Aoms and Molecules class 9 Solved questions For FA3

CHEMISTRY Solved ASSIGNMENT-2 CLASS-9 ATOMS AND MOLECULES

Q1 State law of conservation of mass.

Ans The mass can neither be created nor destroyed in a chemical reaction.

Q2  State law of constant proportion.

Ans A pure chemical compound always consists of the same elements that are combined together in a fixed proportion by mass.

Q4  Define atomic mass unit.

Ans The mass of one twelfth (1/12) of the mass of one atom of carbon taken as 12.

Q5   What are molecules?

Ans  Molecules represents a group of two or more atoms(same or different) chemically  bonded to each other and held tightly by strong attractive forces.    Molecules are of two types

(a) Molecules of elements              (b) Molecules of compound

Q6 What do you mean by molecular mass?

Ans The average relative mass of the molecule as compared to the mass of carbon taken  as 1u.

Q7  Write the relationship between number of moles and atomic mass.

Ans     Number of moles=given mass/gram atomic mass

Q8  Why are chemical reactions according to law of conservation of mass?

Ans In all chemical reactions, there is only exchange of reactants taking place when products are  formed. Since there is no loss or gain of mass,the chemical reactions are according to law of conservation of mass.

Q9 What is basic difference between atoms and molecules?

Ans Atoms except those of noble or inert gas elements cannot exist of their own. However , all molecules can have independent existence.

Q10 The atomic mass of an element is in fraction .What does it mean?

Ans If the atomic mass of an element is in fraction, this mean that it exists in the form of isotopes. The atomic mass is the average atomic mass and is generally fractional.

Q11 What is the difference between the mass of molecule and molecular mass?

Ans:  Mass of a molecule is that of a single molecule also known as its actual mass. But molecular mass is the mass of Avogadro’s number of molecules.

Q12 Where do we use the words mole and mol?

Ans In the text part we use the word mole while as a  unit ,we call it mol.

Q13 How many moles are present in 11.5 g of sodium?

Ans  Gram atomic mass of Na =23g

11.5g of Na represents= (1mol)*(11.5g)/23.5g)=0.5 mol

Q14 Explain why the  number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas?

Ans Hydrogen gas is a diatomic in nature(H2) while helium gas is monoatomic (He).As a result, the number of atoms in one mole of hydrogen(2*NA )are expected to be double as  compared to number of atoms in one mole of helium(NA)

Q15An element Z forms an oxide with formula Z2O3. What is its valency?

Ans  Valency of Z=3                                                                                                  

Q16   The valency of an element A is 4 . Write the formula of its oxide.

Ans The formula of its oxide is A₂O₄ or AO₂.

Q17  An element X has valency 3 while the element Y has valency 2.Write the formula of the compound between XandY .

Ans  X₂Y₃

Q18 What are ions?

Ans Ions are of two types          (a) cations  - positively charged         (b) anions  -  negatively charged

Q19  Which postulate of Dalton’s Atomic theory is the basis of law of conservation of  mass?

Ans “ Atoms can neither be created nor destroyed during a physical or a chemical change”

Q20 Write the formulae of sodium oxide and aluminium chloride

Ans   Sodium oxide = Na₂O             Aluminium chloride =AlCl₃

Q21  Find out the ratio by mass of the combining elements in the following  compounds

(a) MgCO3     (b)  CH3OH     (c) CaCl2                                                                                

Ans21    (a) MgCO3  ------   Mg :C:O  = 24:12:48 = 2:1:4

                (b) CH3OH    ------ C : H : O =12:4:16  =3:1:4

                (c) CaCl2          ----------- Ca :Cl = 40:71

Q22  Define atomicity of the molecule.

Ans   Atomicity of the molecule is the number of atoms present in the molecule.

Q23  What is the atomicity of  oxygen,ozone,neon and sulphur?

Ans   Oxygen O₂  = 2                    Ozone O₃  = 3         Neon  Ne  =1               Sulphur S₈ =8

Q24 What is wrong with the statement '1 mol of hydrogen'

Ans The statement is not correct.We must always write whether hydrogen is in atomic form or molecular form. The correct statement is :        1mole of hydrogen atoms or one mole of hydrogen molecules .

Q25 The atomic mass of an element is in fraction. What does it  mean?

Ans If the atomic mass of an element is in fraction, this mean that it exists in the form of isotopes. The atomic mass is the average atomic mass and is generally fractional.

Related post to 9th Atoms and Molecules

IX(9th)Chemistry Ch 03: Solved Questions
9th Atoms and molecules Study Notes, Q & A
CBSE Test paper-1
CBSE Test paper-2
CBSE Test paper-3
Numerical based on mole concept
Solved questions For FA-3
Self-evaluation on Mole concept
Question bank By CBSE Pathshala For CBSEGuess Questions