Mole Concept (original) (raw)

Last Updated : 8 Apr, 2026

The mole concept is a way to measure the amount of a substance. Experiments have indicated that one gram atom of any element or one gram molecule of any substance contains the same number of particles. This number is called Avogadro’s number and is 6.022 × 10²³. The mole concept makes it easier to calculate the number of atoms or molecules in a given substance and helped solve the problem of finding the absolute atomic masses of elements.

mole_concept

Mole

A mole is the amount of a substance that contains 6.022 × 10²³ particles (atoms, molecules, or ions). Avogadro's number is named after Amedeo Avogadro.

The following formula may be used to calculate the number of moles of a chemical in a given pure sample:

n = \frac{m}{M}

where:

**Example:

1 mole of oxygen molecules (O₂) contains
6.022 \times 10^{23} oxygen molecules.

If we have 18 g of water (H₂O):
Molar mass of water = 18 g/mol

n = \frac{18}{18} = 1 \text{ mole}

So, 18 g of water = 1 mole of water molecules.

Mole Concept

**Avogadro's Number (N A )

**Example:

Mole Concept Formulas

The mole concept is a method where we identify the mass of chemical substances as per requirement. The entire mole concept revolves around 12 g (0.012 kg) of the 12C isotope. In the SI system, the unit of the fundamental quantity 'the amount of substance' is the mole. The symbol of the mole is "mol." Following are the formulas used in the mole concept:

**1) Atomic Mass and Molecular Mass

The value of one amu is 1 g / NA = 1.66056 × 10-24 g.

For example, the atomic mass of carbon is 12.011 amu since carbon mostly contains the carbon-12 isotope. Carbon-12 is 98.9 % & carbon-13 is merely 1.1%, . The atomic mass of these isotopes is different.

**For example:

The molecular mass of water (H2O) =18.015 amu

The atomic mass of a hydrogen atom is 1.007 amu and the atomic mass of oxygen is 15.99 amu.

molecular mass

**2) Molar Mass

**Molar mass of a Substance = \frac{\text{Mass of the substance in grams}}{\text{Number of Moles}}

**For Example: The molar mass of water is 18.015 g/mol, which is the mass of NA number of water molecules.

**3) Gram Atomic Mass and Gram Molecular Mass

Gram atomic mass is the mass of one mole of an atom. Gram molecular mass is the mass of one mole of a molecular substance expressed in grams. It is also known as molar mass. This amount of a substance is also called one gram molecule.

**Example: For **Hydrogen

Atomic mass = 1 u
Gram atomic mass = 1 g

But molecular hydrogen is H2

Gram molecular mass = 2 g

**4) Gram Molecular Volume

The volume occupied by one gram mole of a substance in a vapor state or gaseous state at STP is called gram molecular volume. The standard temperature to obtain GMV is 273K, and the standard pressure is 1 atm. The ideal gas equation is utilized to calculate the corresponding gram molar volume (GMV = 22.4 L).

**5) Relative Molecular Mass (RMM)

Relative molecular mass is the molecular weight of an element or molecule; it is expressed as RMM. It is the number of times a single molecule of a substance remains heavier than one-twelfth the mass of a carbon atom (12C).

Related Formulas

Solved Examples

**Example 1: Calculate the molecular mass of Ammonium Sulphate (NH₄)₂SO₄.

**Solution: Since, the relative atomic masses of N = 14, H = 1, S = 32, O =16

Therefore, the molecular mass of the given compound is,

= 2 (14 × 1 + 1 × 4) + 32 + 16 × 4

= 2(14 + 4) + 32 + 64

= 2 × 18 + 32 + 64

= 36 + 32 + 64

= 132 amu

**Example 2: How many moles are present in 200 g of NaOH?

**Solution: The mass of 1 mole of NaOH = 23 + 16 + 1= 40 g

Therefore, in 200g of NaOH the number of moles present = 200g / 40g mol-1 = 5 mol

**Example 3: Calculate the mass of an atom of the oxygen element.

**Solution: Mass of 1 mole of oxygen = 16g

No. of atoms in 1 mole of oxygen = NA

Therefore, mass of one atom of oxygen = 16g / NA = 16 / (6.022 × 1023) = 2.657 × 10-23 g

**Example 4: Find the ratio of moles of oxygen atoms present in the compounds H₂SO₄, H₂SO₃, and SO₂. .

**Solution: 1 mole of H2SO4 contains 4 × NA atoms of oxygen

1 mole of H2SO3 contains 3 × NA atoms of oxygen

1 mole SO2 contains 2 × NA atoms of oxygen

Therefore the required ratio is 4 × NA : 3 × NA : 2 × NA = 4 : 3 : 2

**Example 5: How many moles of hydrogen and oxygen gas are required to produce 13 moles of water?

**Solution: Chemical equation of water formation:

H2 + O2 ⇢ H2O

Now the balanced equation is

2H2 + O2 ⇢ 2H2O

Thus we can deduce that 2 moles of hydrogen gas and 1 mole of oxygen gas combine together to form 2 moles of water. We can write the equation as :

H2 (1 mole) + O2 (1/2 mole)⇢ H2O (1 mole)

Hence, for production of 13 moles of water the required chemical equation would be:

H2 (13 moles) + O2 (13/2 moles) ⇢ H2O (13 moles)

Hence, for production of 13 moles of water we need 13 moles of hydrogen and 6.5 moles of oxygen gas.

**Example 6: Calculate the moles of electrons present in 104 g of acetylene gas.

**Solution: Now, formula for acetylene gas is C2H2. It's structure is H—C≡C—H. Therefore, number of electrons present in 1 molecule of acetylene are 14.

Now, mass of 1 mole of acetylene is 26g ⇒ 104g of acetylene are 4 moles.

Now, 1 mole of acetylene = NA molecules

⇒ NA molecules have 14 × NA electrons

⇒ 4 moles of acetylene have 4 × 14 × NA electrons = 56 × NA electrons = 56 moles of electrons.