Very Important Question nios Board Chemistry Class 12th, Chapter 1 (Atoms, Molecules, and Stoichiometry)
(MCQ)
Q: What is the smallest particle of an element?
Answer: (Atom)
Q: On which side of a chemical equation are reactants written?
Answer: (On the left side)
Q: How many atoms are present in H₂O?
Answer: 3
Q: What does ‘Na’ represent in NaCl?
Answer: (Sodium)
Q: Why are chemical equations balanced?
Answer: (To uphold the Law of Conservation of Mass)
Q: What is the SI unit of thermodynamic temperature?
Answer: The SI unit of temperature is K (Kelvin).
Q: The SI unit for the mass of a substance is the kilogram (kg). How is this expressed?
Answer: The SI unit for the mass of a substance is the kilogram. It is expressed as ‘kg’.
Q: Define the ‘Mole’.
Answer: A mole is defined as that quantity of a substance which contains the same number of fundamental particles (atoms, molecules, or ions) as there are atoms in exactly 0.012 kg (or 12 grams) of the Carbon-12 isotope.
Q: Define the ‘Molar Mass’ of an element.
Answer: The mass of 1 mole of atoms of any element is called the molar mass of that element.
Q: What is 1 amu?
Answer: 1 amu = 1/12th of the mass of a single Carbon-12 atom.
Q: What is ‘Molar Volume’? What would be the volume of 16.5 moles of an ideal gas at Standard Temperature and Pressure (STP: 273 K, 1 bar)?
Answer: The volume occupied by 1 mole of any substance is called its molar volume. Volume of 1 mole of gas at STP = 22.7 L
Volume of 16.5 moles of gas at STP = 16.5 × 22.7
= 37.55 L
Q: How many picometers are there in one meter?
Ans: 10¹² pm = 1 m; therefore, 1 m = 10¹² pm.
Q: What is the value of Avogadro’s constant?
Ans: 6.022 × 10²³ mol⁻¹
Q: Define the empirical and molecular formulas of a compound.
Ans: Empirical Formula: This is the formula that expresses the simplest ratio of the atoms of the various elements present in a molecule of a compound. It is called the empirical formula.
Molecular Formula: The formula that expresses the actual number of atoms of the various elements present in a molecule of a compound is called the molecular formula.
Q: What information does the empirical formula provide?
Ans: The empirical formula indicates the ratio of the number of atoms of the various elements present in a molecule of a compound.
Chapter 2: Atomic Structure
Q: Compare the masses of a proton and an electron.
Ans: mₚ = 1.672 × 10⁻²⁷ kg; mₑ = 9.11 × 10⁻³¹ kg.
Therefore, a proton is approximately 1040 times heavier than an electron.
Q: What are the fundamental particles of an atom?
Ans: The fundamental particles of an atom are electrons, protons, and neutrons. A proton carries a unit positive charge, an electron carries a unit negative charge, while a neutron carries no charge.
Q: What was the objective of Rutherford’s alpha (α) ray scattering experiment? Answer – The objective of Rutherford’s experiment was to examine Thomson’s Plum-Pudding model.
Explain Rutherford’s model.
Answer – According to Rutherford’s model:
① The nucleus is the positively charged part of the atom that possesses the maximum mass.
② The remaining part of the atom is empty space. Electrons reside in this region.
Q – On what grounds was Rutherford’s model rejected?
Answer – Rutherford’s model was unable to explain the stability of the atom.
Q – Explain Bohr’s model, and describe its limitations.
Answer – Bohr’s model explains the stability of the atom as well as the line spectrum of hydrogen.
Limitations of Bohr’s model:
① Bohr failed to explain the line spectra of other elements.
② It was unable to explain Heisenberg’s Uncertainty Principle.
Q – What is the difference between a quantum and a photon?
Answer – A single quantum of visible light is called a photon. The energy of a quantum is directly proportional to the frequency of the radiation.
Q – State the main postulates of Bohr’s model.
Answer – The main postulates of Bohr’s model are as follows:
① Electrons revolve around the nucleus—located at the center of the atom—in specific circular paths. These are known as stationary orbits.
When an electron moves within a specific orbit, its energy remains unchanged.
(iii) An electron absorbs or emits a photon of energy; thus, E = E₂ – E₁.
(iv) The angular momentum of an electron with mass *mₑ* is equal to the product of its mass, its velocity *v*, and the radius of its orbit, *r*. Therefore, for a single electron in a Bohr atom:
mur = nh
where n = 1, 2, 3, 4, 5…
The energy of a single electron…
Em = – (me Z² e⁴) / (8h²ε₀²)
where me = mass of the electron, Z = nuclear charge, and e = magnitude of the electron’s charge.
