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Sunday, 18 October 2015

Atom and molecules questions with answer

Atom and molecules  questions with answer,class 9 chemistry notes,general science for competition,
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.
Q3 Write name and symbol of first 20 elements of the periodic table.
Atom and molecules  questions with answer,class 9 chemistry notes,general science for competition,

Ans Atomic number Name of the element Symbol
1 Hydrogen H
2 Helium He
3 Lithium Li
4 Beryllium Be
5 Boron B
6 Carbon C
7 Nitrogen N
8 Oxygen O
9 Fluorine F
10 Neon Ne
11 Sodium Na
12 Magnesium Mg
13 Aluminium Al
14 Silicon Si
15 Phosphorous P
16 Sulphur S
17 Chlorine Cl
18 Oxygen O
19 Potassium K
20 Calcium Ca
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=givenmass/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 Avagadro’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
23g of Na represents = 1 mol
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 ofof 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.

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