01 Introduction to Chemistry

02 The Structure of Atoms

2.1 The Basic Concept of Matter

2 Lessons
2.5 The Structure of Atom - Practices

3 Exercises
03 Concept of Mole, Formulae and Equations

3.2 Concept of Mole

3 Lessons
3.3 Chemical Formulae

2 Lessons
04 Periodic Table of Elements

4.5 Group 17 Elements

2 Lessons
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- A mole is defined as the amount of substance which contains the Avogadro Number of particles.
- The Avogadro Number (or Avogadro Constant) is defined as the number of atoms in 12 g of the carbon-12 isotope, which is equal to 6.02 x 10
^{23}.

1 mole = 6.02 x 10^{23}

Example:
1 mol of atoms = 6.02 x 10- 2 mol ferum
- 3.6 mol zink
- 2.8 mol zink
- ¼ mol ferum

- 2 mol iron = 2 x 6.02 x 10
^{23}= 1.204 x 10^{24}iron atoms - 3.6 mol zink = 3.6 x 6.02 x 10
^{23}= 2.167 x 10^{24}zink atoms - 2.8 mol zink = 2.8 x 6.02 x 10
^{23}= 1.686 x 10^{24}zink atoms - ¼ mol iron = ¼ x 6.02 x 10
^{23}= 1.505 x 10^{23}iron atoms.

- Mole is the unit of amount of substance.
- “mol” is the symbol of mole.

- We have just learn that, mole is a quantity, and it is equal to 6.02 x 10
^{23}. The number 6.02 x 10^{23}is called the Avogadro constant. - Therefore, if we are given the number of mole of substance, and asked to find the number of particles (atoms, molecules or ions) in it, we multiply the number of mole by the Avogadro constant.
- Likewise, if we are given the number of particles, and asked to find the number of mole of the particles, we divide the number of particles by the Avogadro constant.

**Example**

Which contains more atoms, 1 mol of helium or 1 mol of uranium? Which has a greater mass? [ RAM: He=4; U=238 ]

**Answer**:

1 mol of helium and 1 mol of uranium has equal number of atoms.

The mass of one Uranium atom ss greater than the mass of one helium atom.

**Example**

Find the number of atoms in 2.5 mol of gold.

**Answer**:

Number of atoms

= Number of mole x Avogadro constant

= 2.5 x 6.02 x 10^{23} = 1.505 x 10^{24}

**Example**

How many moles of magnesium that contain 2.76 x 10^{23} of magnesium atom?

**Answer**:

Number of mole

= Number of atoms ÷ Avogadro constant

= 2.76 x 10^{23 }÷ 6.02 x 10^{23}

= 0.46 mol

- All molecules contain more than one atom.
- For example, in a carbon dioxide molecule (CO
_{2}), there are 3 atoms – 1 carbon atom and 2 oxygen atoms. - Therefore, in 5 carbon dioxide molecules, there will be 15 atoms – 5 carbon atom and 10 oxygen atoms.
- Similarly, in 1 mole carbon dioxide molecules, there will be 3 mole atoms – 1 mole carbon atoms and 2 mole oxygen atoms, and in 3 mole carbon dioxide molecules, there will be 9 mole atoms – 3 mole carbon atoms and 6 mole oxygen atoms, so on and so forth.

**Example**:

- How many hydrogen molecules can be made with 20 mol of hydrogen atoms?
- How many hydrogen atoms are there in 20 mol of hydrogen molecules?

**Answer**:

a.

Each hydrogen molecules (H_{2}) consist of 2 hydrogen atoms.

Therefore, 20 mol of hydrogen atoms can make 10 mole of hydrogen molecules.

b. Number of mole of hydrogen atoms in 20 mole of hydrogen molecules

= 2 x 20 mole = 40 mole.

**Example**:

- Find the number of carbon atom in 0.75 mol of carbon dioxide.
- Find the number of oxygen atom in 0.75 mol of carbon dioxide.

**Answer**:

a.

Each carbon dioxide molecules (CO_{2}) consist of 1 carbon atom and 2 oxygen molecules.

Therefore, 0.75 mole of carbon dioxide contain 0.75 mole carbon atoms.

b.

Number of mole of oxygen atoms in 0.75 mole of carbon dioxide

= 2 x 0.75 mole = 1.50 moles.