CBSE Class 9 Science
Important Questions
Chapter 3
Atoms and Molecules
2 Marks Questions
1. 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?
Ans. As per the given 1:8 ratio mass of oxygen gas required to react completely with 1g of
hydrogen gas is 8g.
Therefore mass of oxygen gas required to react completely with 3g of hydrogen gas will be =
3 X 8 = 24g
2. Which postulate of Dalton’s atomic theory is the result of the law of conservation of
mass?
Ans. The postulate of Dalton’s atomic theory which is the result of the law of conservation of
mass is mentioned as below :
Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
3 .Which postulate of Dalton’s atomic theory can explain the law of definite
proportions?
Ans. The postulate of Dalton’s atomic theory which explains the law of definite proportions
is “Atoms combine in the ratio of small whole numbers to form compounds and the relative
number and kinds of atoms are constant in a given compound.”
4. Why is it not possible to see anatom with naked eyes?
Ans. An atom is an extremely minute particle and as such actual mass of an atom of
hydrogen is considered to be 1.6X10-24 g. That is why it is not possible to see an atom with
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naked eyes.
5. How many atoms are present in a
(i) S molecule and
(ii) ion?
Ans. (i) 2atoms of hydrogen + 1 atom of sulphur = 3 atoms
(ii)1atom of phosphorus + 4atoms of oxygen = 5 atoms
6. Calculate the formula unit masses of ZnO, O, ,given atomic masses of Zn
= 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
Ans. Formula unit mass of :
(i) ZnO = Atomic mass of Zn + atomic mass of O = (65 + 16)u = 81 u
(ii) O = Atomic mass of Na +atomic mass of O = (23x2) + 16 = 46 + 16 = 62 u
(iii) = (39 x 2) + 12 + (16 x3) = 78 + 12 + 48 = 138 u
7. Which has more number of atoms, 100 grams of sodium or100 grams of iron (given,
atomic mass of Na = 23 u, Fe = 56 u)?
Ans. We can find out the element with more number of atoms by calculating number of
moles of each of them :
Number of moles of sodium in 100g = m1/M1 = 100/23 = 4.34
Number of moles of iron in 100g = m2/M2 = 100/56 = 1.79
Therefore, the number of atoms is more for sodium as compared to iron.
8. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced.
What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of
oxygen? Which law of chemical combination will govern your answer?
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Ans. According to the law of chemical combination of constant proportions “in a chemical
compound the elementary constituents always combine in constant proportions by
weight/mass”. Therefore whether 3 g carbon is burnt in 8 g oxygen or 3g carbon is burnt in
50g oxygen in both cases only 11g carbon dioxide will be formed.
9. What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Ans. (a)since 1 mole of O = atomic mass of O = 16u=16g
then 0.2mole of O = 0.2 x 16 = 3.2g
(b) 1mol of O = molecular mass of O = 1x2 + 16 =18 u =18g
then 0.5mol of O = 0.5x18 = 9g
10. Calculate the number of molecules of sulphur (S8) present in16 g of solid sulphur.
Ans. 1mol of S8 = molecular mass of S8 = 32 x 8 = 256u=256g
since 256g of S8= 1mol = 6.022 x 1023 atoms (Avogadro number)
16g of S8= 16x6.022 x 1023/256 = 3.76x1022 molecules
11. Calculate the number of aluminium ions present in 0.051 g ofaluminium oxide.
(Hint: The mass of an ion is the same as that of an atom of thesame element. Atomic
mass of Al = 27 u)
Ans. 1mol of = molecular mass of = 27x2 + 16x3 = 102u=102g
aluminium ions present in = 2Al3+
102 g of contains aluminium ions = 2x6.022x1023
then 0.051 g containsaluminium ions = 2x6.022x1023x0.051/102= 6.022x1020
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12. State law of conservation of Mass?
Ans. According to law of conservation of Mass, mass can neither be created nor be destroyed
in a chemical reaction.
13. Define Valency? Find the Valency of oxygen and Aluminum.
Ans. Valency is defined as the number of electrons that an element has to gain or loose from
its outermost shell so that it can be stable or the combining capacity of an atom.
Oxygen – Atomic number = 8; Electronic configuration = 2, 6 i.e. it has to gain 2e- so that in :
outer most shell has 8e-, Valency of O is -2 Similarly, valency of Al (Aluminum) is +3 (
electronic configuration → 2, 8, 3) so it looses 3e- from its outermost shell.
14. Calculate the molar mass of and CaC O3?
Ans. Molar Mass of =
2× Mass of sodium + 1× Mass of sulphur + 4× Mass of oxygen
= 2×23+1×32+4×16
= 46+32+64
= 142 a.m.u.
Molar Mass of CaC =
1× Mass of calcium +1×Mass of carbon + 3×Mass of oxygen
= 40+12+3×16
= 40+12+48 = 100 a.m.u.
15. Write the chemical formula for
a) Calcium Phosphate b) Magnesium Hydroxide c) Aluminum chloride.
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Ans. Chemical formula for
1) Calcium Phosphate
2) Magnesium Hydroxide
3) Alminium chloride
16. State the law of constant Proportion?
Ans. According to law of constant proportion, whatever the method of its formation, a
chemical compound in its pure state will always contain the same elements combined
together in the fixed ratio by mass.
17. How many molecules of water are present in a drop of water which has a mass of
50mg?
Ans. We know that:-
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1 mole of an compound = 6.023×1023 atoms
= Gram molecular mass
Gram Molecular mass of O = 18g
= 1.673× molecules.
18. Find the number of protons and neutrons in the nucleus of an atom of an element X
which is represented as .
Ans. The element is
Now, 82 = Atomic Number
207 = Mass Number.
(a) Atomic number = number of protons
82 = Number of protons.
(b) Mass number = Number of protons + Number of neutrons
207 = 82 + Number of neutrons
207 – 82 = Number of neutrons
125 = Number of neutrons.
19. Define the terms:- a) Atomic number b) Mass number
Ans. Atomic Number is defined as the total number of protons present in an atom.
Mass number is defined as the sum total of number of protons and the number of neutrons
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present in an atom.
20. What is meant by e/m ratio? What was the value of this ratio for a particle in the
cathode rays?
Ans. ratio is the ratio of charge of the particle to mass of those particle.
The ratio for cathode rays was found out to be 1.759× C / Kg.
21. Complete the following equations which describe nuclear charge –
a)
b)
c)
Ans. In any nuclear reaction, mass number (no. of p+ + no. of no) and atomicnumber (no. of
p+) must be conserved.
(1)
(2)
(3)
22. Stat the properties of cathode rays?
Ans. Properties of cathode rays are :-
(1) They travel in straight line
(2) They have momentum and energy
(3) They are deflected by electric and magnetic fields.
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