Class 9 Chemistry Notes – Chapter 4

Subject: Chemistry

Class: 9th

Board: Punjab Board

Chapter 4: Stoichiometry

Exercise: Multiple Choice Questions (MCQs)

(i) How many atoms are present in one gram of H₂O?

Answer: (c) 0.334 × 10²³ atoms

(ii) Which is the correct formula of calcium phosphide?

Answer: (d) Ca₃P₂

(iii) How many atomic mass units (amu) are there in one gram?

Answer: (c) 6.022 × 10²³ amu

(iv) The structural formula of 2-hexene is CH₃–CH=CH–(CH₂)₂–CH₃. What will be its empirical formula?

Answer: (d) CH₂

(v) How many moles are there in 25 g of H₂SO₄?

Answer: (c) 0.255 moles

(vi) A necklace has 6 g of diamonds in it. What is the number of carbon atoms in it?

Answer: (d) 3.01 × 10²³

(vii) What is the mass of Al in 204 g of aluminium oxide, Al₂O₃?

Answer: (d) 108 g

(viii) Which one of the following compounds has the highest percentage by mass of nitrogen?

Answer: (b) N₂H₄

(ix) When one mole of each of the following compounds reacts with oxygen, which will produce the maximum amount of CO₂?

Answer: (c) Ethane (C₂H₆)

(x) What mass of 95% CaCO₃ will be required to neutralize 50 cm³ of 0.5 M HCl solution?

Answer: (c) 1.32 g

Exercise: Questions for Short Answers

i. Write down the chemical formula of barium nitride.

Answer: Ba₃N₂

ii. Find out the molecular formula of a compound whose empirical formula is CH₂O and its molar mass is 180 g/mol.

Answer:

Empirical formula mass = 12 + (2 × 1) + 16 = 30 g/mol

Ratio, n = Molar mass / Empirical formula mass

n = 180 / 30 = 6

Molecular formula = (CH₂O)₆ = C₆H₁₂O₆

iii. How many molecules are present in 1.5 g of H₂O?

Answer:

Molar mass of H₂O = 18 g/mol

Number of molecules = (Given mass / Molar mass) × Avogadro’s number

= (1.5 / 18) × 6.022 × 10²³

= 5.018 × 10²² molecules

iv. What is the difference between a mole and Avogadro’s number?

Answer:

A mole is the amount of a substance that contains Avogadro’s number of particles. It represents an amount of substance whose mass in grams is numerically equal to its atomic or molecular mass.

Avogadro’s number is the fixed numerical value, 6.022 × 10²³, representing the number of elementary entities in one mole.

v. Write down the chemical equation for the following reaction: Copper + Sulphuric acid → Copper sulphate + Sulphur dioxide + Water.

Answer:

Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O

Exercise: Constructed Response Questions

i. Different compounds will never have the same molecular formula, but they can have the same empirical formula. Explain.

Answer:

The empirical formula represents the simplest whole-number ratio of atoms in a compound, whereas the molecular formula gives the actual number of atoms. Different compounds can possess the same atomic ratio. For example, acetylene (C₂H₂) and benzene (C₆H₆) both have the empirical formula CH, but they have different molecular formulas and molecular masses.

ii. Write down the chemical formulas of the following compounds: Calcium phosphate, aluminium nitride, sodium acetate, ammonium carbonate, and bismuth sulphate.

Answer:

Calcium phosphate: Ca₃(PO₄)₂

Aluminium nitride: AlN

Sodium acetate: CH₃COONa

Ammonium carbonate: (NH₄)₂CO₃

Bismuth sulphate: Bi₂(SO₄)₃

iii. Why does Avogadro’s number have immense importance in chemistry?

Answer:

Avogadro’s number links the submicroscopic scale of individual atoms and molecules to the macroscopic scale of grams measured in the laboratory. It enables quantitative calculations and stoichiometric predictions in chemical reactions.

iv. When 8.657 g of a compound were converted into elements, it gave 5.217 g of carbon, 0.962 g of hydrogen, and 2.478 g of oxygen. Calculate the percentage of each element present in this compound.

Answer:

Percentage of carbon = (Mass of carbon / Total mass of compound) × 100

Percentage of C = (5.217 / 8.657) × 100 = 60.26%

Percentage of hydrogen = (Mass of hydrogen / Total mass of compound) × 100

Percentage of H = (0.962 / 8.657) × 100 = 11.11%

Percentage of oxygen = (Mass of oxygen / Total mass of compound) × 100

Percentage of O = (2.478 / 8.657) × 100 = 28.62%

v. How can you calculate the masses of the products formed in a reversible reaction?

Answer:

The maximum possible masses of products are calculated stoichiometrically using a balanced chemical equation and limiting-reactant analysis, assuming complete conversion. The actual product mass depends on the extent of reaction and the yield achieved under the reaction conditions.

Exercise: Descriptive Questions

i. Which conditions must be fulfilled before writing a chemical equation for a reaction?

Answer:

The formulas and symbols of all reactants and products must be correct. The law of conservation of mass must be satisfied by balancing the total number of atoms of each element on both sides of the equation. The physical states of reactants and products should also be indicated where required.

ii. Explain the concepts of Avogadro’s number and mole.

Answer:

Avogadro’s number (6.022 × 10²³) is the number of particles, such as atoms, ions, or molecules, contained in one mole of a substance.

A mole is the amount of substance containing Avogadro’s number of elementary entities. It serves as a link between laboratory measurements of mass and the number of individual atoms or molecules.

iii. How many grams of CO₂ will be produced when 10 g of CH₄ reacts with excess O₂ according to the following equation?

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Answer:

Given mass of CH₄ = 10 g

Molar mass of CH₄ = 16 g/mol

Molar mass of CO₂ = 44 g/mol

Moles of CH₄ = Given mass / Molar mass

Moles of CH₄ = 10 / 16 = 0.625 moles

From the balanced equation, 1 mole of CH₄ produces 1 mole of CO₂.

Moles of CO₂ produced = 0.625 moles

Mass of CO₂ = Moles × Molar mass

Mass of CO₂ = 0.625 × 44 = 27.5 g

Therefore, 27.5 g of CO₂ will be produced.

iv. How many moles of coal are needed to produce 10 moles of CO according to the following equation?

3C(s) + O₂(g) + H₂O(l) → H₂(g) + 3CO(g)

Answer:

From the balanced equation, 3 moles of carbon produce 3 moles of CO.

The mole ratio of C to CO is 3:3, or 1:1.

Therefore, to produce 10 moles of CO, exactly 10 moles of carbon are needed.

v. How much SO₂ is needed in grams to produce 10 moles of sulphur?

2H₂S(g) + SO₂(g) → 2H₂O(l) + 3S(s)

Answer:

From the balanced equation, 1 mole of SO₂ produces 3 moles of sulphur.

Moles of SO₂ needed = 10 / 3 = 3.333 moles

Molar mass of SO₂ = 32 + (2 × 16) = 64 g/mol

Mass of SO₂ = Moles × Molar mass

Mass of SO₂ = 3.333 × 64 = 213.33 g

Therefore, approximately 213.33 g of SO₂ is needed.

vi. How much ammonia is needed in grams to produce 1 kg of urea fertilizer?

Chemical equation:

2NH₃(aq) + CO₂(aq) → CO(NH₂)₂(aq) + H₂O(l)

Answer:

Target mass of urea, CO(NH₂)₂ = 1000 g

Molar mass of urea = 60 g/mol

Moles of urea = 1000 / 60 = 16.667 moles

From the equation, 1 mole of urea requires 2 moles of NH₃.

Moles of NH₃ needed = 16.667 × 2 = 33.333 moles

Molar mass of NH₃ = 17 g/mol

Mass of NH₃ needed = 33.333 × 17 = 566.67 g

Therefore, approximately 566.67 g of ammonia is required.

vii. Calculate the number of atoms in the following.

(a) 3 g of H₂

Answer:

Molar mass of H₂ = 2 g/mol

Moles of H₂ = 3 / 2 = 1.5 moles

Number of molecules = 1.5 × 6.022 × 10²³

= 9.033 × 10²³ molecules

Each H₂ molecule contains 2 hydrogen atoms.

Total number of atoms = 2 × 9.033 × 10²³

= 1.806 × 10²⁴ atoms

(b) 3.4 moles of N₂

Answer:

Number of molecules = 3.4 × 6.022 × 10²³

= 2.047 × 10²⁴ molecules

Each N₂ molecule contains 2 nitrogen atoms.

Total number of atoms = 2 × 2.047 × 10²⁴

= 4.095 × 10²⁴ atoms

(c) 10 g of C₆H₁₂O₆

Answer:

Molar mass of C₆H₁₂O₆ = 180 g/mol

Moles = 10 / 180 = 0.0556 moles

Number of molecules = 0.0556 × 6.022 × 10²³

= 3.345 × 10²² molecules

Total atoms per molecule = 6 + 12 + 6 = 24

Total number of atoms = 24 × 3.345 × 10²²

= 8.028 × 10²³ atoms

Exercise: Investigative Questions

i. It is generally believed that drinking eight glasses of water every day is required to keep oneself hydrated, especially in summer. If a glass contains an average of 400 cm³ of water, how many moles of water are needed for a single adult?

Answer:

Number of glasses = 8

Volume of one glass = 400 cm³

Total volume of water = 8 × 400 = 3200 cm³

Since the density of water is approximately 1 g/cm³:

Mass of water = 3200 g

Molar mass of H₂O = 18 g/mol

Moles of water = Mass / Molar mass

Moles of water = 3200 / 18 = 177.78 moles

Therefore, the amount of water described in the question is approximately 177.78 moles.

ii. The chemical formula for sand is SiO₂, but sand does not exist in the form of discrete molecules like H₂O. How has its formula been determined, keeping its structure in view?

Answer:

Silicon dioxide (SiO₂) forms a giant three-dimensional covalent network crystal lattice rather than existing as separate molecules. Each silicon atom is covalently bonded to four oxygen atoms in a tetrahedral arrangement, and each oxygen atom bridges two silicon atoms.

The formula SiO₂ represents the simplest 1:2 ratio of silicon to oxygen atoms throughout the continuous lattice structure. It is therefore an empirical formula representing the composition of the network.

Notes by Sir Dilawar

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