# Atoms and Molecules MCQ Chapter 3 Class 9 Science

Atoms and Molecules MCQ/Objective questions Chapter 3 Class 9 Science

#### Law of chemical combination says:

• a. The total mass of reactants equals the total mass of products.
• b. A found always has its constituent in a fixed proportion
• c. A reaction happens only if there is the liberation of energy
• d. Both a and b
d. Both a and b
Explanation
Option a depicts law of conservation of mass while option b is law of constant proportion. Both together make law of chemical combination.

#### Who gave the law of chemical combination?

• a. Pakudha Katayama
• b. Democritus and Leucippus
• c. Antonie L. Lavoisier
• d. John Dalton
c. Antonie L. Lavoisier
Explanation
Pakudha said all matters consist of tiny particle that can not be further divided. He called it “parmanu”. Democritus and Leucippus used the term atom for the smallest molecule. Both these were philosophical ideas with no real explanation. Lavoisier first observed the reactions and gave laws of chemical combination. Dalton further clarified the laws of chemical combination by giving Dalton’s atomic theory

#### Dalton theory applies for:

• a.  Alloys
• b. Mixtures
• c. Chemical compounds
• d. All of the above
c. Chemical compounds

#### How many grams of water make one mole?

• a. 16gm
• b. 18gm
• c. 2gm
• d. 160gm
b. 18gm
Explanation
One mole of water = 2 mole of hydrogen and one mole of oxygen atom. Here atomic weight of hydrogen atom is 1 and that of oxygen is 16gm, so, 1+1+16 = 18gm.

#### One mole of water contain how many molecules of water?

• a. 6.022 X 1020
• b. 6.022 X 1021
• c. 6.022 X 1022
• d. 6.022 X 1023
d. 6.022 X 1023

#### One mole of oxygen contain how many atoms of oxygen

• a. 1 X 6.022 X 1022
• b. 2 X 6.022 X 1022
• c. 3 X 6.022 X 1022
• d. 4 X 6.022 X 1022
b. 2 X 6.022 X 1022

#### How many grams of common salt (NaCl) will be formed when 11.5gm of sodium (Na) reacts with 35.5gm of chlorine gas(Cl2)? Atomic weight of sodium is  23 and that of chlorine gas is 71.

• a. 47 (11.5+35.5)
• b. 29.25 (11.5+35.5/2)
• c. 41.25 (11.5/2+35.5)
• d. None of the above
b. 29.25 (11.5+35.5/2)
Explanation
Here half mole of sodium is limiting factor. It will react with only half mole of chlorine atom.

#### Carbon dioxide in air and the carbon dioxide dissolved in water, both contain carbon and oxygen in ratio of 12:32 gram. This shows:

• a. Law of conservation of mass
• b. Law of constant proportion
• c. Law of combination
• d. Law of conservation of energy
b. Law of constant proportion

#### The mass of constituent atoms A and B of compound AxBy are in the ratio of 7:16 but their atomic weight is in the ratio of 14 and 16. The possible formula of A and B will be

• a. AB
• b. A2B
• c. AB2
• d. A2B2
c. AB2 • a. CaCl
• b. Ca2Cl
• c. CaCl2
• d. Ca2Cl4
c. CaCl2

#### What will be  the numer of molecules of carbon dioxide if 2 mole of carbon dioxide is taken and Avogadro Number is 6.022 X 1022

• a. 6.022 X 1022
• b. 2 X 6.022 X 1022
• c. 3  X 6.022 X 1022
• d. 1/2 X 6.022 X 1022
b. 2 X 6.022 X 1022

#### The molecular mass of a compound is 100 grams. The possible compound is:

(assume at. wt of c= 12, Ca= 40, Cl=35.5, P=40, O=16)

• a. CaCl2
• b. Ca3(PO4)2
• c. CaCO3
• d. CaO
c. CaCO3

#### Pick up the statement which does not belong to Dalton theory.

• a. All compounds are made up of atoms.
• b. Atoms of an element are identical always
• c. Atoms combine in whole-number rato with another atom
• d. Atoms reacts as per their valence electrons.
d. Atoms reacts as per their valence electrons.
Explanation
Option b is correct for chemical combination but belong to valence bond theory

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Atoms and Molecules MCQ/Objective questions Chapter 3 Class 9 Science

Ref: Chapter 3, NCERT.

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