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All questions of Atoms and Molecules for Class 9 Exam

Which constituent of air is monoatomic?
  • a)
    Argon
  • b)
    Water vapour
  • c)
    Oxygen
  • d)
    Nitrogen
Correct answer is option 'A'. Can you explain this answer?

Avinash Patel answered
Argon is a Noble gas. Noble gases are not very reactive since they have a very stable electron configuration. It is only about 1% of the air in the atmosphere but it is more abundant than any other element on the Periodic Table behind Nitrogen and Oxygen.
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Which one of the following is the modern symbol of Gold?
  • a)
    Gl
  • b)
    Go
  • c)
    Si
  • d)
    Au
Correct answer is option 'D'. Can you explain this answer?

Arvind Singh answered
Gold is a chemical element with symbol Au (from Latin: aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal.

The combining capacity of an element is called 
  • a)
    Valency 
  • b)
    Atomicity 
  • c)
    Atomic number 
  • d)
    Valence electron
Correct answer is option 'A'. Can you explain this answer?

Aditi Sharma answered
  • The combining capacity of an element is known as its valency.
  • Valency is the number of valence electrons in outer shell that take part in the chemical reaction.

Can you explain the answer of this question below:

Which is the smallest particle of an element that can take part in a chemical reaction?

  • A:

    Molecule

  • B:

    Mixture

  • C:

    Compound

  • D:

    Atom

The answer is d.

Arvind Singh answered
The basic unit of matter is the atom. It is the smallest particle of an element which can take part in chemical reactions and may or may not exist separately. It consists of sub-atomic particles, i.e., protons, neutrons and electrons.

What is the atomic mass of nitrogen?
  • a)
    16a.m.u.
  • b)
    7a.m.u.
  • c)
    14a.m.u.
  • d)
    8 a.m.u.
Correct answer is option 'C'. Can you explain this answer?

Anchal Singh answered
Atomic Mass is determined by total number of nucleons in nucleus. Total number of nucleons is equal to sum of proton and neutrons in a atom .So, number of protons in Nitrogen is 7 as well as Neutron present are also 7. As 7 +7 is 14, the atomic mass is 14 u,(small u) is the atomic mass unit .

Identify the correct symbol of Gold. 
  • a)
    Go 
  • b)
    Gd 
  • c)
    Ge 
  • d)
    Au 
Correct answer is option 'D'. Can you explain this answer?

Aditi Sharma answered
The chemical symbol for gold is Au, taken from the Latin word for gold- Aurum. Its atomic number is 79.

Identify the symbol used to represent Avogadro’s number.​
  • a)
    MA
  • b)
    AO
  • c)
    AN
  • d)
    NA
Correct answer is option 'D'. Can you explain this answer?

Arvind Singh answered
Avogadro's number is defined as the number of elementary particles (molecules, atoms, compounds, etc.) per mole of a substance. It is equal to 6.022x10^23 mol-1 and is expressed as the symbol NA. Avogadro's number is a similar concept to that of a dozen or a gross.

The atom is indivisible, was proposed by
  • a)
    Einstein
  • b)
    Lavoisier
  • c)
    Dalton
  • d)
    Proust
Correct answer is option 'C'. Can you explain this answer?

Deepnarayan Chauhan answered
Yes john Dalton say this first but this was expect after because this is made up of electron protein and nuetron

The number of atoms present in one molecule of an element is called as:
  • a)
    Empirical formula
  • b)
    Molecule
  • c)
    Atomicity
  • d)
    Compound
Correct answer is option 'C'. Can you explain this answer?

Ananya Sharma answered
Atomicity is the total number of atoms present in one molecule of an element or a compound. 
For example: one molecule of hydrogen (H2) contains two atoms of hydrogen. Therefore, atomicity of hydrogen is 2. Similarly, 1 molecule of O3 contains 3 atoms. Therefore, its atomicity is 3. molecule of Argon exist as Ar and hence its a monoatomic compound.

One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the mass of one atom of ___________.
  • a)
    Carbon-12
  • b)
    Nitrogen -14
  • c)
    Carbon-1
  • d)
    4.Silicon-14
Correct answer is option 'A'. Can you explain this answer?

Jyoti Kapoor answered
An atomic mass unit (symbolized AMU or amu) is defined as precisely 1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus. In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass.

The reaction used in the experiment to verify the law of conservation of masses is included in which of the following categories?
AgNO3(aqueous) + KCl(aqueous) —–AgCl(precipitate) + KNO3(aqueous)
  • a)
    Precipitation reaction
  • b)
    Double displacement reaction
  • c)
    Neutralization reaction
  • d)
    Both A and B
Correct answer is option 'D'. Can you explain this answer?

Imk Pathshala answered
The correct answer is D: Both A and B.

- Precipitation Reaction: AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq) forms a solid precipitate.
- Double Displacement Reaction: AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq) involves the exchange of ions between two compounds.
- Explanation: The given reaction demonstrates both characteristics, forming a precipitate (A) and involving double displacement of ions (B), making it both a precipitation and a double displacement reaction.

NaCl molecule is made of which of the following ions?
  • a)
    Na cation and Cl anion
  • b)
    Cl cation and Na anion
  • c)
    Both Na and Cl cation
  • d)
    Both Na and Cl anion
Correct answer is option 'A'. Can you explain this answer?

Jyoti Kapoor answered
An oxygen molecule (O2) is a good example of a molecule with a covalent bond. Ionic bonds occur when electrons are donated from one atom to another. Table salt (NaCl) is a common example of a compound with an ionic bond.

The number of moles of oxygen atoms in two moles of nitric acid is:
  • a)
    2
  • b)
    4
  • c)
    8
  • d)
    6
Correct answer is option 'D'. Can you explain this answer?

Deepshikha Mohanty answered
Oxygen is 1/18th and A mole is a unit of measurement used to express the amount of substance contained in a certain amount of any given elemental chemical unit, such as atoms, molecules or ions. The number of these units contained in one mole of any substance is a constant, which is known as Avogadro’s number and is equal to 6.22x10^23 units. Moles and molecular mass play an essential role in calculating chemical reactions and the magnitudes involved in them.

Which one of the following phrases would be incorrect to use?
  • a)
    A mole of an element
  • b)
    A mole of a compound
  • c)
    An atom of an element
  • d)
    An atom of a compound
Correct answer is option 'D'. Can you explain this answer?

Krishna Iyer answered
 it  will be incorrect to say that  an  atom of  a compound   because, 
Every combination of atoms is a molecule. A compound is a molecule made of atoms  from different elements. All compounds are molecules , but not all molecules are compounds.

Valency of Calcium element is:
  • a)
    4
  • b)
    5
  • c)
    2
  • d)
    3
Correct answer is option 'C'. Can you explain this answer?

Amit Sharma answered
The valency of Calcium of 2+. Calcium has 2 electrons in its outermost shell, its electronic configuration being (2,8,8,2). In order to attain stability it loses 2 electrons to become stable according to the octet rule.

Which of the following is the chemical symbol for nitrogen gas?
  • a)
    N2
  • b)
    N
  • c)
    N3
  • d)
    Ni
Correct answer is option 'A'. Can you explain this answer?

Mahi Chauhan answered
The chemical formula of Nitrogen is N but Nitrogen exists in the molecule of two ions hence the chemical symbol of Nitrogen gas is written as N2.

1.0 mole CO2 contains
  • a)
    6.022 x 10-23 atoms of carbon
  • b)
    6.022 X 10-23 atoms of oxygen
  • c)
    6.022 x 1023 molecules of CO2
  • d)
    3g atom of CO2
Correct answer is option 'C'. Can you explain this answer?

Swati Verma answered
  • One mole of CO2 = one mole of C = 6.02×10 23 atoms of carbon.
  • One mole of CO2 = two moles of O = 2×6.02×1023= 12.04×10 23 atoms of oxygen.
  • One mole of CO2​ = 6.02×1023 molecules of CO2
​Note: 6.02×1023 is Avogadro's number and represents number of atoms/molecules present in one mole of substance.

Identify the correct one from the following statements. 
  • a)
    Elements can be changed into simpler substances. 
  • b)
    Compound has variable composition 
  • c)
    The mixture has properties similar to its components. 
  • d)
    Brass is a compound of Copper and Zinc
Correct answer is option 'C'. Can you explain this answer?

Geetika Shah answered
option ( c) The mixture has properties similar to its compound is the correct answer. 
 
Explanation:- 
This is because in a mixture, the constituent particles are not mixed chemically.
They are mixed physically and can be separated fairly easily by physical Methods.
Thus they show the properties of components.
Example can be alloys.

How do atoms usually exist in nature?
  • a)
    In the form of molecules
  • b)
    In the form of ions
  • c)
    In the free state
  • d)
    In the form of molecules and Ions
Correct answer is option 'D'. Can you explain this answer?

Khushi answered
Most of the elements in environment do not have 8 electrons in their valance shells(because of which they are more or less reactive and unstable ) so they react with the atoms of other metals to form molecules or ion to achieve noble gas configuration and become stable. The atoms of only a few elements like helium , Neon ..etc have their valance shells already filled with 8 electrons.so they can exist in free state. But matter mostly exist in the form of molecules and ions.

Which atom is the smallest atom of all?
  • a)
    Lithium
  • b)
    Helium
  • c)
    Hydrogen
  • d)
    Carbon
Correct answer is option 'C'. Can you explain this answer?

Gayatri Deshpande answered
Explanation:
The size of an atom is determined by its atomic radius, which is the distance from the center of the nucleus to the outermost electron. Therefore, the smallest atom will have the smallest atomic radius.


Factors Affecting Atomic Radius:

  • Number of protons in the nucleus

  • Number of energy levels or electron shells

  • Effective nuclear charge



Comparison of Atomic Radii:
Based on the factors affecting atomic radius, we can compare the atomic radii of the four given elements:


  • Hydrogen has only one proton and one electron, making it the smallest atom.

  • Helium has two protons and two electrons, making it slightly larger than hydrogen.

  • Lithium has three protons and electrons, and is larger than both hydrogen and helium.

  • Carbon has six protons and electrons, and is larger than all three previous elements.



Conclusion:
Therefore, the smallest atom of all is Hydrogen, with only one proton and one electron.

The atomic masses and chemical properties of Sulphur and Phosphorus are different. This can be explained on the basis of:
  • a)
    Law of conservation of mass
  • b)
    Laws of Chemical combination
  • c)
    Law of constant proportions
  • d)
    Dalton’s Atomic theory
Correct answer is option 'D'. Can you explain this answer?

Saumay Dagar answered
The answer is Dalton's atomic theory , which states  that atoms of different elements have different masses and chemical properties, but a drawback to this are IsoBars like argon and calcium both having mass no. equal to 40

Atom of Hydrogen is
  • a)
    10−10m.
  • b)
    10−9m
  • c)
    10−8m
  • d)
    10−1m
Correct answer is option 'A'. Can you explain this answer?

Swara malhotra answered
The diameter of Hydrogen atom is 10−10m. it is the smallest atom among all elemetns.

Calculate the relative molecular mass of H2S.
  • a)
    16 a.m.u.
  • b)
    64 a.m.u.
  • c)
    18 a.m.u.
  • d)
    34 a.m.u.
Correct answer is option 'D'. Can you explain this answer?

Vikram Khanna answered
Atomic mass of hydrogen = 1 a.m.u.

Atomic mass of Sulphur = 32 a.m.u.

Thus, the molecular mass of hydrogen sulphide, which contains two atoms of hydrogen and one atom of sulphur is = 2 X 1 + 1 X 32 = 34 a.m.u.

How many molecules are present in 9g of water -
  • a)
    3.01 × 1023
  • b)
    6.022 × 1023
  • c)
    6.08 × 1023
  • d)
    3.82 × 1023
Correct answer is option 'A'. Can you explain this answer?

Debanshi Chopra answered
No. of molecules in 9g of water:

Molecular mass of water(H2O)= (1x2)+16=18g/mol

1 mole of water=18g of water contains=6.022x10^23 molecules of water

then 9g of water will have= (6.022x10^23/18)x9 = 3.011x10^23 molecules

Who gave the Law of constant proportions?
  • a) 
    Cavendish
  • b) 
    Proust
  • c) 
    Dalton
  • d) 
    Lavoisier
Correct answer is option 'B'. Can you explain this answer?

Arvind Singh answered
A molecule of water will always have two hydrogen atoms and one oxygen atom, meaning that in a bottle of pure water the hydrogen to oxygen atom ratio will always be 2:1. This seems very logical to us today. But 300 years ago, scientists didn't understand compounds this way. However, in 1794, Joseph Proust published the Law of Constant Proportions, which says that a chemical molecule will always contains the same elements in the same proportion.
There are two parts to the law of constant proportions: 1) there will always be the same elements that make up a compound, and 2) the mass of these compounds will always be in the same proportion.

The number of carbon atoms in 1g of CaCO3 is-
  • a)
    6.022 × 1023
  • b)
    6.022 × 1021
  • c)
    3.0125 × 1022
  • d)
    1.204 × 1023
Correct answer is option 'B'. Can you explain this answer?

Dr Manju Sen answered
First we calculate the number of moles of CaCO3
Molecular weight of CaCO3 = 100g
Therefore, 100g of CaCO3 will have Avogadro number of molecules (6.022x10²³).
so 1g will have
   (6.022x10²³) x 1/100
= 6.022x10²¹
Now one molecule of CaCO3 has 1 carbon atom. So, 6.022x10²¹ molecules will have 6.022x10²¹ atoms of Carbon.
Answer is (b) 6.022x10²¹

1 mole of water molecules has:
  • a)
    2 moles of oxygen atoms
  • b)
    1 mole of hydrogen atom
  • c)
    2 moles of hydrogen atoms
  • d)
    2 moles of hydrogen molecule
Correct answer is option 'C'. Can you explain this answer?

Arvind Singh answered
So, 1 mole H2O = 1.2044X10^24 hydrogen atoms. Therefore 2 mole H2O will have 2.4088X10^24 hydrogen atoms. Since one mole is defined as 6.022 x 10^23, and there are two hydrogen per molecule, that would mean that there are exactly 12.044 x 10^23 molecules of hydrogen in one mole of water.

A student heats 25g of reactant ‘A’ with 50g of reactant ‘B’. He obtains 50g of product ‘C’ and recovers 25 g of unreacted ‘B’. Which of the following law is confirmed in the following reaction?
  • a)
    Law of constant proportion
  • b)
    Law of conservation of mass
  • c)
    Law of conservation of mass and Law of constant proportion
  • d)
    Law of multiple proportion
Correct answer is option 'C'. Can you explain this answer?

Arvind Singh answered
Two laws are followed in this question.
First law is Law of definite proportion as follows: 
Here 25 g of reactant A reacts with 50 g of reactant B to produce 50 g of product C. That means 25 g of A consumes only 25 g of B. The ratio is 1:1. Even if there would have been say 75 g B is present, still only 25 g of B will react with 25 g of A. Or if 10 g of B is present then only 10 g of A will react and remaining 15 g of A will remain unreacted. 
Second law is Law of conservation of mass as follows :
There is 25 g of A and 50 g of B. That means total 75 g of reactant is present. On product side 50 g of C and 25 g of unreacted B is present. That means total 75 g of product is present. Hence, the correct option will be " Law of conservation of mass and Law of constant proportion ".

The percentage of hydrogen in water is______.
  • a)
    1.11%
  • b)
    11.11%
  • c)
    8.89 %
  • d)
    88.9%
Correct answer is option 'B'. Can you explain this answer?

Arun Sharma answered
There is also one mole of oxygen atoms weighing 16.00 grams in the mole of water and that of hydrogen is 2.00 gram.
To get the percentage of hydrogen, divide the 2 by 18 and multiply by 100.
i.e. Percentage of hydrogen = (2/18) x 100 = 11.11%

Valency of hydrogen is 1 and that of sulphur is 2. What should be the formula of hydrogen sulphide?​
  • a)
    HS
  • b)
    H2S2
  • c)
    HS2
  • d)
    H2S
Correct answer is option 'D'. Can you explain this answer?

Jyoti Kapoor answered
Hydrogen sulfide is the chemical compound with the chemical formula H2S. It is a colorless gas with the characteristic foul odor of rotten eggs. It is very poisonous, corrosive, and flammable.

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