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What is a mole in chemistry?

Class 11 ChemistryNCERT chapter: Some Basic Concepts of Chemistryइसे हिंदी में पढ़ें

Exam-ready answer

A mole (symbol mol) is the SI unit of amount of substance. One mole contains exactly 6.02214076 × 10²³ elementary entities, such as atoms, molecules or ions. This number is the Avogadro constant (NA). The mass of one mole of a substance in grams is its molar mass, numerically equal to its atomic or molecular mass in u.

Explanation

Atoms and molecules are far too small to count one by one, so chemists count them in large fixed groups, just as eggs are counted in dozens. The group used is the mole. Earlier, one mole was defined as the number of atoms in exactly 12 g of the carbon-12 isotope. Today the mole is defined by fixing the value of the Avogadro constant: one mole contains exactly 6.02214076 × 10²³ entities.

The mass of one mole of a substance in grams is its molar mass. It equals the atomic or molecular mass written in grams. For example, the molecular mass of water (H₂O) is 18.02 u, so one mole of water, that is 6.022 × 10²³ molecules, has a mass of 18.02 g. One mole of carbon-12 atoms has a mass of 12 g, and one mole of sodium chloride (NaCl) has a mass of 58.5 g.

Two simple relations connect the mole to measurable quantities. Number of moles n = given mass ÷ molar mass, and number of particles N = n × NA. So 36 g of water is 36 ÷ 18 = 2 mol, which contains 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules. In a balanced equation, the coefficients give mole ratios: in 2H₂ + O₂ → 2H₂O, 2 mol of hydrogen react with 1 mol of oxygen to form 2 mol of water.

Points to remember

  1. Mole: the SI unit of amount of substance; symbol mol.
  2. 1 mol = exactly 6.02214076 × 10²³ entities (Avogadro constant NA, usually taken as 6.022 × 10²³ mol⁻¹).
  3. Molar mass: the mass of one mole of a substance in grams; unit g mol⁻¹.
  4. Number of moles n = mass (g) ÷ molar mass (g mol⁻¹).
  5. Number of particles N = n × NA; coefficients in a balanced equation give mole ratios.

Exam tip

Write the definition with the exact value 6.02214076 × 10²³, then show n = mass ÷ molar mass with units. In numericals, write the molar mass calculation first, then convert moles to particles.

Questions students also ask

What is the difference between molecular mass and molar mass?
Molecular mass is the mass of one molecule, expressed in atomic mass units (u); for water it is 18.02 u. Molar mass is the mass of one mole of molecules, expressed in g mol⁻¹; for water it is 18.02 g mol⁻¹. The numbers are equal but the units differ.
How many atoms are there in one mole of oxygen gas (O₂)?
One mole of O₂ contains 6.022 × 10²³ molecules. Each molecule has two oxygen atoms, so the number of atoms is 2 × 6.022 × 10²³ = 1.2044 × 10²⁴. Always check whether a question asks for molecules or atoms.
Why do chemists use the mole instead of counting particles?
Even a small sample contains an enormous number of particles, of the order of 10²³ in a few grams. The mole links this huge number to a mass that can be weighed on a balance, so chemists can measure out substances in the exact ratios a reaction needs.

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