dm3)    i.e. Vm = v  dm3mol-1

            Amount (mole)             n

Amount = Reacting mass (g)

                Molar mass (gmol-1)

Also, Amount of substance = Number of particles

                Avogadro’s constant

But, Avogadro’s constant = 6.02 x 1023

Combining the two expressions:

Reacting mass = Number of particles

 Molar mass         6.02 x 1023

CALCULATIONS

1. What is the mass of 2.7 mole of aluminium (Al=27)?

Solution:

Amount = Reacting mass

                Molar mass

Reacting mass = Amount x Molar mass

                       = 2.7mole x 27 gmol-1   = 72.9g.

2. What is the number of oxygen atoms in 32g of the gas (O=16, NA = 6.02 x 1023)?

Solution:

Reacting mass= Number of atoms

       Molar mass    6.02 x 1023

Number of atoms = Reacting mass x 6.02 x 1023

    Molar mass

Molar mass of O2 = 16x2 =32gmol-1

Number of atoms = 32g x 6.02 x 1023

            32gmol-1

    = 6.02 x 1023

The number of oxygen atoms is 6.02 x 1023

EVALUATION

1. Define the molar volume of a gas
2. How many molecules are contained in 1.12dm3 of hydrogen gas at s.t.p?

PERCENTAGE OF AN ELEMENT IN A COMPOUND

The percentage composition of an atom in a compound is the amount of the atom expressed in percentage.

Percentage of an element in a compound = Mass of element in the compound x 100

                        Molar mass of compound               1

CALCULATIONS

1. What is the percentage by mass of nitrogen in NH4NO3 ( H=1, N=14, 0=16)?

Solution:

Molar mass of NH4NO3 = 14x2 + 1x4 + 16x3 = 80gmol-1

Percentage by mass of N2 = Mass of N2 x 100

            Molar mass of NH4NO3 1

            = 28 x 100= 35%

            80      1

2. Calculate the percentage by mass of water of crystallization in MgSO4.7H2O 

(Mg=24, S=32, 0=16, H=1)

Solution:

Molar mass of MgSO4.7H20 = 24 + 32 + 16x4 + 9(2+16) = 246gmol-1

7 moles of water of crystallization = 126g

Percentage by mass of water = Mass of H2O          x 100

                    Molar mass of MgSO4.7H2O      1

                = 126g           x 100

                    246gmol-11

                = 51.2%

GENERAL EVALUATION

1. What is the number of molecules in 6.4g of SO2 (NA = 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, NA = 6.02 X 1023)

1. How many moles are there in 10g of iron (II) tetraoxosulphate (VI)?