Class 11 Chemistry - First Term Exam (Practice Paper) with Solution

 

Class 11 Chemistry - First Term Exam (Practice Paper) with Solution 

Q.1. Objective Type Questions (1 mark each) (10 marks)

(A) Multiple Choice Questions:

1. The number of moles in 11.2 L of CO₂ at STP is —
    (a) 0.25 (b) 0.5 (c) 1 (d) 2
    Answer: (b) 0.5

2. Which law is related to the combining volumes of gases?
    (a) Boyle’s Law (b) Charles’ Law (c) Gay Lussac’s Law (d) Avogadro’s Law
    Answer: (c) Gay Lussac’s Law

3. The SI unit of molar mass is —
    (a) g/mol (b) kg/mol (c) mol/g (d) kg/L
    Answer: (b) kg/mol

4. The number of protons in ¹⁷Cl³⁵ is —
    (a) 17 (b) 18 (c) 35 (d) 52
    Answer: (a) 17

5. The principal quantum number denotes —
    (a) shape (b) orientation (c) energy level (d) spin
    Answer: (c) energy level

(B) True or False:

6. The atomic number of oxygen is 8. — True

7. The Avogadro number is $6.022 × 10^{23}$ mol⁻¹. — True

8. Covalent bonds are formed by transfer of electrons. — False

9. The p-orbital has spherical shape. — False

10. Hydrogen bonding increases boiling point. — True

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Q.2. Very Short Answer Type (2 marks each) (10 marks)

1. Define atomic number and mass number.
    Answer: Atomic number (Z) = number of protons in the nucleus.
    Mass number (A) = sum of protons and neutrons in the nucleus.

2. State Boyle’s Law.
    Answer: At constant temperature, the pressure of a fixed amount of gas is inversely proportional to its volume.
    $P \propto \frac{1}{V}$ or $PV = constant$.

3. Write the electronic configuration of sodium (Na, Z=11).
    Answer: 1s² 2s² 2p⁶ 3s¹.

4. Define ionization enthalpy.
    Answer: The amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form cations.

5. Write the formula of aluminium oxide and sodium carbonate.
    Answer:
    Aluminium oxide – Al₂O₃
    Sodium carbonate – Na₂CO₃

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Q.3. Short Answer Type (3 marks each) (15 marks)

1. State and explain Avogadro’s Law with an example.
    Answer:
    Avogadro’s Law: Equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.
    Example: 1 L of H₂ and 1 L of O₂ at STP contain the same number of molecules.

2. Differentiate between ionic bond and covalent bond.

Properties	Ionic Bond	Covalent Bond
Formation	Transfer of electrons	Sharing of electrons
Force	Electrostatic attraction	Mutual sharing
Example	Nacl	H₂, CH₄

3. Calculate the number of moles in 9 g of (H2O)water.
    Answer:
    Molar mass of H₂O = 2 + 16 = 18 g/mol
    Moles = mass / molar mass = 9 / 18 = 0.5 mol

4. State three postulates of Dalton’s Atomic Theory.
    Answer:
    - Matter is made of indivisible atoms.
    - Atoms of the same element are identical.
    - Atoms combine in simple whole number ratios to form compounds.

5. Explain hydrogen bonding with one example.
    Answer:
    Hydrogen bond is a weak attractive force between a hydrogen atom bonded to a highly electronegative atom (F, O, N) and another electronegative atom.
    Example: In water, intermolecular hydrogen bonding exists between H and O atoms.

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Q.4. Long Answer Type (5 marks each) (20 marks)

1. Explain in detail Bohr’s Atomic Model and his limitations.
    Answer:
    Bohr’s postulates:
    - Electrons revolve around the nucleus in fixed circular orbits.
    - Each orbit has a definite energy (energy levels).
    - Energy is absorbed/emitted when electron moves between orbits (ΔE = hν).

 Limitations:
 - Could not explain spectra of multi-electron atoms.
 - Failed to explain Zeeman and Stark effects.
 - Could not justify elliptical orbits or the wave nature of electrons.

2. Describe the periodic trends in the modern periodic table.
    Answer:
    - Atomic radius: Decreases across a period, increases down a group.
    - Ionization enthalpy: Increases across a period, decreases down a group.
    - Electronegativity: Increases across a period, decreases down a group.
    - Metallic character: Decreases across a period, increases down a group.

3. Explain molecular orbital theory (MOT) for formation of H₂ molecule.
    Answer:
    According to MOT, atomic orbitals combine to form molecular orbitals — bonding and antibonding.
    For H₂: 1s + 1s → σ1s (bonding) and σ*1s (antibonding).
    Electrons fill bonding orbital → bond order = ½(2−0)=1 → stable molecule.

4. Derive the ideal gas equation and state its significance.
    Answer:
    From Boyle’s law: $P \propto \frac{1}{V}$,
    From Charles’ law: $V \propto T$,
    From Avogadro’s law: $V \propto n$,
    Combining: $V \propto \frac{nT}{P}$,
    So $PV = nRT$.
    Significance: Relates pressure, volume, temperature, and number of moles for ideal gases.

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Q.5. Numerical Problems (5 marks) (15 marks)

1. Calculate the volume occupied by 2 moles of a gas at STP.
    Answer:
    At STP, 1 mol = 22.4 L
    ∴ 2 mol = 2 × 22.4 = 44.8 L

2. Calculate the number of molecules in 9 g of water.
    Answer:
    Moles = 9/18 = 0.5 mol
    Molecules = 0.5 × 6.022×10²³ = 3.011×10²³ molecules

3. Calculate the empirical formula of a compound containing 40% carbon, 6.7% hydrogen, and 53.3% oxygen.
    Answer:
    C: 40/12 = 3.33 H: 6.7/1 = 6.7 O: 53.3/16 = 3.33
    Ratio: 3.33 : 6.7 : 3.33 → 1 : 2 : 1
    Empirical formula: CH₂O

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Q.6. Answer any one (5 marks)

(a) Explain the shapes of s, p, and d orbitals.
 Answer:
 - s-orbital: Spherical.
 - p-orbital: Dumbbell shaped, oriented along x, y, z axes.
 - d-orbital: Cloverleaf shaped.

(b) Explain types of hybridization with examples.
 Answer:
 - sp → Linear (BeCl₂)
 - sp² → Trigonal planar (BF₃)
 - sp³ → Tetrahedral (CH₄)