Atoms And Molecules(9th Grade > Chemistry ) Questions and answers for exam Preparation

9th Grade > Chemistry

ATOMS AND MOLECULES MCQs

Total Questions : 85 | Page 4 of 9 pages

Question 31. Why was carbon-12 preferred over oxygen-16 to be chosen as the standard reference for measuring atomic masses? [2 MARKS]

Discuss Question

:
Each point : 1 Mark
1. Oxygen is a mixture of O-16 , O-17 and O-18 isotopes, which gives the average mass if oxygen is taken as the standard.
2. C-12 is the most abundant isotope of carbon which constitutes 98.9% of available isotopes of carbon.
So it was better to choose Carbon-12 isotope as the standard.

Question 32. Calculate the molar mass of the following substances: [5 MARKS]
(a) Ethyne, C₂H₂
(b) Sulphur molecule, S₈
(c) Phosphorus molecule, P₄ (atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO₃

Discuss Question

:
Each point: 1 Mark
(a) Ethyne, C₂H₂
Molar mass of ethyne, C₂H₂ = 2 x 12 + 2 x 1 = 26 g
(b) Sulphur molecule, S₈
Molar mass of sulphur molecule, S₈ = 8 x 32 = 256 g
(c) Phosphorus molecule, P₄ (atomic mass of phosphorus = 31)
Molar mass of phosphorus molecule, P₄ = 4 x 31 = 124 g
(d) Hydrochloric acid, HCl
Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5 g
(e) Nitric acid, HNO₃
Molar mass of nitric acid, HNO₃ = 1 + 14 + 3 x 16 = 63 g

Question 33. What is the mass of: [3 MARKS]
1. 0.2 mole of elemental oxygen atom
2. 0.5 mole of water molecules
3. 2 mole of cabon dioxide

Discuss Question

:
Each point: 1 Mark
1. Mass of one mole of elemental oxygen atom = 16 g
Then, mass of 0.2 mole of oxygen atoms = 0.2 x 16g = 3.2 g
2. Mass of one mole of water molecule = (2 x 1 g)+ 16 g=18 g
Then, mass of 0.5 mole of water molecules = 0.5 x 18 g = 9 g
3. Mass of one mole of carbon dioxide molecule = 12 g + (2 x 16 g) = 44 g
Then, mass of two mole of carbon dioxide molecule = 2 x 44 g = 88 g

Question 34. What is the full form of IUPAC? [1 MARK]

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:
Expansion: 1 Mark
IUPAC stands forInternational Union of Pure and Applied Chemistry. It is an international scientific organisation which approves names of elements, symbols and units.

Question 35. What is the chemical formula of aluminium oxide?

AlO
Al3O2
Al2O3
AlO2

Discuss Question

Answer: Option C. -> Al2O3
:
C

∙

The combining elements of aluminium oxide are aluminium and oxygen.

∙

The chemical formula of a compound is obtainedby a short-cut method of cross-multiplication of their valencies.

\begin{matrix} Aluminium & Oxygen Symbol & Al & O Valency & 3 & 2 \end{matrix}

∙

After cross-multiplication of the valencies, we get the formula of the compound as

{Al}_{2} O_{3}

Question 36. The chemical formula of calcium hydroxide is _________.

CaOH2
Ca(OH)2
Ca2(OH)3
CaO2

Discuss Question

Answer: Option B. -> Ca(OH)2
:
B
Calcium hydroxideconsist of calcium ion and hydroxideion i.e.

C a^{2 +}

and

(O H)^{-}

, which have a charge of +2 and -1respectively.

\begin{matrix} Symbol & Ca & OH Charge & +2 & -1 \end{matrix}

On cross-multiplication and balancing the charge, we get the formula as

C a (O H)_{2}

Question 37. What is the number of oxygen atoms present in 5 moles of sulphuric acid

(H_{2} S O_{4})

120.44×1023
90.33×1023
60.22×1023
75.55×1023

Discuss Question

Answer: Option A. -> 120.44×1023
:
A
The molecular formula of sulphuric acid is

H_{2} {SO}_{4}

.
Onemolecule of

H_{2} {SO}_{4}

has 4 atoms of oxygen. So, one mole of sulphuric acid will have 4 moles of oxygen. Therefore, 5 moles of sulphuric acid will have
4

\times

5 = 20moles of oxygen atoms.
One mole of any substance will have

6.022 \times 10^{23}

particles of that substance. Hence, thetotal numberof oxygen atoms in 5 moles of

H_{2} {SO}_{4}

= 20 \times 6.022 \times 10^{23}

= 120.44 \times 10^{23}

atoms.

Question 38. What is the molecular mass of the compound

S O_{2}

16 u
48 u
32 u
64 u

Discuss Question

Answer: Option D. -> 64 u
:
D
Molecular mass of a substanceis the sum of the atomic masses of its constituent atoms.
Molecular mass of

S O_{2}

=Atomic mass of S+ 2

\times

(Atomic mass of O)
=

32 + (2 \times 16)

= 64 u.

Question 39. According to __________, during a chemical reaction, the total mass of the products remains equal to that of reactants

law of conservation of mass
law of multiple proportions
avogadro's law
law of constant proportions

Discuss Question

Answer: Option A. -> law of conservation of mass
:
A
According to law of conservation of mass,during a chemical reaction, the total mass of the products remains equal to the that of reactants. In other words, the law states thatmasscan neither be created nor destroyed in a chemical reaction.

Question 40. Select the correct formula for the hydride ion.

H+
H−
OH−
H2

Discuss Question

Answer: Option B. -> H−
:
B
A monoatomic negative ion's name will always end with -ide suffix while a positive ion's name is always same as that of the element. Hydride ion is represented as