Chapter 4.2 – Mole Concept | Chapter Solution Class 9

This means 35.46 = (Average mass of an atom of Cl in natural isotopic mixture / Mass of an atom of C¹²) × 12

We know, 1 amu = 1.6603 × 10⁻²⁴ g

Weight of one atom of oxygen = Atomic mass × 1 amu

= 15.994 × 1.6603 × 10⁻²⁴ g

= 2.656 × 10⁻²³ g

One atomic mass unit (amu or u) is defined as 1/12th of the mass of one atom of C¹² isotope.

1 amu = (Mass of one atom of C¹²) / 12

In grams, 1 amu = 1.6603 × 10⁻²⁴ g.

The molecular mass of ammonia (NH₃) is 17 u, which means that one molecule of ammonia is 17 times heavier than 1/12th of the mass of one C¹² atom.

One gram atom of oxygen is 16 g, which means the atomic mass of oxygen is 16 u, and when expressed in grams, it is called one gram atom of oxygen

No. of moles = \text{given mass} \over \text{molar mass}

For ammonia, molar mass = 17 g/mol

No. of moles = 100 \over 17

= 5.88 moles

So, 100 g of ammonia contains 5.88 gram-molecules of ammonia.

Avogadro’s number is defined as the number of particles (atoms, molecules, or ions) present in one mole of a substance.

Its value is 6.022 × 10²³ particles per mole.

At STP, 1 mole of hydrogen gas (H₂) = 2 g occupies 22.4 L.

No. of moles in 4 g of H₂ = 4 g / 2 g = 2 moles

Volume = 2 × 22.4 L = 44.8 L

Thus, 4 g of hydrogen occupies 44.8 L at STP.

At STP, 1 gram-molecule (1 mole) of O₂ occupies 22.4 L.

Volume of 1.5 gram-molecules of O₂ = 1.5 × 22.4 L
= 33.6 L

Thus, 1.5 gram-molecules of oxygen occupy 33.6 L at STP.

(d) Avogadro’s number of any particle.

Explanation:

A mole represents 6.022 × 10²³ particles (atoms, molecules, or ions) of a substance, which is known as Avogadro’s number. It is a fixed quantity that helps in calculations related to atoms and molecules.

(b) 6.022 × 10²³

Explanation:

0.012 kg = 12 g of C-12 isotope is defined as 1 mole of C-12 atoms.
Since 1 mole = Avogadro’s number = 6.022 × 10²³ atoms, 12 g of C-12 contains 6.022 × 10²³ atoms.

(d) chemistry, physics, and biology

Explanation:

Avogadro’s number is used in:

• Chemistry (for mole concept, atomic and molecular calculations)
• Physics (for gas laws and calculations related to atomic theory)
• Biology (for understanding molecular biology and counting molecules like DNA, proteins, etc.)

(d) no unit

Explanation:

Relative atomic mass is a ratio of the mass of an atom to 1/12th of the mass of a C-12 atom. Since it is a ratio, it has no unit.

6.022 × 10²³

1 amu = 1.6603 × 10⁻²⁴ g

5.6 L

Explanation:

Molecular mass of N₂ = 2 × 14 = 28 g

1 mole (28 g) of N₂ gas occupies 22.4 L at STP

Volume of 7 g of N₂ = (22.4 / 28) × 7 = 5.6 L

22.4 litres

Explanation:

At STP (0°C, 1 atm pressure), 1 mole of oxygen occupies 22.4 L. At higher temperatures, the volume increases due to gas expansion.

0.25 moles

Explanation:

Molar mass of O₂ = 32 g

No. of moles = 8 g / 32 g = 0.25 moles

Since 1 amu = 1.6603 × 10⁻²⁴ g, multiplying atomic mass with 1 amu gives the actual mass of an atom in grams.

gram molecular mass

Explanation:

1 mole of a substance contains Avogadro’s number of molecules, so its mass is equal to the gram molecular mass.

only 1 atom of carbon

Explanation:

12U (atomic mass unit) refers to the mass of a single carbon atom, not grams of carbon.

12g of C

Explanation:

Since 1 mole of carbon contains Avogadro’s number of atoms (6.022 × 10²³ atoms), its mass is 12 g

False

Explanation:

IUPAC has accepted C-12 isotope as the standard for atomic mass measurement, not hydrogen.

True

Explanation:

• 1 gram-molecule (1 mole) of H₂O contains 1 mole of H₂O molecules.
• Each H₂O molecule has 2 hydrogen atoms, so 1 mole of H₂O contains 2 moles of hydrogen atoms.

False

Explanation:

A gram atom of nitrogen (N) is 14 g (since atomic mass of nitrogen is 14 u).

But 1 mole of N₂ molecules (gas) is 28 g. The statement is incorrect because it confuses atomic nitrogen (N) with molecular nitrogen (N₂).

A mole is the amount of substance that contains 6.022 × 10²³ elementary particles (atoms, molecules, or ions). It is a fundamental unit in chemistry for counting large numbers of particles.

Example:

If we take 1 mole of sand grains, the number of grains would be 6.022 × 10²³, which is so vast that it could cover the entire Earth with a thick layer of sand.

Atomic Mass Unit (amu or u) is defined as 1/12th of the mass of one atom of carbon-12 (C¹²) isotope.

Molecular mass is the sum of the atomic masses of all the atoms present in a molecule, measured relative to 1/12th of the mass of a C¹² atom.

Formula mass is used for ionic compounds that do not exist as discrete molecules but as a lattice of ions.

Examples:

• NaCl (Sodium chloride) → Formula mass = (23 + 35.5) = 58.5 u
• MgCl₂ (Magnesium chloride) → Formula mass = (24 + 2 × 35.5) = 95 u

Molar volume is the volume occupied by 1 mole of a gas at standard temperature and pressure (STP).

At STP (0°C and 1 atm pressure), molar volume = 22.4 L.

This means 1 mole of any gas occupies 22.4 L at STP.

Molar mass of S₈ = 8 × 32 = 256 g

Number of moles in 24 g of S₈ = \frac{24}  {256}

= 0.09375 moles

Number of molecules = moles × Avogadro’s number

= 0.09375 × 6.022 × 10²³

= 5.64 × 10²² molecules of S₈

Atomic mass is the mass of an atom of an element relative to 1/12th of the mass of a C¹² atom. It is a dimensionless number.

Gram molecular volume is the volume occupied by one mole of any gas at STP, which is 22.4 L.

Mass of one mole of CO₂:

Molecular mass of CO₂ = (1 × 12) + (2 × 16) = 44 g

So, mass of 1 mole of CO₂ = 44 g

(a) A mole is a unit used to measure the amount of substance containing Avogadro’s number of particles (6.022 × 10²³ particles).

Mathematically,

1 mole = 6.022 × 10²³ atoms/molecules/ions

(b) If we take 1 mole of marbles, they would cover the entire surface of the Earth with a thick layer, showing how large 6.022 × 10²³ is.

(a) The volume of gases depends on temperature and pressure. At different conditions, gas expands or compresses. So, for consistency, molar volume is measured at standard temperature and pressure (STP), which is 0°C and 1 atm.

(b) At STP, 1 mole of any gas occupies 22.4 L.

1 millimole = 1/1000 moles = 10⁻³ moles

Volume at STP = 10⁻³ × 22.4 L = 0.0224 L