Atomic Foundations of Matter Class 9 MCQ Questions Science Chapter 9

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Class 9 Science Chapter 9 Atomic Foundations of Matter MCQ

Atomic Foundations of Matter MCQ

Class 9 Science Chapter 9 MCQ online test

Question 1.
If 12 g of C is burnt in the presence of 32 g of O₂, how much CO₂ will be formed?
(A) 40 g CO₂
(B) 44 g C0₂
(C) 30 g CO₂
(D) 22 g CO₂
Answer:
(B) 44 g CO₂
CO₂ is formed by the following reaction
C (12 g) + O₂ (32 g) → CO₂
By the law of conservation of mass,
Mass of reactants = Mass of products
Mass of C + Mass of O₂ = Mass of CO₂
12g + 32g = 44g
Therefore, 44 g of CO₂ will be formed.

Question 2.
Which laws of chemical combination apply to these reactions?
Reaction 1:
C( s) + O₂ (g) → CO₂ (g)
(Carbon burns in oxygen to form carbon dioxide.)
Reaction 2:
2H₂ (g) + O₂ (g) → 2H₂O (l)
(Hydrogen and oxygen combine to form water.)
(A) Only reaction 1 follows the law of conservation of mass
(B) Both reactions follow the law of constant proportions
(C) Only reaction 2 follows the law of conservation of mass
(D) Both reactions follow the law of conservation of mass
Answer:
(D) Both reactions follow the law of conservation of mass
which states that the total mass of reactants is equal to the total mass of product.
Reaction 1:
C (s) + O₂ (g) → CO₂ (g) → Balanced
Reaction 2:
2H₂ (g) + O (g) → 2H₂O (l) → Balanced

Question 3.
Four students performed an experiment to verify the law of conservation of mass by reacting 10.6 g of sodium carbonate with 12.0 g of ethanoic acid. The products formed were sodium acetate, carbon dioxide and water. They recorded the total mass of the products.

Student	Observation (Total Mass of Products)
A	22.6 g
B	22.0 g
C	21.8 g
D	22.4 g

Which of the following observations is correct according to the law of conservation of mass?
(A) A
(B) B
(C) C
(D) D
Answer:
(A) A
According to the law of conservation of mass, the total mass of reactants (10.6 g + 12.0 g = 22.6 g) must equal the total mass of products.
So, correct observation will be of Student A (22.6 g).

Question 4.
What can be concluded from the result of the experiment?

(A) Total mass of the chemicals remain the same.
(B) Total volume of the chemicals remain the same.
(C) State of matter of the chemicals remain the same.
(D) Composition of the chemicals remain the same.
Answer:
(A) Total mass of the chemicals remain the same.
According to the law of conservation of mass, the total mass of the products remains equal to the mass of the reactants during a chemical reaction.

Question 5.
Four students A, B, C and D verified the law of conservation of mass in a chemical reaction of barium chloride and sodium sulphate. All of them took 107.2 g barium chloride solution and 116.1 g of sodium sulphate solution and mixed them in a beaker of mass 150 g. They reported their results as follows.

Students	Colour of Reaction Mixture after Mixing	Mass of reaction mixture in the Beaker Including Mass of Beaker
A	White precipitate	383.3 g
B	Brown precipitate	393.3 g
C	White precipitate	373.3 g
D	Brown precipitate	363.3 g

The correct observation is that of student
(A) A
(B) B
(C) C
(D) D
Answer:
(C) C
Mass of empty beaker = 150 g
Mass of barium chloride solution =107.2 g
Mass of sodium sulphate solution = 116.1 g
Using law of conservation of mass, in any chemical reaction
Total mass of the reactants = total mass of the products
107.2 g + 116.1 g + 150 g = 373.3 g
Thus, correct observation was made by student C.

Question 6.
The table lists some compounds and their mass ratio.

Compound	Combining Elements	Ratio by Mass
Water (H2O)	Hydrogen, oxygen	1 : 8
Carbon dioxide (CO2)	Carbon,oxygen	3 : 4
Nitrogen dioxide (NO2)	Nitrogen, oxygen	7 : 16
Magnesium sulphide (MgS)	Magnesium, sulphur	3 : 4

Which compound does not support the law of constant proportions?
(A) H₂O
(B) CO₂
(C) NO₂
(D) MgS
Answer:
(B) CO₂
The ratio of masses in carbon dioxide (CO₂) = 12 : 32 = 3 : 8
Therefore, among the given compounds, on the basis of ratio by mass, CO₂ does not support the law of constant proportions.

Question 7.
Four statements about atoms are listed.
i. All matter is made up of atoms.
ii. Atoms are tiny indivisible particles which can neither be created nor be destroyed in a chemical reaction.
iii. Atoms combine in simple whole number ratios to form compounds.
iv. Atoms of different elements can have the same mass.
Choose the statements which are the postulates of Dalton’s atomic theory.
(A) Only i and iv
(B) Only ii and iii
(C) i, ii and iv
(D) i, ii and iii
Answer:
(D) i, ii and iii
Atoms of different elements have different mass. The statements (i), (ii) and (iii) are the postulates of Dalton’s atomic theory.

Question 8.
The most remarkable concept that Dalton’s atomic theory proposed was that of the
(A) atomic weight
(B) atomic mass
(C) molar mass
(D) atomicity
Answer:
(B) atomic mass
Atomic mass was the most remarkable concept proposed by Dalton’s atomic theory.

Question 9.
Which of the following statement is not true about an atom?
(A) Atoms are not able to exist independently.
(B) Atoms are the basic units from which molecules and ions are formed.
(C) Atoms are always neutral in nature.
(D) Atoms aggregate in large numbers to form the matter that we can see, feel or touch.
Answer:
(D) Atoms aggregate in large numbers to form the matter that we can see, feel or touch. is not true.

The corrected statement is written as :
The molecules and ions aggregate together in large number to form the matter. We cannot see the individual molecules/ions with our eyes, only we can see the various substances which are a big collection of molecules/ions.

Question 10.
Which of the following correctly represents a molecule of an element?
(A) H₂O
(B) NH₃
(C) O₂
(D) HCl
Answer:
(C) O₂
A molecule of an element contains atoms of the same element only. O₂ contains two oxygen atoms, so it is a molecule of an element.

Question 11.
Look closely at the provided diagram of the oxygen molecule. After the bond is formed, how many non-bonding (lone pair) electrons remain in the outermost shell of each individual oxygen atom?

(A) 2
(B) 4
(C) 6
(D) 8
Answer:
(B) 4
Each oxygen atom has 6 valence electrons. In an oxygen molecule (O₂), two pairs of electrons are shared to form a double covalent bond. Thus, out of 6 electrons, 2 electrons are used in bonding, and the remaining 4 electrons stay as non-bonding (lone pair) electrons on each oxygen atom.

Question 12.
Which of the following is the correct name of the compoundCO₂?
(A) Carbon oxide
(B) Carbon dioxide
(C) Carbon monoxide
(D) Dicarbon oxide
Answer:
(B) Carbon dioxide
In naming covalent compounds, prefixes are used to show the number of atoms. CO₂ contains one carbon atom and two oxygen atoms, so it is called carbon dioxide.

Question 13.
Which of the following is correctly matched?
(A) H₂S – Hydrogen monosulphide
(B) PCl₃ – Phosphorus trichloride
(C) CO – Carbon dioxide
(D) NH₃ – Nitrogen trihydride
Answer:
(B) PCl₃ – Phosphorus trichloride
PCl₃ contains one phosphorus atom and three chlorine atoms, so its correct name is phosphorus trichloride.
H₂S is hydrogen sulphide, CO is carbon monoxide, and NH₃ is commonly called ammonia.

Question 14.
Match Column I with Column II.

Column I	Column II
A. Covalent bond	(i) Transfer of electrons
B. Molecule of element	(ii) Same kind of atoms
C. Molecule of compound	(iii) Different kinds of atoms
D. Ionic bond	(iv) Sharing of electrons

(A) A-(iv), B-(ii), C-(iii), D-(i)
(B) A-(i), B-(iii), C-(ii), D-(iv)
(C) A-(iv), B-(iii), C-(ii), D-(i)
(D) A-(ii), B-(iv), C-(iii), D-(i)
Answer:
(A) A-(iv), B-(ii), C-(iii), D-(i)

Question 15.
Which of the following statements best describes an ionic bond?
(A0 Sharing of electrons between atoms
(B) Transfer of electrons from one atom to another
(C) Equal sharing of electrons
(D) Overlapping of atomic orbitals
Answer:
(B) Transfer of electrons from one atom to another
In an ionic bond, one atom loses electrons to form a cation and another gains electrons to form an anion. The electrostatic attraction between these oppositely charged ions forms the bond.

Question 16.
Element X has valency 1 and element Y has valency 2. What will be the formula of the compound formed between X and Y?
(A) XY
(B) XY₂
(C) X₂Y
(D) X₂Y₃
Answer:
(C) X₂Y
Element X forms X⁺ and Y forms Y^2-. To balance charges, two X⁺ ions combine with one Y^2- ion, giving the formula X₂Y.

Question 17.
Which of the following represents a correct chemical formula ? [NCERT Exemplar]
(A) CaCl
(B) BiPO₄
(C) NaSO₄
(D) NaS
Answer:
(B) BiPO₄, is the correct formula, its name is bismuth phosphate.
Correct chemical formula of remaining species are
CaCl₂ – Calcium chloride
Na₂SO₄ – Sodium sulphate
Na₂S – Sodium sulphide

Question 18.
The valency of hydrogen is 1 and that of sulphur is 2. The chemical formula of hydrogen sulphide is
(A) HS₂
(B) HSi₂
(C) H₂S
(D) H₂Si
Answer:
(C)

Formula: H₂S
Thus, the formula of hydrogen sulphide is H₂S.

Question 19.
An element ‘A’ has valency equal to 3. What will be its formula with carbonate ions?
(A) A₂CO₃
(B) ACO₃
(C) A₂(CO₃)₃
(D) A(CO₃)₃
Answer:
(C)

Formula: A₂(CO₃)₃

Question 20.
The formula of chloride of a metal M is MCl₃ , then the formula of the phosphate of metal M will be
(A) M₂PO₄
(B) MPO₄
(C) M₂(PO₄ )₃
(D) M₃ PO₄
Answer:
(B)

Formula: MPO₄
∴ The formula of phosphate of metal M will be MPO₄.

Question 21.
A student compares two substances. Substance P dissolves in water and its molten form conducts electricity. Substance Q does not dissolve in water and does not conduct electricity in molten state. Which statement is correct?
(A) Both P and Q are ionic compounds
(B) P is likely ionic, while Q is likely covalent
(C) P is covalent, while Q is ionic
(D) Both P and Q are metals
Answer:
(B) P is likely ionic, while Q is likely covalent
Substance P is likely ionic because it dissolves in water and conducts electricity when molten due to mobile ions, whereas substance Q is likely covalent as it lacks these properties.

Question 22.
Which of the following correctly distinguishes ionic and covalent compounds?
(A) Ionic compounds are formed by sharing of electrons.
(B) Covalent compounds are formed by transfer of electrons.
(C) Ionic compounds generally have high melting points, while covalent compounds have low melting points.
(D) Both have similar physical properties.
Answer:
(C) Ionic compounds generally have high melting points, while covalent compounds have low melting points.
Strong electrostatic forces in ionic compounds require more energy to break, leading to high melting points, whereas covalent compounds have weaker intermolecular forces and thus have low melting points.

Question 23.
Four statements about atomic and molecular masses are listed.
i. A molecule of nitrogen (N₂) has a mass equal to twice the atomic mass of nitrogen.
ii. A molecule of oxygen (O₂) has a mass equal to twice the atomic mass of oxygen.
iii. Neon is a monoatomic gas, so its atomic mass is equal to its molecular mass.
iv. A molecule of chlorine (Cl₂) has a mass equal to twice the atomic mass of chlorine.
Which of the above statements correctly describes an element whose atomic mass and molecular mass are equal?
(A) i and ii
(B) ii and iii
(C) iii only
(D) iii and iv
Answer:
(C) iii only
Neon is a noble gas and exists as single atoms (monoatomic). Hence, its atomic mass and molecular mass are the same. All other elements mentioned (N₂, O₂, Cl₂) exist as diatomic molecules, so their molecular mass is twice their atomic mass.
But they do not describle elements whose atomic mass and molecular mass are equal.

Question 24.
The table list atomic masses and atomic numbers of three elements.

Elements	Atomic number	Atomic Mass (amu)
Mg	12	24.305
Fe	26	55.845
O	8	16.0

A student studies that magnesium (Mg) and iron (Fe) react with oxygen (O) to form MgO and FeO. Although both compounds contain two atoms each of their molecular masses are different. What is the reason for the difference in their molecular masses?
(A) Difference in the atomic mass of iron and magnesium.
(B) Difference in the number of iron and magnesium atoms.
(C) Difference in the atomic number of iron and magnesium.
(D) Difference in the number of O atoms in the two compounds.
Answer:
(A) Difference in the atomic mass of iron and magnesium.
MgO and FeO, although both compounds contain two atoms but each have different molecular masses. This is because of the difference in the atomic mass of iron and magnesium as both compounds have oxygen in common. The atomic masses of Mg and Fe are 24.3 g and 55.8 g respectively.
∴ FeO > MgO

Question 25.
What is the formula unit mass of Na₂O?
(A) 20 u
(B) 39 u
(C) 55 u
(D) 62 u
Answer:
(D) 62 u
Formula unit mass of Na₂O = 2 × atomic mass of Na + atomic mass of O
= 2 × 23 + 16
= 46 + 16 = 62 u.

Question 26.
A sample of pure water, irrespective of its source, contain 11.1% hydrogen and 88.9% oxygen. The data supports
(A) law of multiple proportions
(B) law of reciprocal proportions
(C) law of constant proportions
(D) law of conservation of mass
Answer:
(C) Law of constant proportions.
According to the law of constant proportions, a given compound always contains the same elements in the same fixed proportion by mass, regardless of its source or method of preparation.

Question 27.
What is the ratio of mass between carbon and oxygen in carbon dioxide?
(A) 3 : 8
(B) 5 : 6
(C) 2 : 9
(D) 7 : 4
Answer:
(A) 3 : 8
In CO₂, carbomoxygen mass ratio is 12 : 32 = 3 : 8

Question 28.
Which of the following is a monoatomic element?
(A) Nitrogen
(B) Hydrogen
(C) Helium
(D) Oxygen
Answer:
(C) Helium
It exists as a single atom (monoatomic).

Question 29.
In which of the following, the valency of each of the constituent elements is equal to the total number of atoms in one molecule of the compound?
(A) CH₄
(B) H₂S
(C) MgCl₂
(D) CaO
Answer:
(D) CaO
Valency equals to total atoms in the molecule only in this option (D).

Question 30.
An element X is divalent and another element Y is tetravalent. The compound formed by these two
elements will be
(A) XY
(B) XY₂
(C) X₂Y
(D) XY₄
Answer:
(C) X₂Y
Divalent X and tetravalent Y form X₄Y₂ which simplified to give X₂Y.

Question 31.
A student learns that aluminium forms compound with chlorine and oxygen. She records the valencies of the three elements.

Element	Valency
Aluminium	3 +
Chlorine	1 –
Oxygen	2 –

Which option gives the correct formula of aluminium oxide and aluminium chloride?
(A) Aluminium oxide: Al₂O₃; aluminium chloride: AlCl₃
(B) Aluminium oxide: 3 AlO₂; aluminium chloride: 3 AlCl₂
(C) Aluminium oxide: Al₃O₂; aluminium chloride: Al₃Cl₂
(D) Aluminium oxide: 3 (AlO)₂; aluminium chloride: 2 (Al₃Cl)
Answer:
(A) Aluminium oxide: Al₂O₃; aluminium chloride: AlCl₃
It is the correct combination using given valencies.

Atomic Foundations of Matter Class 9 Assertion and Reason Questions

Direction (Question Nos. 1 – 11): The following questions consist of two statements-Asssertion (A) and Reason (R):. Answer these questions by selecting the appropriate option given below.
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
(C) Assertion is true, but Reason is false.
(D) Assertion is false, but Reason is true.

Question 1.
Assertion (A): On burning magnesium in oxygen, magnesium oxide is formed. The mass of magnesium oxide formed is equal to the total mass of magnesium and oxygen.
Reason (R): In a chemical substance, the elements are always present in a definite proportion.
Answer:
(B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.

When magnesium burns in oxygen, magnesium oxide is formed. The total mass of the products equals the total mass of reactants, which follows the law of conservation of mass, not the law of definite proportions. Hence, both Assertion and Reason are true but Reason is not the correct explanation of Assertion.

Question 2.
Assertion (A): Pure water obtained from different sources such as river, well, spring, sea, etc. always contains hydrogen and oxygen in the ratio of 1: 8 by mass.
Reason (R): A chemical compound always contains elements combined in a fixed proportions by mass.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

The law of constant proportion states that in a chemical substance, the elements are always present in definite proportion by mass. So, pure water obtained from different sources such as river, well, spring, etc. contain H and O in the ratio 1: 8. Hence, both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Question 3.
Assertion (A): Atoms always combine to form molecules and ions.
Reason (R): Atoms of most element are not able to exist independently.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Except for noble gases (like He, Ne, Ar), most atoms are unstable alone and thus combine with other atoms to form molecules or ions to attain stability.

Question 4.
Assertion (A): When a sodium atom loses one electron, it forms a cation (Na⁺).
Reason (R): Loss of an electron makes the number of electrons less than the number of protons.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
On losing one electron, sodium has more protons than electrons, resulting in a net positive charge and formation of a cation (Na⁺). Hence, both assertion and reason are true and reason is the correct explanation of assertion.

Question 5.
Assertion (A): Magnesium ion and chloride ion combine to form a compound having chemical formula MgCl₂.
Reason (R): Magnesium ion and chloride ion contain +1 and -2 charges respectively.
Answer:
(C) Assertion is true, but Reason is false.
Magnesium ion (Mg²⁺) has a charge of +2 and chloride ion (Cl^–) has a charge of -1.
Two chloride ions are needed to balance one magnesium ion, forming MgCl₂.

Question 6.
Assertion (A): The chemical formula of aluminium oxide is Al₂O₃.
Reason (R): The valency of aluminium is 2 and oxygen is 3.
Answer:
(C) Assertion is true, but Reason is false.
The chemical formula of aluminium oxide is

Formula: Al₂O₃
Thus, valency of aluminium is 3 and oxygen is 2. Hence, Assertion is true, but Reason is false.

Question 7.
Assertion (A): Ionic compounds are generally soluble in water, whereas covalent compounds are usually insoluble in water.
Reason (R): Water is a polar solvent and can separate ions, while covalent compounds are mostly non-polar.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Ionic compounds dissolve in water as polar water molecules attract and separate ions. Covalent compounds are mostly non-polar, so they do not dissolve easily in water. Hence, both assertion and reason are true and reason is the correct explanation of assertion.

Question 8.
Assertion (A): Molecular weight of oxygen is 16 u.
Reason (R): Atomic weight of oxygen is 16 u.
Answer:
(D) Assertion is false, but Reason is true.
Oxygen exists as a diatomic molecule (O₂).
Molecular weight of oxygen (O₂) = 16 × 2 = 32 u
Hence, Assertion is false, but Reason is true.

Question 9.
Assertion (A): When 10 g of CaCO₃ is decomposed, 5.6 g of residue is left and 4.4 g of CO₂ escapes.
Reason (R): Law of conservation of mass is followed.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
The total mass before (10 g) equals the total mass after (5.6 g + 4.4 g = 10 g), which obeys the law of conservation of mass. Hence, both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Question 10.
Assertion (A): In carbon dioxide the ratio of mass of carbon to the mass of oxygen is always 3 : 8.
Reason (R): According to law of constant proportion, the elements are always present in definite proportion by mass in a chemical substance.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
CO₂ always has a fixed carbon-to-oxygen mass ratio of 3 : 8, which is a direct application of the law of constant proportions. Hence, both Aassertion and Reason are true and Reason is the correct explanation of Assertion.

Question 11.
Assertion (A): Atomicity of O₃ is 3.
Reason (R): Atomicity is the combining power of an element with other atom.
Answer:
(C) Assertion is true, but Reason is false.
The atomicity of O₃ is 3. Atomicity is not the combining capacity but it refers to the number of atoms in a molecule. Thus, Assertion is true but Reason is false.