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Atoms and Molecules - Mole Concept

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A mole is defined as the amount of substance that contains as many particles (atoms, molecules, or ions) as there are atoms in exactly 12 g12\text{ g} of carbon-12 isotope.

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The number of particles present in 1 mole1\text{ mole} of any substance is fixed, with a value of 6.022×10236.022 \times 10^{23}. This is known as the Avogadro Constant or Avogadro Number (NAN_A or N0N_0).

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The mass of 1 mole1\text{ mole} of a substance is called its Molar Mass (MM). It is numerically equal to the atomic mass or molecular mass but expressed in grams (g/molg/mol).

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Atomic mass is expressed in atomic mass units (uu), whereas molar mass of an atom (Gram Atomic Mass) is expressed in gg. For example, the atomic mass of Nitrogen is 14u14u, and its molar mass is 14 g/mol14\text{ g/mol}.

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The mole concept acts as a link between the mass of a substance and the number of particles.

📐Formulae

Number of moles (n)=Given mass (m)Molar mass (M)\text{Number of moles } (n) = \frac{\text{Given mass } (m)}{\text{Molar mass } (M)}

Number of moles (n)=Given number of particles (N)Avogadro number (N0)\text{Number of moles } (n) = \frac{\text{Given number of particles } (N)}{\text{Avogadro number } (N_0)}

Mass (m)=n×M\text{Mass } (m) = n \times M

Number of particles (N)=mM×N0\text{Number of particles } (N) = \frac{m}{M} \times N_0

Mass of one atom/molecule=Molar MassN0\text{Mass of one atom/molecule} = \frac{\text{Molar Mass}}{N_0}

💡Examples

Problem 1:

Calculate the number of moles in 52 g52\text{ g} of HeHe (Helium).

Solution:

n=mM=52 g4 g/mol=13 molesn = \frac{m}{M} = \frac{52\text{ g}}{4\text{ g/mol}} = 13\text{ moles}

Explanation:

The atomic mass of HeHe is 4u4u, so its molar mass MM is 4 g/mol4\text{ g/mol}. Using the formula n=m/Mn = m/M, we divide the given mass by the molar mass.

Problem 2:

Calculate the mass of 0.5 moles0.5\text{ moles} of N2N_2 gas.

Solution:

m=n×M=0.5×(2×14)=0.5×28=14 gm = n \times M = 0.5 \times (2 \times 14) = 0.5 \times 28 = 14\text{ g}

Explanation:

The molar mass of N2N_2 is calculated as 2×14=28 g/mol2 \times 14 = 28\text{ g/mol}. Multiplying the number of moles by the molar mass gives the total mass.

Problem 3:

Calculate the number of molecules in 36 g36\text{ g} of water (H2OH_2O).

Solution:

N=mM×N0=3618×6.022×1023=2×6.022×1023=1.2044×1024 moleculesN = \frac{m}{M} \times N_0 = \frac{36}{18} \times 6.022 \times 10^{23} = 2 \times 6.022 \times 10^{23} = 1.2044 \times 10^{24}\text{ molecules}

Explanation:

First, find the molar mass of H2OH_2O: (2×1)+16=18 g/mol(2 \times 1) + 16 = 18\text{ g/mol}. Then, find the number of moles (36/18=236/18 = 2). Finally, multiply the moles by Avogadro's number to find the number of molecules.

Mole Concept - Revision Notes & Key Formulas | CBSE Class 9 Science