CBSE Class 9 Science Atoms and Molecules assignment

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

Question : Write down the formulae of

(i) sodium oxide

(ii) aluminium chloride

(iii) sodium sulphide

(iv) magnesium hydroxide

Solution:

(i)Na₂O

(ii)NH₄Cl

(iii)Na₂SO₄

(iv)Mg(OH)₂

 

Question : Write down the names of compounds represented by the following formulae.

(i) AL₂(SO₄)₃ (ii) CaCL₂ (iii) K₂SO₄ (iv) KNO₃(v) CaCO₃

Solution:

(i) Aluminium sulphate

(ii) Calcium chloride

(iii) Potassium sulphate

(iv) Potassium nitrate

(v) Calcium carbonate.

 

Question : What is meant by the term chemical formula?

Solution:  Chemical formula of a compound (or element) is the symbolic representation of its composition. It represents

(i) The number and kind of atoms present per molecule of the compound,

(ii) One mole of the compound,

(iii) Molar mass of the compound.

 

Question : How many atoms are present in a 

(i) H₂S molecule and (ii) PO₄³⁻ion?

Solution:

(i) 2 atom of hydrogen + 1 atom of sulphur = three (3) atoms (in a H₂S molecule).

(ii) 1 atom of phosphorus + 4 atoms of oxygen = five (5) atoms (in aPO₄³⁻ion).

 

Question : Calculate the molecular masses of

H₂, O₂, CL₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.

Solution:

(i) Molecular mass of H₂(hydrogen)

= Atomic mass of hydrogen × 2

= 1 × 2 = 2 u

 

(ii) Molecular mass of O₂ (oxygen)

=Atomic mass of oxygen × 2

= 16 × 2 = 32 u

 

(iii) Molecular mass of CI₂(chlorine)

= Atomic mass of chlorine × 2

= 35.5 × 2 = 71 u

 

(iv) Molecular mass of CO₂(carbon dioxide)

= (Atomic mass of carbon × 1)+ (Atomic mass of oxygen × 2)

= 12 + (16 × 2) = 12 + 32 = 44 u

 

(v) Molecular mass of CH₄(methane)

= (Atomic mass of carbon × 1) + (Atomic mass of hydrogen × 4)

= 12 + (1 × 4) = 12 + 4 = 16 u

 

(vi) Molecular mass of C₂H₆(ethane)

= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 6)

= (12 × 2) + (1 × 6) = 24 + 6 = 30 u

 

(vii) Molecular mass of C₂H₄(ethene)

= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 4)

= (12 × 2) + (1 × 4) = 24 + 4 = 28 u

 

(viii) Molecular mass of NH₃(ammonia)

= (Atomic mass of nitrogen × 1) + (Atomic mass of hydrogen × 3)

= (14 × 1) + (1 × 3) = 14 + 3 = 17 u

 

(ix) Molecular mass of CH₃OH (methanol or methyl alcohol)

= (Atomic mass of carbon × 1) + (Atomic mass of hydrogen × 3)+ (Atomic mass of oxygen × 1) + (Atomic mass of hydrogen × 1)

= 12 + 3 + 16 + 1 = 32 u

 

Question : Calculate the formula unit masses of Zn0, Na₂0, K₂C0₃.

Given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u

C = 12 u and 0 = 16 u.

Solution:

(i) Formula unit mass of ZnO (zinc oxide) =

65 + 16 = 81 u

(ii) Formula unit mass of Na₂O (sodium oxide) =

(23 × 2) + (16 × 1) = 46 + 16 = 62 u

(iii) Formula unit mass of K₂C0₃(potassium carbonate) =

(39 × 2) + (12 × 1) + (16 × 3) = 78 + 12 + 48 = 138 u

 

Question : If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?

Solution:

1 mole carbon atom = 6.022 x 10²³ atoms

Molar atomic mass = 12 g

6.022 x 10²³ carbon atoms weigh = 12 g

1 carbon atom weighs ----12/6.022 x 10²³ = 1.99 x 10-23g

 

Question : 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)?

Solution:

Molar mass of sodium = 23 g

1 mole atom = 6.022 x 10²³ atoms

23 g sodium contains = 6.022 x 10²³ atoms

1 g sodium contains = 6.022 x 10²³ atoms

23

100 g sodium contains = 6.022 x 10²³ atoms

= 2.618 x 10²⁴ atoms

By the above method or by formula we find number of atoms in 100 g

Fe;

Number of atoms of an element in a given mass

 

=  Given mass / Gram atomic mass 100g x Avogadro's number 

= 100g / 56g x 6.022 x 10²³ 

= 1.075 x 10²⁴ atoms

Hence, 100 g of sodium has more number of atoms as compared to 100 g of iron.

 

Exercises

Question : 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.

Solution:

Mass of the compound = 0.24 g

Mass of boron = 0.096 g

Mass of oxygen = 0.144 g

Mass of boron 0.096 g

 

Alternative method

Percentage of oxygen =100 percentage of boron

=100 - 40 = 60%

 

Question : 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?

Solution:

First we find the proportion of mass of carbon and oxygen in carbon dioxide.

In CO₂ , C : O = 12 : 32 or 3 : 8

In other words, we can say that

12.00 g carbon reacts with oxygen = 32.00 g

3.00 g carbon will react with oxygen = 32 x 3 /12 = 8g

 

Therefore, 3.00 g of carbon will always react with 8.00 g of oxygen to form CO₂ (11g), even if large amount (50.00 g) of oxygen is present.

This answer will be governed by ‘the law of constant proportions’.

 

Question : What are polyatomic ions? Give examples.

Solution: The group of atoms which carry a fixed charge (either positive or negative) on them and behave as ions are called polyatomic ions.

Example

(i) Carbonate ion (ii) Sulphate ion

(iii) Ammonium ion (iv) Phosphate ion

 

Question : Write the chemical formulae of the following.

(i) Magnesium chloride (ii) Calcium oxide

(iii) Copper nitrate (iv) Aluminium chloride

(v) Calcium carbonate

Solution:

(i)  Formula = MgCl₂ (Magnesium chloride)

(ii)  Formula = CaO (Calcium oxide)

(iii) Formula = Cu(NO₃)₂ (Copper nitrate)

(iv) Formula = AICI₃ (Aluminium Chloride)

(v)  Formula = CaCO₃ (Calcium carbonate)

 

Question : Give the names of the elements present in the following compounds.

(a) Quick lime (b) Hydrogen bromide

(c) Baking powder (d) Potassium sulphate

Solution:

(a) Quick lime- Calcium oxide - CaO

Elements–Calcium, oxygen.

(b)Hydrogen bromide- HBr

Elements- Hydrogen, bromine.

(c) Baking powder- Sodium hydrogen carbonate -NaHCO₃

Elements- Sodium, hydrogen, carbon, oxygen.

(d)Potassium sulphate- K₂SO₄

Elements- Potassium, sulphur, oxygen.

 

Question : 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

Solution:

(a) Molar mass of C₂H₂

= (2 × Atomic mass of C) + (2 × Atomic mass of H)

= (2 × 12) + (2 × 1)

= 26 u

(b) Molar mass of S8

= (8 × Atomic mass of S)

= 8 × 32 = 256 u

(c) Molar mass of P4

= 4 × Atomic mass of P

= 4 × 31 = 124u

(d) Molar mass of HCI

= Atomic mass of hydrogen + Atomic mass of CI

= 1 + 35.5 = 36.5 u

(e) Molar mass of HN0₃

= Atomic mass of H + Atomic mass of N + (3 × Atomic mass of 0)

= 1 + 14 + (3 × 16) = 15 + 48 = 63 u

 

Question : 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 (Na₂SO₃)?

Solution:

(a) Molar mass of N atom = Atomic mass of N.

Mass of 1 mol of N atoms = 14 g

(b) Mass of 1 mole AI atoms = 27 g

Mass of 4 moles of AI atoms = 27 × 4 = 108 g

(c) Mass of 1 mole of Na₂SO₃ = (23 ×2 ) + 32 + (16 × 3)

= 46 + 32 + 48 = 126 g

Mass of 10 moles of Na₂SO₃ = 126 × 10 = 1260 g

 

Question : Convert into mole.

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide.

Solution:

 

Question : What is the mass of

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

Solution:

(a) Mass of 1 mole O-atoms = 16 g

Mass of 0.2 mole O-atoms = 16 × 0.2 = 3.2 g

(b) Mass of 1 mole of H₂O molecules = 18 g

Mass of 0.5 mole of H₂O molecules = 18 × 0.5 = 9.0 g

 

Question : Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur

Solution:

Molar mass of sulphur (S8 ) = 32 x 8 = 256 g

Number of S8 molecules in 256 g of solid sulphur = 6.022 x 10²³

Number of S8 molecules in 16 g of solid sulphur=6.022 x10²³x 16g 256g

=3.76 x 10²³ molecules

 

Question : Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.

[HintThe mass of an ion is the same as that of an atom of the same element. Atomic mass of AI= 27 u.]

Solution:

 

Exam Questions NCERT Class 9 Science Chapter 3 Atoms And Molecules

Question. Define law of conservation of mass.
Ans : Law of conservation of mass states that, ‘Mass is neither created nor destroyed in a chemical reaction.’

Question. Name the scientists whose experimentation established laws of chemical combination. Name the laws also.
Ans : Antoine Laurent Lavoisier and Joseph L. Proust experimented and established two laws of chemical combination. These laws are : (i) Law of conservation of mass, (ii) Law of constant proportions.

Question. Give one example each of (i) Monovalent cation, (ii) Bivalent cation, (iii) Monovalent anion, (iv) Bivalent anion.
Ans : (i) K+ or Na+
         (ii) Mg+2 or Ca+2
         (iii) F– or Cl–
         (iv) O2– or S2–

Question. How can Dalton’s atomic theory explain the Law of Constant Proportions?
Ans : According to Dalton’s atomic theory, atoms of the same elements are same. Also atoms combine in whole number. This means that the atoms can combine with each other in a simple fixed ratio to form molecules.

Question. What is ion?
Ans : An ion is a charged particle. It can be positive or negative.

Question. What do you mean by symbols of elements?
Ans : Each element is represented by a letter or group of two letters to write the chemical reactions conveniently. It is called symbol.

Question. How did the scientist lay the foundation of chemical sciences? Name the scientist.
Ans : Antoine Laurent Lavoisier laid the foundation of chemical science by establishing two important laws of chemical combination.

Question. An element has 8 electrons in its valence shell. What is its general name?
Ans : Noble gas.

Question. Write the names of the following compounds :
(a) NiS
(b) Mg(NO₃)₂
(c) Na₂SO₄
(d) Al(NO₃)₃
(e) K3PO4 (f) Ca3N₂
Ans : (a) Nickel sulphide,
         (b) Magnesium nitrate,
         (c) Sodium sulphate,
         (d) Aluminium nitrate,
         (e) Potassium phosphate,
         (f) Calcium nitride.

Question Give the derivation source of symbol of sodium (Na).
Ans : The symbol of ‘Na’ for sodium is derived from its Latin name ‘Natrium’.

Question. Give the full form of amu?
Ans : The full from of amu is atomic mass unit.

Question. Write the symbols of tungsten and iron.
Ans : (i) Tungsten (W) and
         (ii) Iron (Fe)

Question. All elements have charged valency. Explain.
Ans : No, all elements do not form ions thus they, do not have a charge.
For example : Carbon has a valency of 4 and nitrogen has a valency of 3. Non-metals are formed without a charged valency. Example : In carbon tetrachloride, carbon has valency of 4 and chlorine has a valency of 1.

Question. What are molecules? Give brief explanation of the arrangement of the constituent atoms in the molecules.
Ans : A molecule is the smallest particle of an element or compound which is stable in normal conditions. And it can freely show all the properties of that element or compound. It may be made up of one, two or more atoms. Molecule with one atom called monoatomic. E.g. helium, neon, etc. Molecule with two atoms called diatomic. E.g.
Cl₂, O₂. Similarly, there are molecules containing three atoms (CO₂), four atoms (P4) and so on.

Mark (1)

Q 1 144 grams of pure water is decomposed by passing electricity. 16 grams of hydrogen and 128 grams of oxygen are obtained.
Which chemical law is illustrated by this statement?

Q 2 What do you mean by Molar Mass ?

Q 3 What is atomic mass unit ?

Q 4 What are molecules?

Q 5 What name is given to the number 6.023 x 10²³?

Q 6 Name the building blocks of matter.

Q 7 How many metres are there in 1 nanometer (nm) ?

Q 8 Define atomicity.

Q 9 Write the chemical formula of glucose.

Q 10 Which subatomic particle was not present in Thomson‘s model of the atom?

Q 11 Write the symbol of element lead and iron.

Marks (2)

Q 12 Write down the formulae of aluminium sulphate and ethanol.

Q 13 What is the atomicity of –
(a) Ozone (b) Nitrogen
(c) Neon (d) Sulphur

Q 14 Write the symbols of the following elements :
a. Sodium
b. Calcium
c. Gold
d. Iron

Q 15 What weight (in grams) is represented by (a) 2 moles of CO₂ (b) 5 moles of NH3.

Q 16 Define law of conservation of mass.

Q 17 What is an atom ?

Q 18 How many moles are there in 1 litre of water?

Q 19 Write the chemical symbols of –
(a) Silver
(b) Potassium
(c) Tin
(d) Mercury

Q 20 Write two differences between an atom and its ion.

Q 21 What precentage of nitrogen is present in aluminium nitride? (Al = 27, N = 14) .

Q 22 Calculate the number of moles for the following:
(a) 12.046 x 10²³ number of He atoms
(b) 56g of He

Q 23 Write the formula of the following compound and name the elements present in them.
(a) Ammonia (b) Sulphur dioxide
(c) Ethanol (d) Methane

Q 24 What do we call those species which have-
(a) less electrons than the normal atoms.
(b) more electrons than the normal atoms.

Q 25 What is the mass of 5 moles of aluminium atoms? (Atomic mass of Al = 27 u)

Q 26 How many times is a proton heavier than an electron? Write the absolute mass of a proton.

Q 27 Which of the following are isotopes and which are isobars?
Calcium, Protium, Argon, Deuterium

Q 28 Calculate the mass of 0.8 mole of NaCl.
(Atomic masses : Na = 23u; Cl = 35.5 u)

Q 29 If one mole of oxygen atom weighs 16 grams, then what will be the mass (in grams) of 1 atom of oxygen?

Q 30 Write the chemical formula of:
(i) Nitogen oxide
(ii) Barium nitride

Q 31 (a) Calculate the molecular masses of
(i) H₃PO₄
(ii) H₂O₂ 
(Atomic masses: H=1, O =16, P = 31)

Q 32 Write the name of four elements which show variable valencies. Also write their valencies.

Q 33 Calculate the mass of 6.022 x 10²⁴ atoms of carbon.

Q 34 Calculate the mass of 1 molecule of Nitrogen. (Atomic mass of Nitrogen = 14)

Marks (3)

Q 35 Calculate the number of molecules present in a drop of water weighing 0.75 g. (H = 1, O = 16) .

Q 36 Write the formula of :
(i) Ammonium sulphate
(ii) Magnesium nitrate
(iii) Aluminium bromide

Q 37 What do you understand by Formula Unit Mass? Calculate the formula unit masses of K2CO3 and ZnO. (Atomic masses of Zn = 65u, K = 39u, C = 12u)

Q 38 An element X has a valency of 4
(a) What will be the formula of its chloride ?
(b) What will be the formula of its sulphide ?
(c) What will be the formula of its bromide?

Q 39 Calculate the mass of 3.011 X 10²⁴ molecules of nitrogen gas. (Atomic mass of N = 14 u)

Q 40 Distinguish between the molecule of an element and the molecule of a compound with the help of suitable example.

Q 41 How many grams of hydrogen gas contain the same number of molecules as 22 grams of carbon dioxide gas?

Q 42 What the following abbreviations stands for.
(a) C (b) 2H (c) N₂
(d) 3N₂ (e) O₃ (f) O_2-

Q 43 Write the name of the following compounds:
(a) Ca₃(PO₄)₂ (b) NaHCO₃
(c) (NH₄) 2SO₄ (d) MgCO₃
(e) FeSO₄ (f) AlCl₃

Q 44 The mass of an atom of element (Y) is 5.30 x 10^-23g.
(i) Calculate its atomic mass.
(ii) Name this element.

Q 45 Calculate the number of particles in each of the following:
(i) 92g of Na atoms
(ii) 7g of N₂ molecules
(iii) 0.1 mole of oxygen atoms

Q 46 Which of the following weighs the most?
(i) 32g of oxygen (ii) 10g atoms of hydrogen
(iii) 0.5 of iron (iv) 6.022 x 1022 atoms of C
(Atomic mass: O =16, N = 14, Fe = 56, C = 12)

Marks (5) 

Q 47 Calculate the number of atoms of each type in 2.65 g of Na₂CO₃.

Q 48 If 2 g of water contains z molecules, what will be number of molecules in 4 g of carbon dioxide?

Q 49 (a) In 0.25 mol of P4, calculate
(i) The number of P4 molecules
(ii) The number of P atoms
(iii) The number of moles of P atoms
(b) Calculate the number of moles of magnesium in its 200 gram. (Atomic mass of Mg = 24 u)
(c) Calculate the number of molecules present in 36 grams of water.

Q 50 Calculate the number of chromium atom in 1.47 g of potassium dichromate (K₂Cr₂O₇) [Atomic mass: K = 39, Cr = 52, O = 16].

Q 51 Write any four significances of symbol. Give one example also.

Q 52 Give the significance of the formula CO₂.

Q 53 (i) Write the atomicity of the following:
(a) Ozone (b) Chlorine
(c) Helium (d) Sulphur
(e) Oxygen (f) Phosphorous
(ii) Give one word answer
The Latin name of iron from which its symbol is derived.
An atom or group of atom having positive charge.

Most Important Questions

Q 1 State main points of Dalton atomic theory.

Q 2 State the law of constant proportion.

Q 3 state the law of conservation of mass

Q 4 If 90g of pure water is obtained through 80g of oxygen and Xg of hydrogen.
a. Find the value of X.
b. Which chemical law is illustrated by this statement?

Q 5 Calculate the molecular mass of the following:
a. H₂O
b. HCl
c. H₂S

Q 6 If 6g of hydrogen combine with 48g of oxygen, how many grams of water is obtained?

Q 7 If carbon and oxygen combine in a definite ratio of 3:8 to form carbon dioxide. What mass of oxygen gas is required in carbon dioxide with 10 g of carbon?

Q 8 Give symbol of the followinga. oxygen
b. hydrogen
c. carbon

Q 9 If atomic mass of carbon is 1u then what is the atomic mass of nitrogen (N = 14, C = 12) ?

Q 10 Identify the anions, cations and charge present on them in following molecules: HCl , NaCl , H₂O , Al₂O₃

Q 11 Write the chemical formula of the following:
a. Oxygen
b. Carbon dioxide
c. Ammonia
d. Calcium carbonate

Q 12 Give the names of the following compounds:
a. H₂SO₄
b. He
c. CO

Q 13 Write the formula unit mass of the following:
a. NH₃
b. CO₂
c. K₂Cr₂O₇
d. SO₂
e. C₁₂H₂₂O₁₁

Q 14 Calculate the mass in grams of 0.8 mole of CO_2.

Q 15 In which of the following cases the number of hydrogen atoms is more?
a. One mole of CH₄
b. Two mole of NH₃

Q 16 Calculate the number of molecules in 20g of water.

Q 17 How many moles are present in 6g of sulphuric acid?

Q 18 What is the number of molecules in a drop of water weighing 0.2g?

Q 19 Calculate the number of S atoms in 2 mole of S8

Q 20 How many moles are there in 70g of potassium dichromate,( K₂Cr₂O₇) ?

Q 21 How many moles of calcium carbonate are present in its 5g ?

Q 22 What mass of hydrogen chloride, (HCl) , will contain the same number of molecules as in
a. 6.022 x 10²³ molecules of water
b. 2.0 g of methane, CH₄