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

9th Grade > Chemistry

ATOMS AND MOLECULES MCQs

Total Questions : 85 | Page 7 of 9 pages

Question 61.

The chemical formula of sodium carbonate is:

$N a C O_{2}$
$N a C O_{3}$
${N a}_{2} C O_{3}$
${N a}_{3} C O_{2}$

Discuss Question

Answer: Option C. ->

{N a}_{2} C O_{3}

:
C

The constituent ions present in sodium carbonate are $N a^{+}$ and ${CO}_{3}_{3}^{2 -}$

Symbol: Na $C O_{3}$

Charge: +1 -2

To balance the charges, we cross multiply the valencies and get the chemical formula of sodium carbonate as ${N a}_{2} C O_{3}$ .

Question 62.

The chemical formula of calcium hydroxide is _________.

$C a O H_{2}$
$C a (O H)_{2}$
$C a_{2} (O H)_{3}$
$C a O_{2}$

Discuss Question

Answer: Option B. ->

C a (O H)_{2}

:
B
Calcium hydroxide consist of calcium ion and hydroxide ion i.e.

C a^{2 +}

and

(O H)^{-}

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

\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 63.

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 substance is 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 64.

Which of the following elements has its symbol given in the correct format?

Aluminium - Al
Zinc - ZN
Fluorine - f
Boron - B

Discuss Question

Answer: Option A. -> Aluminium - Al
:
A and D
Symbols of elements are written as per the IUPAC guidelines. The first letter of a symbol must always be a capital letter (upper case) and the second letter should be small letter (lower case).

\begin{matrix} Element & Chemical Symbol Aluminium & Al Zinc & Zn Fluorine & F Boron & B \end{matrix}

Question 65.

What is the chemical formula of aluminium oxide?

$A l O$
${A l}_{3} O_{2}$
${A l}_{2} O_{3}$
$A l O_{2}$

Discuss Question

Answer: Option C. ->

{A l}_{2} O_{3}

:
C

∙

The combining elements of aluminium oxide are aluminium and oxygen.

∙

The chemical formula of a compound is obtained by 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 66.

Calculate the molecular mass of Na₂CO_3.

51 u
74 u
130 u
106 u

Discuss Question

Answer: Option D. -> 106 u
:
D

Molecular mass of a compound is equal to the sum of the masses of its constituent atoms. The atomic mass of sodium, carbon and oxygen are 23u, 12u and 16u respectively.

Therefore, the molecular mass of ${N a}_{2} {C O}_{3}$
$= (2 \times atomic mass of Na)$ + $(1 \times atomic mass of C)$ + $(3 \times atomic mass of O)$
$= (2 \times 23) + (1 \times 12) + (3 \times 16)$
$= 46 + 12 + 48 = 106 u$ .

Question 67.

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 that mass can neither be created nor destroyed in a chemical reaction.

Question 68.

What is the number of oxygen atoms present in 5 moles of sulphuric acid $(H_{2} S O_{4})$ ?

$120.44 \times 10^{23}$
$90.33 \times 10^{23}$
$60.22 \times 10^{23}$
$75.55 \times 10^{23}$

Discuss Question

Answer: Option A. ->

120.44 \times 10^{23}

:
A

The molecular formula of sulphuric acid is $H_{2} {SO}_{4}$ .
One molecule 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 = 20 moles of oxygen atoms.

One mole of any substance will have $6.022 \times 10^{23}$ particles of that substance. Hence, the total number of oxygen atoms in 5 moles of $H_{2} {SO}_{4}$
$= 20 \times 6.022 \times 10^{23}$
$= 120.44 \times 10^{23}$ atoms.

Question 69.

Select the correct formula for the hydride ion.

$H^{+}$
$H^{-}$
$O H^{-}$
$H_{2}$

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

H^{-}

Question 70.

Name the international organization which approves names of the elements:

IUPAC
ISPAC
IPAUC
UIPAC

Discuss Question

Answer: Option A. -> IUPAC
:
A
International Union of Pure and Applied Chemistry(IUPAC) is an international organization which takes up the responsibility of approving names of elements, units and symbols. Elements are usually named after the scientist, or the minerals where its extracted from or a place to name a few.