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

9th Grade > Chemistry

ATOMS AND MOLECULES MCQs

Total Questions : 85 | Page 5 of 9 pages

In a biogas plant, methane gas ( $C H_{4}$ ) is produced as a fuel. Find the mass ratio of carbon to hydrogen in methane.

$3 : 1$
$2 : 1$
$1 : 3$
$4 : 2$

Discuss Question

Answer: Option A. ->

3 : 1

:
A

Mass of one carbon atom = 12u
Mass of each hydrogen atom = 1u
Mass of 4 hydrogen atoms = 4u
Mass ratio of carbon and hydrogen atoms in methane $= 12 : 4 = 3 : 1$

Question 42.

Which of the following is the chemical symbol for iron?

Discuss Question

Answer: Option B. -> Fe
:
B

Fe is the chemical symbol for iron, I is the chemical symbol for iodine, Zn is the chemical symbol for zinc and Na is the chemical symbol for sodium.

Question 43.

What does the abbreviation amu stand for?

Atomic molecular unit
Atomic mass unit
Atomic mole unit
Atomic measuring unit

Discuss Question

Answer: Option B. -> Atomic mass unit
:
B

According to IUPAC recommendations, mass of the atom is expressed as atomic mass unit (amu) or unified atomic mass unit (u). 1 amu is defined as one-twelth the mass of a carbon-12 atom.

Question 44.

Atomicity of oxygen molecule is 1.

True
False
Atomic mole unit
Atomic measuring unit

Discuss Question

Answer: Option B. -> False
:
B

Atomicity means the number of atoms present in one molecule. Oxygen exists as $O_{2}$ . The atoms in one molecule of oxygen is 2. Hence, it is diatomic and the given statement is false.

Question 45.

What is the number of oxygen atoms present in 126 g of ${H N O}_{3}$ ?

$36.132 \times 10^{23}$
$9.033 \times 10^{23}$
$120.44 \times 10^{23}$
$16.132 \times 10^{23}$

Discuss Question

Answer: Option A. ->

36.132 \times 10^{23}

:
A
Molecular mass of

H N O_{3}

= atomic mass of hydrogen + atomic mass of nitrogen + 3

\times

atomic mass of oxygen = 1 + 14 + (3

\times

16) = 63 u.

Number of moles = $\frac{G i v e n m a s s}{G r a m m o l e c u l a r m a s s}$
Moles of $H N O_{3}$ = $\frac{126}{63}$ = 2 moles

∙

In 1 molecule of

H N O_{3}

we have 3 atoms of oxygen.

∙

1 mole of

H N O_{3}

contains 3 moles of oxygen atoms. So, in 2 moles of

H N O_{3}

we will have 6 moles of oxygen atoms.

∙

One mole of a substance contains

6.022 \times 10^{23}

particles. So, the number of atoms in 126 g of

H N O_{3}

6 \times 6.022 \times 10^{23}

36.132 \times 10^{23}

atoms.

Question 46.

Which theory has provided an explanation for the law of conservation of mass and the law of definite proportions?

Dalton's atomic theory
Lavoisier's theory
J.J Thomson's atomic theory
Democritus atomic theory

Discuss Question

Answer: Option A. -> Dalton's atomic theory
:
A

Daltons' atomic theory was based on the laws of chemical combinations i.e., law of conservation of mass and the law of definite proportions. The two postulates which supports the law of combinations are:

Atoms combine in the ratio of small whole numbers to form compounds.

Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction

Question 47.

What is the term used for the molecular mass of ionic compounds?

Ionic mass
Ionic molecular weight
Formula unit mass
Molecular Mass

Discuss Question

Answer: Option C. -> Formula unit mass
:
C
The word formula unit is used for the substances of which constituent particles are ions.
For example, magnesium chloride has a formula unit

M g C l_{2}

Its formula unit mass can be calculated as:

(1 \times A t o m i c m a s s o f m a g n e s i u m) + (2 \times A t o m i c m a s s o f c h l o r i n e)

(1 \times 24) + (2 \times 35.5) = 95 u

Question 48.

What is the mass of one atom of neon if its gram atomic mass is 20 grams?

$2.65 \times 10^{- 23}$ grams
$3.32 \times 10^{- 23}$ grams
$5.32 \times 10^{- 23}$ grams
$4.32 \times 10^{- 23}$ grams

Discuss Question

Answer: Option B. ->

3.32 \times 10^{- 23}

grams
:
B
1 mole of an element contains

6.022 \times 10^{23}

atoms.
1 mole of a compound contains

6.022 \times 10^{23}

molecules.

Neon is monoatomic in nature.
It is given that the gram atomic mass of Neon is 20 g

So, $6.022 \times 10^{23}$ atoms of Neon weigh = 20 grams

The mass of one atom
= $\frac{20}{6.022 \times 10^{23}}$
= $3.32 \times 10^{- 23}$ g

Question 49.

Find the number of hydrogen atoms in $9.033 \times 10^{23}$ molecules of water.

$18.066 \times 10^{23}$
$9.033 \times 10^{23}$
$4.52 \times 10^{23}$
$6.022 \times 10^{23}$

Discuss Question

Answer: Option A. ->

18.066 \times 10^{23}

:
A
The chemical formula of water is

H_{2} O

.
One molecule of water has 2 hydrogen atoms.

9.033 \times 10^{23}

molecules of water will have

2 \times 9.033 \times 10^{23} = 18.066 \times 10^{23}

atoms of hydrogen.

Question 50.

According to the Dalton, which element does the following symbol represent?

Hydrogen
Carbon
Oxygen
Iron

Discuss Question

Answer: Option A. -> Hydrogen
:
A
Dalton was the first scientist to use the symbols for elements in a very specific sense. The symbols are the unique pictorial representation of each element. The given symbol was used for representing the element "hydrogen".