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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 113 marksmedium
Some Basic Concepts of Chemistry
How many molecules and how many atoms are present in 0.25 mol of CO2? (Avogadro number = 6.022 x 10^23)
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Number of molecules = 0.25 x 6.022 x 10^23 = 1.506 x 10^23 molecules. Each CO2 molecule has 3 atoms (1 C + 2 O), so number of atoms = 3 x 1.506 x 10^23 = 4.52 x 10^23 atoms.
N = n x N_A
Marking-scheme points
- ✓molecules = n x N_A = 0.25 x 6.022e23 = 1.506e23
- ✓Atoms per CO2 molecule = 3
- ✓atoms = 3 x 1.506e23 = 4.52e23
ChemistryClass 112 markseasy
Some Basic Concepts of Chemistry
Define one mole and state the value of Avogadro's number.
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Some Basic Concepts of Chemistry
A compound contains 24.27% carbon, 4.07% hydrogen and 71.65% chlorine by mass. Its molar mass is 98.96 g/mol. Find its empirical and molecular formula.
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Some Basic Concepts of Chemistry
Define molarity and molality. State their units.
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Some Basic Concepts of Chemistry
Calculate the molarity of a solution prepared by dissolving 5 g of NaOH in enough water to make 450 mL of solution.
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Some Basic Concepts of Chemistry
3.0 g of H2 reacts with 29 g of O2 to form water (2H2 + O2 -> 2H2O). Identify the limiting reagent and calculate the mass of water formed.
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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
State the main postulates of Bohr's model of the hydrogen atom.
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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
The radius of the first Bohr orbit of hydrogen is 0.529 Angstrom. Calculate the radius of the third orbit.
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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 de Broglie wavelength of an electron moving with a velocity of 2.05 x 10^7 m/s. (mass of electron = 9.1 x 10^-31 kg, h = 6.626 x 10^-34 J s)
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Structure of Atom
What are the four quantum numbers? State what each one describes.
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Structure of Atom
Write the electronic configurations of chromium (Z = 24) and copper (Z = 29). Explain why they are exceptions to the expected order.
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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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