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3. Atoms and Molecules Science class 9 exercise Additional -Questions 3

3. Atoms and Molecules Science class 9 exercise Additional -Questions 3 ncert book solution in english-medium

NCERT Solutions ⇒ Class 9th

NCERT Books Subjects for class 9th Hindi Medium

3. Atoms and Molecules

Chapter Review

Chapter Review:

  • The smallest tiny particles of matter which cann't be divided further is called atom (Parmanu).
  • The law of conservation of mass and the law of constant proportions these two laws of chemical combination were established by Lavoisier and Joseph L. Proust.
  • Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
  • The law of constant proportions which is also known as the law of definite proportions.
  • In a chemical substance the elements are always present in definite proportions by mass, this law is called the law of constant proportions.
  • Dalton's theory was based on the laws of chemical combination.
  • According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.
  • Dalton's second postulate states "Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction".
  • Dalton's third postulate states "atoms of a given element are identical in mass and chemical properties".
  • Dalton's third postulate states "atoms of different elements have different masses and chemical properties".
  • Atomic radius is measured in nanometres.
  • IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.
  • The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).
  • Each element has a name and a unique chemical symbol.
  • The atomic mass unit is writen as ‘u’ – unified mass.
  • Carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
  • One atomic mass unit is a mass unit equal to exactly onetwelfth
    (1/12th) the mass of one atom of carbon-12.
  • Atoms form molecules and ions.
  • These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.
  • A molecule can be defined as the smallest particle of an element or a
    compound that is capable of an independent existence and shows all the properties of that substance.
  • Atoms of the same element or of different elements can join together to form molecules.
  • A molecule of oxygen consists of two atoms of oxygen and hence it
    is known as a diatomic molecule, O2.
  • 3 atoms of oxygen unite into a molecule these form ozone e.i O3.
  • The number of atoms constituting a molecule is known as its atomicity.
  • Atoms of different elements join together in definite proportions to form molecules of compounds.
  • The charged atoms are known as ions.
  • An ion is a charged particle and can be negatively or positively
    charged.
  • A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’.
  • Ions may consist of a single charged atom or a group of atoms that have a net charge on them.
  • A group of atoms carrying a charge is known as a polyatomic ion.
  • The chemical formula of a compound is a symbolic representation of its composition.
  • The combining power (or capacity) of an element is known as its valency, in other hand the number of valnce electron is known as valancy of an atom. 
  • The quantity of a substance can be characterised by its mass or the number of molecules.
  • One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.
  • The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 1023 ,  This number is called the Avogadro Constant or Avogadro Number.
  • The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.
  • Mass of 1 mole of a substance is called its molar mass.
  • The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.

 

Text-book Questions

Text-book questiuons with solutions.

From Page 32-33 (NCERT Book)

Q1.  In a reaction, 5.3 g of sodium carbonate reacted with 6g of ethanoic acid. The products were 2.2g of carbon dioxide, 0.9g water and 8.2g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Answer: 

sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

Mass of Sodium carbonate = 5.3g 

Mass of ethanoic Acid = 6g

Mass of Sodium ethanoic acid = 8.2g 

Mass of water = 0.9g  

We knock that according to the law of conservation of mass-

Mass of reactants = Mass of products

5.3g + 6g = 8.2g + 2.2g + 0.9g 

11.3g = 11.3g 

Hence, the given observations are satisfied the law of conservation of mass.

Q2.  Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Answer: Given ratio of Hydrogen and Oxyden - 1:8 

It means- 1g of Hydrogen gas required 8g Oxygen to reaction with Hydrogen

Therefore, 3g of Hydrogen gas required = 3 x 8 = 24g 

So, 24g Oxygen is required to react completely with Hydrogen gas.

Q3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer: The second postulate of Dalton's atomic theory is the result of the law of conservation of mass. 

That is as follow- 

"Atoms are indivisible particles, which can neither be created nor be destroyed in a chemical reaction". 

Q4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer: 

The sixth postulate of Dalton's atomic theory explains the law of definite proportions. 

That is as follow- 

"The ralative number and kind of atoms in a given compound remains constatnt".

From Page 35 (NCERT Book)

Q1. Define the atomic mass unit.

Answer: 

An atomic mass unit or  amu is one twelfth of the mass of an unbound atom of carbon -12 isotope. This is approximetely 1.67377 x 10 -27 kilogram (kg). 

The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Atomic mass unit is abbreviated as 'amu' but on IUPAC latest recommendation, it is writen as 'u'. 

Q2. Why is it not possible to see an atom with naked eyes?

Answer: Atoms are very small tiny particles. It is so small that can not be seen with a nacked eyes. Also, an another reason - the atom of an element does not exist independently. 

From Page 39 (NCERT Book)

Q1. Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide

Answer: 

(i) Sodium oxide : Na2O

(ii) Aluminium chloride : AlCl3

(iii) Sodium Sulphate : Na2S

(iv) Magnesium hydroxide : Mg(OH)2

Q2. Write down the names of compounds represented by the

 

From Page 40 (NCERT Book)

Q1. Calculate the molecular masses of

        H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.

Q2. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

From Page 42 (NCERT Book)

Q1. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

Q2. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?

 

Exercises

Q1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
Answer: Boron and oxygen compound —> Boron + Oxygen
0.24 g —> 0.096 g + 0.144 g
 

Q2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
Answer: The reaction of burning of carbon in oxygen may be written as:
It shows that 12 g of carbon bums in 32 g oxygen to form 44 g of carbon dioxide. Therefore 3 g of carbon reacts with 8 g of oxygen to form 11 g of carbon dioxide. It is given that 3.0 g of carbon is burnt with 8 g of oxygen to produce 11.0 g of CO2. Consequently 11.0 g of carbon dioxide will be formed when 3.0 g of C is burnt in 50 g of oxygen consuming 8 g of oxygen, leaving behind 50 – 8 = 42 g of O2. The answer governs the law of constant proportion.

Q3. What are poly atomic ions? Give examples.
Answer: The ions which contain more than one atoms (same kind or may be of different kind) and behave as a single unit are called polyatomic ions e.g., OH–, SO42-, CO32-.

Q4. Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Answer: (a) Magnesium chloride
Symbol —> Mg Cl
Change —> +2 -1
Formula —> MgCl2
(b) Calcium oxide
Symbol —> Ca O
Charge —> +2 -2
Formula —> CaO
(c) Copper nitrate
Symbol —> Cu NO
Change +2 -1
Formula -4 CU(N03)2
(d) Aluminium chloride
Symbol —> Al Cl
Change —> +3 -1
Formula —> AlCl3
(d) Calcium carbonate
Symbol —> Ca CO3
Change —> +2 -2
Formula —> CaC03

Q5. Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Answer: (a) Quick lime —> Calcium oxide
Elements —> Calcium and oxygen
(b) Hydrogen bromide
Elements —> Hydrogen and bromine
(c) Baking powder —> Sodium hydrogen carbonate
Elements —> Sodium, hydrogen, carbon and oxygen
(d) Potassium sulphate
Elements —> Potassium, sulphur and oxygen

Q6. Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Answer: The molar mass of the following: [Unit is ‘g’]
(a) Ethyne, C2H2 = 2 x 12 + 2 x 1 = 24 + 2 = 26 g
(b) Sulphur molecule, S8 = 8 x 32 = 256 g
(c) Phosphorus molecule, P4=4 x 31 = i24g
(d) Hydrochloric acid, HCl = 1 x 1 + 1 x 35.5 = 1 + 35.5 = 36.5 g
(e) Nitric acid, HN03 = 1 x 1 + 1 x 14 + 3 x 16 = 1 + 14 + 48 = 63 g

Q7. What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2S03)?
Answer: (a) Mass of 1 mole of nitrogen atoms = 14 g
(b) 4 moles of aluminium atoms
Mass of 1 mole of aluminium atoms = 27 g
∴ Mass of 4 moles of aluminium atoms = 27 x 4 = 108 g
(c) 10 moles of sodium sulphite (Na2SO3)
Mass of 1 mole of Na2SO3 = 2 x 23 + 32 + 3 x 16 = 46 + 32 + 48 = 126 g
∴ Mass of 10 moles of Na2SO3 = 126 x 10 = 1260 g

Q8. Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of Carbon dioxide.
Answer: (a) Given mass of oxygen gas = 12 g
Molar mass of oxygen gas (O2) = 32 g
Mole of oxygen gas 12/32 = 0.375 mole
(b) Given mass of water = 20 g
Molar mass of water (H2O) = (2 x 1) + 16 = 18 g
Mole of water = 20/18 = 1.12 mole
(c) Given mass of Carbon dioxide = 22 g
Molar mass of carbon dioxide (CO2) = (1 x 12) + (2 x 16)
= 12 + 32 = 44 g
∴ Mole of carbon dioxide = 22/44 = 0.5 mole

Q9. What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Answer: (a) Mole of Oxygen atoms = 0.2 mole
Molar mass of oxygen atoms = 16 g
Mass of oxygen atoms = 16 x 0.2 = 3.2 g
(b) Mole of water molecule = 0.5 mole
Molar mass of water molecules = 2 x 1 + 16= 18 g .
Mass of H2O = 18 x 0.5 = 9 g

Q10. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Answer:Molar mass of S8 sulphur = 256 g = 6.022 x 1023 molecule

Given ma:ss of sulphur = 16 g

Q11. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
Answer: Molar mass of aluminium oxide Al203
= (2 x 27) + (3 x 16)
= 54 + 48 = 102 g.
 

 

Additional -Questions

Chapter– 3 ATOMS AND MOLECULES

Additional And Important Questions with Solution: 

Ques.1 – Who had found two important laws of chemical combination.

Ans. - Antoine L. Lavoisier .

Ques.2 – Write the name of two important laws of chemical combination which is found by Antoine L. Lavoisier.

Ans. – (i) Law of conservation of mass.    (ii)    Law of constant proportions.

Ques.3 – Write the law of conservation of mass.

Ans. - Mass can neither be created nor destroyed in a chemical reaction.

Ques.4 – Write the law of constant proportions.

Ans. - In a chemical substance the elements are always present in definite proportions by mass.

Ques.5 – Define atoms according to Dalton atomic theory.

Ans. – All matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Ques.6 – Write the Dalton’s atomic theory.

Ans. – Dalton’s atomic theory as follow.

        (i)  All matter is made of very tiny particles called atoms.

        (ii)  Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

        (iii) Atoms of a given element are identical in mass and chemical properties.

        (iv) Atoms of different elements have different masses and chemical properties.

        (v) Atoms combine in the ratio of small whole numbers to form compounds.

        (vi) The relative number and kinds of atoms are constant in a given compound.

Ques.7 - Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Ans. – The second postulate of Dalton’s atomic theory is the result of the law of conservation of mass.

Ques.8 - Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Ans. – The forth postulate of Dalton’s atomic theory can explain the law of definite proportions.

Ques.9 – Define atomic mass.

Ans. – The sum of numbers of protons and neutrons presence in a nucleus of an atom is known as atomic mass.

Ques.10 – Define atomic number.

Ans. – The number of protons presence in a nucleus of an atom is known as atomic number.

Ques.11 – What is ion?

Ans. – The charged atom is known as ion.

Example – Na +, Mg+, Cl-.

Ques.12 – Write the differences between atom and ion.

Ans –

                                    Atom

                             Ions
  1. It is neutralized.
  2. All the atoms are active insteed of  noble gas.
  3. It has uncompleted electronic structure in spite of noble gas.
  1. It is charged.
  2. All ions are reactive.
  3. It has completed electronic structure.

 

 

Additional -Questions 2

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Additional -Questions 3

Additional -Questions:

Q1. What is atomicity?

Ans: The number of atoms contained in a molecule of a substance is called its atomicity. 

Q2. How molecules are categrised on the base of atomicity?

Ans:

1. Monoatomic Molecules (Molecule having 1 atom)

2. Diatomic Molecules (molecule having 2 atoms)

3. Triatomic Molecules (molecules having 3 atoms)

4. Tetraatomic Molecules (molecules having 4 atoms)

5. Ployatomic Molecules (molecules having more than 4 molecules) 

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