Previous Year Questions Answers: Atoms and Molecules - Science Class 9

Short Answer Type Questions

Q.1. (a) Write the chemical formula of magnesium chloride and sodium hydroxide.
(b) Calculate the molecular mass of nitric acid (HNO₃).    [H = 1 u, N = 14 u, O = 16 u]     [NCT 2019]
Ans. 

(b) Molecular mass of nitric acid HNO₃
= (1 x 1 u) + (1 x 14 u) + (3 x 16 u)
= 1 u + 14 u + 48 u

= 63 u

Q.2. Find which would weigh more 2 mole of CaCO₃ or 10 mole of H₂O.    [NCT 2019]
Ans. 1 mole of CaC0₃ weighs = (1 x 40 g) + (1 x 12 g) + (3 x 16 g)

= 40 + 12 + 48 = 100 g
2 moles of CaCO₃ weigh = 2 x 100 g = 200 g
1 mole of H₂O weighs = (2 x 1 g) + (1 x 16 g)
= 2 g + 16 g = 18 g
10 moles of H₂O weigh = 10 x 18 g = 180 g
Therefore, 2 mole of CaCO₃ weigh more.

Q.3. (i) Write the chemical formula of sodium oxide.
(ii) Calculate formula unit mass of ZnO. [Zn = 65, O = 16]
(iii) Define ion.    [NCT 2018]
Ans. (i) Na₂O

(ii) Formula unit mass of ZnO = (65 u + 16 u) = 81 u
(iii) An atom or group of atoms carrying positive or negative charge is called an ion. For example, CI^-, chloride ion.

Q.4. Find the number of moles in:    [NCT 2018]
(i) 176 g carbon dioxide.
(ii) 12.044 x 10²³ number of carbon dioxide molecules.
(iii) Oxygen atoms in 44 g of carbon dioxide. [Atomic mass: C = 12 u, O = 16 u]
Ans. (i) Molecular mass of CO₂ =  12 + 32 = 44 u
44 g of CO₂ = 1 mole
176 g of CO₂ =
(ii) 6.022 x 10²³ molecules of CO₂ = 1 mole
12.044 x 10²³ molecules of CO₂ 
(iii) 44 g CO₂ means 1 mole of CO₂

= 12.044 x 10²³ atoms of oxygen

Q.5. State any four postulates of Dalton’s atomic theory of matter. Which of his postulates does not hold correct at present?    [CBSE 2016]
Ans. The postulates of Dalton’s atomic theory of matter are:
(i) All matter is made of very tiny particles called an atom.
(ii) Atoms of a given element are identical in mass and chemical properties.
(iii) Atoms of different elements have different masses and chemical properties.
(iv) Atoms combine in the ratio of simple whole numbers to form compounds.
(v) Atoms are indivisible particles that cannot be created or destroyed in a chemical reaction.
The postulate that atoms are indivisible particles does not hold good at present. We know that an atom is made up of protons, neutrons and electrons.

Q.6. (a) Define atomicity.
(b) State the atomicity of the following molecules:
(i) Oxygen
(ii) Phosphorous
(iii) Sulphur
(iv) Argon    [CBSE 2016]
Ans. (a) The number of atoms in one molecule of the substance is called atomicity.
(b) Atomicity of the substances is given as under:


Q.7. What is Avogadro’s constant?    [CBSE 2016]
Ans. Avogadro’s constant is the number of particles (atoms, ions or molecules) present in one mole of the substance. Its value is 6.022 x 10²³.

Q.8. The atomic number of three elements A, B and C are 9, 10 and 13 respectively. Which of them will form a cation?    [CBSE 2016]
Ans. The element with the atomic number 13 will form a cation. It has electronic configuration 2, 8, 3. It loses three electrons to attain a stable octet and forms the cation.

Q.9. Name the isotope of an element that is used in the treatment of cancer.    [CBSE 2016]
Ans. An isotope of cobalt is used in the treatment of cancer.

Q.10. The atomic mass of Calcium is 40 u. What will be the number of Calcium atoms in 0.4 u of Calcium?    [CBSE 2016]
Ans. 40 u of calcium contains 1 atom
0.4 u of calcium contains 1/40 x 0.4 = 10^-2 atoms

Q.11. (i) Write the full form of IUPAC.
(ii) Hydrogen and oxygen combine in the ratio of 1: 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?    [CBSE 2016]
Ans. (i) International Union of Pure and Applied Chemistry
(ii) 1 g of H₂ combines with 8 g of oxygen. 3 g of H₂ combines with 8 x 3 = 24 g of oxygen.

Q.12. Name the international organisation which approves the name given to the elements.    [CBSE 2016]
Ans. IUPAC, the International Union of Pure and Applied Chemistry.

Q.13. As per the law of definite proportions carbon and oxygen combine in a ratio of 3:8. Compute the mass of oxygen gas that would be required to react completely with 6 g carbon.     [CBSE 2016]
Ans. 3 g of carbon combines with 8 g of oxygen
∴ 6 g of carbon will combine with = 8/3 x 6 =16 g of oxygen.

Q.14. (a) Which among the following has more number of molecules:
1 g of hydrogen (H₂) or 1 g of methane (CH₄)?
(Atomic mass of H = 1 u, C = 12 u)
(b) Calculate the number of particles in 46 g of Na atoms.
(Atomic mass of Na = 23 u)    [CBSE 2016]
Ans. (a) 1 g of H₂ contains number of molecules
= 1/2 x 6.022 x 10²³
= 3.011 x 10²³ molecules
1 g of CH₄ contains number of molecules
= 1/16 x 6.022 x 10²³

∴ 1 g of H₂ contains more number of molecules.
(b) The atomic mass of Na atom = 23 g
∵ 23 g Na atom contains 6.022 x 10²³ particles
∴ 46 g Na atom will contain

Q.15. The oxide of aluminium has a chemical formula AI₂O₃. State the valency of AL.    [CBSE 2015]
Ans. The valency of AI = 3.

Q.16. State three points of difference between anion and cation.    [CBSE 2015]
Ans. 

Q.17. What is AMU? Which quantity is measured in terms of AMU?    [CBSE 2015]
Ans. AMU stands for Atomic Mass Unit.
Atomic mass is measured in terms of AMU.

Q.18. Who gave the laws of chemical combination?    [CBSE 2015]
Ans. Lavoisier and Proust

Q.19. What do you understand by an octet of electrons in the valence shell?    [CBSE 2015]
Ans. If there are eight electrons in the outermost shell of an atom, we say that it has an octet of electrons. All atoms have a tendency to acquire octet of electrons in the valence shell.

Q.20. (a) Define one mole. How is it related to Avogadro’s constant?
(b) Find the number of sodium ion in one mole of sodium sulphate.    [CBSE 2015]
Ans. (a) 1 mole is defined as equal to 6.022 x 10²³ particles. It is also equal to molar mass in grams.
1 mole = Avogadro’s constant
= 6.022 x 10²³
(b) 1 mole of Na₂SO₄ contains 2 moles of Na⁺
2 moles of Na⁺ = 2 x 6.022 x 10²³ 
= 12.044 x 10²³ = 1.2044 x 10²⁴Na⁺ ions.

Q.21. Define atomicity. Give an example of each of monoatomic, diatomic, tetra-atomic and polyatomic molecules.    [CBSE 2015]
Ans. Atomicity is defined as the number of atoms present in a molecule. He is monoatomic, H₂ is diatomic, P₄ is tetra-atomic and S₈ is polyatomic molecules.

Long Answer Type Questions

Q.1. (a) What is meant by molecular formula? State with example what information can be derived from a molecular formula?
(b) Write the names of the compounds represented by the following formulae:

(i) Mg(NO₃)₂    
(ii) K₂SO₄
(iii) Ca₃N₂    [CBSE 2016]
Ans. (a) Molecular formula of a compound shows the constituent elements and the number of atoms of each element present in one molecule of the compound.
Take the example of H₂SO₄. It is the molecular formula of sulphuric acid. It shows that
(i) Sulphuric acid molecule is constituted of hydrogen, sulphur and oxygen.
(ii) One molecule of sulphuric acid contains two atoms of hydrogen, one atom of sulphur and four atoms of oxygen.


Q.2. (i) Write chemical formulae of all the compounds that can be formed by the combination of the following ions:

(b) The molar mass of nitrogen is 14u. What will be the mass of one atom of nitrogen in grams?    [CBSE 2016]
Ans. (a)

(b) 1 mole of nitro g en atoms = 14 g
1 mole of nitrogen atoms
= 6.022 x 10²³ atom
6.022 x 10²³ atoms of nitrogen weigh = 14 g
1 atom of nitrogen weighs


Q.3. (a) Calculate the number of oxygen atoms in 0.10 mole of Na₂CO₃.10H₂O.
(b) If one mole of sulphur weighs 32 grams, what is the mass (in grams) of 1 atom of sulphur?
(c) Identify the correct formula for ammonium sulphate from the following formula:
    [CBSE 2015]
Ans. (a) 1 mole of Na₂CO₃.10H₂O contains 13 x 6.022 x 10²³ atoms of oxygen
∴ 0.10 moles of Na₂CO₃.10H₂O will contain= 0.10 x 13 x 6.022 x 10²³ 
= 7.83 x 10²³ oxygen atoms.
(b) 1 mole of sulphur = 32 g
Also 1 mole of sulphur
= 6.022 x 10²³ atoms
Now, 6.022 x 10²³ atoms of sulphur weigh = 32g
∴ 1 atom of sulphur weighs

(c) (NH₄)₂SO₄