CBSE Class 9 Science Atoms and Molecules MCQs Set 03

Question. A chemical equation is always balanced to fulfil the condition of
(a) Dalton’s atomic theory
(b) law of constant composition
(c) law of multiple proportions
(d) law of conservation of mass

Answer: d

Question. How many elements are present in one formula unit of Al(OH)3?
(a) 3
(b) 4
(c) 5
(d) 6

Answer: a

Question. An atom is the
(a) smallest particle of matter known
(b) smallest particle of a gas
(c) smallest particle of an element that can take part in a chemical change
(d) radioactive emission

Answer: c

Question. The atomic mass of sodium is 23. The number of moles in 46 g of sodium are
(a) 1
(b) 2
(c) 2.3
(d) 4.6

Answer: b

Question. Identify the incorrect statement.
(a) The building blocks of all matter are atoms.
(b) Atoms are very small. They cannot be seen by the naked eye.
(c) The size of an atom is expressed in metres.
(d) An atom of hydrogen has the radius of the order of 10-10 m.

Answer: c

Question. What mass of carbon-di-oxide ^CO₂h will contain 3.011 × 10²³ molecules?
(a) 11.0 g
(b) 22.0 g
(c) 4.4 g
(d) 44.0 g

Answer: b

Question. Which of the following is a triatomic molecule?
(a) Carbon-di-oxide
(b) Ammonia
(c) Helium
(d) Sugar

Answer: a

Question. The total number of electrons present in 16 g of methane gas is
(a) 96.352 × 10²³
(b) 48.176 × 10²³
(c) 60.22 × 10²³
(d) 30.110 × 10²³

Answer: c

Question. Chemical formula of ferric oxide is
(a) FeO
(b) Fe₂O₃
(c) Fe₃O₄
(d) none of these

Answer: b

Question. All samples of carbon-di-oxide contain carbon and oxygen in the mass ratio 3 : 8. This is in agreement with the law of
(a) conservation of mass
(b) constant proportions
(c) multiple proportions
(d) gaseous volumes

Answer: b

Question. The molecular formula P₂O₅ means that
(a) a molecule contains 2 atoms of P and 5 atoms of O
(b) the ratio of the mass of P to the mass of O in the molecule is 2 : 5
(c) there are twice as many P atoms in the molecule as there are O atoms
(d) the ratio of the mass of P to the mass of O in the molecule is 5 : 2.

Answer: a

Question. Identify the correct statements.
1. In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 8 : 1.
2. If 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.
3. In ammonia, nitrogen and hydrogen are always present in the ratio 3 : 14 by mass.
4. Many compounds are composed of two or more elements and each such compound has the same elements in the same proportions.
(a) 1 and 3
(b) 1, 2 and 3
(c) 2 and 4
(d) All of these

Answer: c

Question. Select the incorrect match.
1. N₂O₄ -Dinitrogen tetroxide
2. HCl-Hydrogen chloride
3. CO-Carbon dioxide
4. PCl₅-Phosphorus trichloride
(a) 1 and 2
(b) 3 and 4
(c) 1 and 3
(d) 2 and 4

Answer: b

Question. Molecular mass is defined as the
(a) mass of one atom compared with the mass of one molecule
(b) mass of one atom compared with the mass of one atom of hydrogen
(c) mass of one molecule of any substance compared with the mass of one atom of C-12
(d) none of the above

Answer: c

FILL IN THE BLANK

Question. Those ions which are formed from single atoms are called ………
Answer: simple ions.

Question. Mole is link between the ………. and ………
Answer: mass of atoms & number of atoms.

Question. The valency of an ion is ……… to the charge on the ion.
Answer: equal.

Question. Clusters of atoms that act as an ion are called ……… ions.
Answer: polyatomic

Question. The abbreviation used for lengthy names of elements are termed as their ………
Answer: symbol.

Question. Ionic compounds are formed by the combination between ………. and ………
Answer: metal and non-metals.

TRUE/FALSE

Question. Those particles which have more or less electrons than the normal atoms are called ions.
Answer: True

Question. Formula mass of Na₂O is 62 amu.
Answer: True

Question. Formula for sulphur dioxide is SO₃.
Answer: False

Question. 22 gm of CO₂ consists of 1 mole.
Answer: False

ASSERTION AND REASON

Question. Assertion : Number of gram-molecules of SO2Cl2 in 13.5 g of sulfuryl chloride is 0.1.
Reason : Gram molecular mass is equal to one gram molecule.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: a

Question. Assertion : A molecule is the smallest particle of an element or a compound which is capable of free existence.
Reason : The number of atoms present in one molecule of the substance is called its atomicity.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: b

Question. Assertion : Atomicity of O₃ is 3.
Reason : 1 mole of an element contains 6.023 × 10²³ atoms.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: b

Question. Assertion : Atomic mass of aluminium is 27.
Reason : An atom of aluminium is 27 times heavier than 1/12th of the mass of carbon-12 atom.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: a

Question. Assertion : 1 amu equals to 6 × 10-24 g.
Reason : 1.66 × 10-24 g equals to 1/12th of mass of a C – 12 atom.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: b

Question. Assertion : Pure water obtained from different sources such as river, well, spring, sea always contains hydrogen and oxygen combined in the ratio of 1 : 8 by mass.
Reason : A chemical compound always contains same elements combined in same fixed proportion by mass.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: a

Question. Assertion : One mole of molecules has mass equal to gram molecular mass and contains Avogadro’s number of molecules or has a volume of 22.4 L at STP if the substance is a gas.
Reason : One mole of an ionic compound has mass equal to gram formula unit mass and contains Avogadro’s number of formula units.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: b

Question. Assertion : The molecular mass and formula unit mass of a substance is the sum of atomic masses of all the atoms in the molecular formula or formula unit of a compound.
Reason : The only difference between the molecular mass and formula unit mass is that, former is for molecular compounds (covalent compounds) and latter is for ionic compounds. However, their numerical value is the same.
Options:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Answer: b