CBSE Class 9 Science Atoms and Molecules MCQs Set 06

Question. What are the elements present in common salt?
(a) Sodium and Chlorine
(b) Potassium and Chlorine
(c) Sulphur and Nitrate
(d) Sodium and sulphate
Answer: (a) Sodium and Chlorine

Question. Atoms of most elements are not able to exist independently. Name the elements that exist as independent atoms.
(a) Sodium
(b) Calcium
(c) Oxygen
(d) Sulphur
Answer: (d) Sulphur

Question. Sum of the atomic masses of all the atoms in a formula unit of a compound is called __________.
(a) Molecular mass
(b) Formula unit mass
(c) Molar gram
(d) Mole
Answer: (b) Formula unit mass

Question. Sum of the atomic masses of all the atoms in a molecule of the substance is called __________.
(a) Molecular mass
(b) Molar mass
(c) Molar gram
(d) Mole
Answer: (a) Molecular mass

Question. __________ is equal to \( 1/12\text{-th} \) the mass of one atom of carbon-12 atom.
(a) Atomic mass unit
(b) Molecular mass
(c) Molar gram
(d) Mole
Answer: (a) Atomic mass unit

Question. How many atoms are there in \( 1\text{ gm} \) of carbon?
(a) \( 12\text{ u} \)
(b) \( 2\text{ u} \)
(c) Avogadro number
(d) \( 4\text{ u} \)
Answer: (c) Avogadro number

Question. Find the number of moles of oxygen atoms in water.
(a) \( 1 \)
(b) \( 2 \)
(c) \( 3 \)
(d) \( 4 \)
Answer: (a) \( 1 \)

Question. Which of the following statements best defines the law of conservation of mass?
(a) Mass can neither be created nor destroyed in a chemical reaction.
(b) Volume can neither be created nor destroyed in a chemical reaction.
(c) Mass can neither be created nor destroyed in a physical reaction.
(d) Mass can either be created nor destroyed in a physical reaction.
Answer: (a) Mass can neither be created nor destroyed in a chemical reaction.

Question. Which of the following statements best defines the law of constant proportions or law of definite proportions?
(a) In a chemical substance, the elements are not always present in a definite proportion by mass.
(b) In a chemical substance, the elements are always present in a definite proportion by mass.
(c) Mass can neither be created nor destroyed in a chemical reaction.
(d) Mass can neither be created nor destroyed in a physical reaction.
Answer: (b) In a chemical substance, the elements are always present in a definite proportion by mass.

Question. What is the full form of IUPAC?
(a) International Union of Pure and Applied Chemistry
(b) Indian Union of Pure and Applied Chemistry
(c) International Union of Pure and Applied Chemist
(d) International Understanding of Pure and Applied Chemistry
Answer: (a) International Union of Pure and Applied Chemistry

Question. What is the symbol of gold?
(a) \( \text{Au} \)
(b) \( \text{Ag} \)
(c) \( \text{Al} \)
(d) \( \text{Go} \)
Answer: (a) \( \text{Au} \)

Question. What is the Latin name of sodium?
(a) Natrium
(b) Kalium
(c) Ferrum
(d) Cuprum
Answer: (a) Natrium

Question. What is atomicity?
(a) Number of hydrogen atoms constituting a molecule
(b) Number of bonds constituting a molecule
(c) Number of molecules constituting an atom
(d) Number of atoms constituting a molecule
Answer: (d) Number of atoms constituting a molecule

Question. A charged particle can be negatively or positively charged is called as.
(a) Ion
(b) Molecule
(c) Atom
(d) Atomicity
Answer: (a) Ion

Question. A group of atoms carrying a fixed charge on them are called.
(a) Radicals
(b) Polyatomic ions
(c) Ion
(d) Both (a) and (b)
Answer: (d) Both (a) and (b)

Question. What is atomic number?
(a) Number of electrons in an atom of an element.
(b) Number of protons in an atom of an element.
(c) Number of molecules in an atom of an element.
(d) Number of atoms constituting a molecule
Answer: (b) Number of protons in an atom of an element.

ASSERTION REASONING QUESTIONS

Question. Assertion: CO is a compound
Reason: Carbon monoxide (CO) formed by carbon and oxygen combined in a fixed ratio by mass
(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
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Question. Assertion: The relative atomic masses of all elements have been found with respect to an atom of carbon-12
Reason: oxygen reacted with a large number of elements and formed compounds.
(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
Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Question. Assertion: A molecule of oxygen consists of two atoms of oxygen
Reason: It is known as a diatomic molecule.
(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
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Question. Assertion: The molecular mass of a substance is the sum of atomic masses of all the atoms in the molecular formula.
Reason: Total mass of the reactants is equal to the total mass of the products in a chemical reaction
(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
Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Question. Assertion: Aluminium sulphate consists of Al, S and O
Reason: The formula of Aluminium sulphate is \(\text{Al}_2(\text{SO}_4)_3\)
(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
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Question. Assertion: Atomic mass of aluminium is 27.
Reason: An atom of aluminium is 27 times heavier than 1/12th of the mass of carbon-12 atom.
(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
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Question. Assertion: Molecular weight of \(\text{SO}_2\) is double to that \(\text{O}_2\).
Reason: One mole of \(\text{SO}_2\) contains double the number of molecules present in one mole of \(\text{O}_2\).
(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
Answer: (c) Assertion is true but reason is false.

Question. Assertion: Mg forms cation.
Reason: Cations are negatively charged
(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
Answer: (c) Assertion is true but reason is false.

Question. Assertion: Formula of Calcium hydroxide is \(\text{Ca}(\text{OH})_2\)
Reason: Ca ion has a charge of +2 and hydroxide ion has a charge of -1.
(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
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Question. Assertion: Relative atomic masses were determined using the laws of chemical combinations and the compounds formed
Reason: Determining the mass of an individual atom was a relatively difficult task.
(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
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

CASE BASED QUESTIONS

READ THE FOLLOWING PASSAGE AND ANSWER THE QUESTIONS

I. THE MOLE CONCEPT

The mole concept is a convenient method of expressing the amount of a substance. Any measurement can be broken down into two parts – the numerical magnitude and the units that the magnitude is expressed in. For example, when the mass of a ball is measured to be 2 kilograms, the magnitude is ‘2’ and the unit is ‘kilogram’.

When dealing with particles at an atomic (or molecular) level, even one gram of a pure element is known to contain a huge number of atoms. This is where the mole concept is widely used. It primarily focuses on the unit known as a ‘mole’, which is a count of a very large number of particles. One mole of a pure carbon-12 (\( ^{12}\text{C} \)) sample will have a mass of exactly 12 grams and will contain \( 6.02214076 \times 10^{23} \) (\( N_A \)) number of \( ^{12}\text{C} \) atoms. The number of moles of a substance in a given pure sample can be represented by the following formula:

\( n = N/N_A \)

Where n is the number of moles of the substance (or elementary entity), N is the total number of elementary entities in the sample, and \( N_A \) is the Avogadro constant.

The word “mole” was introduced around the year 1896 by the German chemist Wilhelm Ostwald, who derived the term from the Latin word moles meaning a ‘heap’ or ‘pile.

Question. How many molecules of water are present in \( 36\text{ grams} \) of water?
Answer:
No. of molecules = No. of moles \( \times \) Avogadro number
No. of moles = Mass/ Molar mass = \( 36\text{g}/18 = 2 \)
\( \therefore \) No. of molecules = \( 2 \times 6.022 \times 10^{23} = 12.044 \times 10^{23} \)

Question. Find no. of moles in \( 10\text{ gms} \) of \( \text{H}_2 \)?
Answer:
No. of moles = Mass/ Molar mass
= \( 10/2 = 5 \)

Question. Define mole
Answer: It is defined as exactly \( 6.022 \times 10^{23} \) elementary entities, which may be atoms, molecules, ions, or electrons.

Question. What is meant by molar mass?
Answer: Mass of one mole of a substance is called molar mass. It is equal to molecular mass expressed in grams.