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800 most-asked Class 11 & 12 (+1 / +2) questions across Physics, Chemistry, Maths and Biology — each with a model answer and the exact marking-scheme points examiners reward. Revise smart, walk in calm.
ChemistryClass 112 markseasy
Some Basic Concepts of Chemistry
State the law of conservation of mass and the law of definite proportions.
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Law of conservation of mass: matter can neither be created nor destroyed in a chemical reaction; the total mass of reactants equals the total mass of products. Law of definite proportions: a given chemical compound always contains the same elements combined in the same fixed proportion by mass, irrespective of its source or method of preparation.
Marking-scheme points
- ✓Conservation of mass: mass of reactants = mass of products
- ✓Definite proportions: fixed ratio of elements by mass
- ✓Example: water always 1:8 H:O by mass
The concept
Two founding laws: mass is conserved in a reaction, and a compound always has the same fixed ratio of elements.
Why it matters
They gave birth to the atomic theory and balanced equations - the start of modern chemistry.
💡 Memory trick
Mass stays (conservation); ratio stays (definite proportions) - 'both stay the same'.
ChemistryClass 112 markseasy
Some Basic Concepts of Chemistry
Calculate the number of moles present in 11 g of carbon dioxide (CO2).
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Molar mass of CO2 = 12 + 2(16) = 44 g/mol. Number of moles = given mass / molar mass = 11 / 44 = 0.25 mol.
n = m / M
Marking-scheme points
- ✓Molar mass of CO2 = 44 g/mol
- ✓moles = mass / molar mass
- ✓n = 11/44 = 0.25 mol
ChemistryClass 112 markseasy
Some Basic Concepts of Chemistry
Define one mole and state the value of Avogadro's number.
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One mole is the amount of a substance that contains as many elementary entities (atoms, molecules, ions) as there are atoms in exactly 12 g of carbon-12. This number of entities is Avogadro's number, N_A = 6.022 x 10^23 mol^-1.
1 mol = 6.022 x 10^23 particles
Marking-scheme points
- ✓Mole = amount containing N_A entities
- ✓Reference: atoms in 12 g of C-12
- ✓N_A = 6.022 x 10^23 per mole
ChemistryClass 112 markseasy
Some Basic Concepts of Chemistry
Define molarity and molality. State their units.
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Some Basic Concepts of Chemistry
Define mole fraction and mass percent of a component in a solution.
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Structure of Atom
Calculate the energy of a photon of light of wavelength 4000 Angstrom. (h = 6.626 x 10^-34 J s, c = 3 x 10^8 m/s)
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Structure of Atom
State Heisenberg's uncertainty principle and give its mathematical form.
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Structure of Atom
Calculate the wave number of the spectral line when an electron in a hydrogen atom jumps from n = 3 to n = 2. (Rydberg constant R = 1.097 x 10^7 m^-1)
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Structure of Atom
State Pauli's exclusion principle and Hund's rule of maximum multiplicity.
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Structure of Atom
State the Aufbau principle and the (n + l) rule for filling of orbitals.
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Classification of Elements and Periodicity
State the modern periodic law. How does it differ from Mendeleev's periodic law?
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Classification of Elements and Periodicity
Why is the first ionization enthalpy of nitrogen greater than that of oxygen?
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Classification of Elements and Periodicity
Why is the electron gain enthalpy of chlorine more negative than that of fluorine?
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Classification of Elements and Periodicity
Define electronegativity. How does it vary across a period and down a group?
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Classification of Elements and Periodicity
Arrange the isoelectronic species O2-, F-, Na+ and Mg2+ in increasing order of ionic radius. Justify.
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Chemical Bonding and Molecular Structure
Distinguish between an ionic bond and a covalent bond with one example each.
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Chemical Bonding and Molecular Structure
Why is the bond angle of H2O (104.5 deg) less than that of NH3 (107 deg)?
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Chemical Bonding and Molecular Structure
Calculate the bond order of the nitrogen molecule (N2) using molecular orbital theory.
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Chemical Bonding and Molecular Structure
What is a coordinate (dative) bond? Explain with the example of the ammonium ion.
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Chemical Bonding and Molecular Structure
What is a hydrogen bond? State the conditions for its formation and its two types.
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