Basic Concepts of Chemistry Class 11 Chapter 1 | Important Questions and MCQ’s

Important Basic Concepts of Chemistry Definitions:

Stoichiometry :

Stoichiometry is a quantitative chemistry. “Calculations based on law of conservation of mass, law of constant composition and balanced chemical equations are known as stoichiometric calculations.” 

Atom:

Atom is derived from Greek word “Atomos” meaning indivisible.

“smallest particle of matter which possesses all the properties

of that matter but which cannot exist independently”

is known as an atom e.g. O, H, N, Cl, Br etc.

Fundamental Sub-atomic particles of an atom:

Fundamental sub-atomic particles of atoms are as follow :

  • Electrons
  • Protons
  • Neutrons
What is Atom Definition with Example ( in Urdu / Hindi)

Molecule:

It is defined as “smallest particle of matter which possesses all the properties of that matter and which can exist independently.” e.g. O2, Cl2, C6H12O6, S8, H2O, He etc.

What is a Molecule and its Types ( in Urdu / Hindi)

Ions:

Charge carrying species is called ion.

Types of Ions:

Cation:

Ions bearing positive charge are called cations

Anion:

Ions bearing negative charge are called anions.

Formation of cations is endothermic process while formation of anions is exothermic process. Ions are of three types.

Types of Ions (In Urdu/ Hindi)

(i) simple ion: If a single atom bears +ve or –ve charge then it is called simple ion e.g. H+, O-2. (ii) compound ion: When a group of atoms bear +ve or –ve charge then it is called compound ion e.g. NH4+, SO4-2 etc.

(iii) molecular ion: When a molecule gains or loses electrons, the resulting species is called molecular ion e.g. CH4+, O2+, CO+, N2+ etc. Cationic molecular ions are more common than anionic molecular ions.

Recommended video:

Difference between atom and ion ( in Urdu / Hindi)

Element:

A substance which is made up of only one kind of atoms is called element e.g. O2, O3, Cl2, S8, He etc. There are 92 natural elements while total elements are more than 110.

Compound:

A substance which is made up of more than one kind of atoms chemically combined together, is called compound e.g. CO2, H2SO4, C6H12O6 etc.

Isotope:

Atoms having same atomic number but different atomic masses are called isotopes. e.g. Hydrogen has three isotopes protium 1H1, deuterium 1H2, tritium 1H3, Carbon has three isotopes

6C12, 6C13, 6C14, oxygen also has three isotopes 8O16, 8O17, 8O18 etc. More than 300 isotopes occur in nature they include 40 radioactive isotopes. Often, elements with even atomic number have more number of isotopes and are abundant.

Isotopes Important Concepts for Exams

Atomic Number (Z)  

“Number of protons in the nucleus of an atoms” or “Number of electrons in an atom” is called atomic number.

Number of electrons in an atom          = Z

Number of protons in an atom            = Z

Number of neutrons in an atom          = A – Z

Atomic masses:

Atomic masses are expressed in three ways:

(i)         Mass Number (A):   

    Sum of protons and neutrons in an atom is called its mass number or nucleon number.

                          A = Protons + Neutrons

(ii)        Relative atomic mass (Ar)   

  The atomic mass of an atom as compared to mass of Carbon-12, is called relative atomic mass e.g. relative atomic mass of Hydrogen (protium i.e. 1H1) is 1.0078 amu.

(iii)       Average atomic mass (Ar) 

    The weight average of isotopic masses of an element is called average atomic mass of the element.   

e.g. (i)                          Relative isotopic mass             Relative abundance

            Carbon-12                   12.00000 amu                         98.89%

            Carbon-13                   13.00335 amu                         1.11%

Average atomic mass =      12.00000 x 98.89   +   13.00335 x 1.11 / 100

                                                                               =  12.01 amu        

   amu: This is a unit to measure atomic masses. It is one-twelfth of the mass of carbon-12.

                                 1 a.m.u = 1.66 x 10-24g            or 1.66 x 10-27kg

Relative molecular masses (Mr): Sum of relative atomic masses of atoms present in a molecule gives relative molecular masses

Chemical Formulae:

            The symbolic representation of a compound that shows elements in the compound and also the ratio of their atoms is called chemical formula.

Types of Chemical formulae

Chemical formulae are of three types:

(i)   Empirical formula or Simple formula:            

            The formula of a compound that shows elements in the compound and their simple atomic ratio is called empirical or simple formula.

(ii)   Molecular formula:      

            The formula of a compound that shows elements in the compound and actual number of their atoms is called molecular formula.

(iii)  Structural formula:       

            The formula of a compound that shows elements in the compound, actual number of their atoms and arrangement of the atoms in the molecule is called structural formula.

Important MCQ’s question answers with brief explanation:

  • A species that contains one proton, one neutron and two electron is
    1. H-1
    2. D-1
    3. He
    4. He+2

Answer ) D-1 ( Deuterium )

Brief Explanation :

Deuterium atom has one electron, one proton and one neutron. Thus D-1 ion wills one contains one proton, one neutron and two electrons.


  • A substance that can be separated into components only by chemical method?
    1. Air
    2. Petroleum
    3. Bleaching Powder
    4. Molten NaCl

Answer ) Molten NaCl

Brief Explanation :

Molten NaCl is a chemical compound and chemical compound can be separated into its components by chemical methods like electrolysis


  • Atoms which are chemically same and differ only in mass-dependent properties are
    1. 6C12 , 6C13 , 6C14
    2. 1H1 , 2He4 , 6C12
    3.  14Si30 , 15P31 , 16S32
    4. 18Ar40 , 19K40 , 20Ca40

Answer ) 6C12 , 6C13 , 6C14

Brief Explanation :

6C12 , 6C13 , 6C14 are isotopes having the same atomic numbers but different mass numbers. Isotopes have the same chemical property due to same atomic number but different mas dependent physical properties and rates of reactions due to different mass number


  • Elements are arranged in periodic table, in increasing order of their
    1. Valency
    2. Mass Number
    3. Oxidation State
    4. Charge on nucleus

Answer ) Charge on nucleus

Brief Explanation :

Atomic number (charge on Nucleus) is the fundamental property and elements in the periodic table have been arranged in order of their increasing atomic numbers


  • H2O, CH4, HF and Na+1 have the same
    1. Number of protons
    2. Number of electrons
    3. Number of neutrons
    4. Average mass

Answer ) Number of electrons

Brief Explanation :

H2O, CH4, HF and Na+1 ions, all have the same number of electrons (10)


  • One atomic mass unit (a.m.u) stands for
    1. One C12 atom
    2. One H-atom
    3. 1/12th of the mass of H-atom
    4. 1/12th of the mass of C12-atom

Answer ) 1/12th of the mass of C12-atom

Brief Explanation :

1 a.m.u = 1.66 x 10-24 g

Mass of 6.023 x 1023 C12 atoms = 12 g

Mass of one C12 atom = 12 x 1 /6.023 x 1023

                                        = 1.9927 x 10-23 g

We have one atomic mass unit is 1/12th of the mass of C12 atoms

1 amu    = 1 /12 x mass of C12 atom

1 amu    = 1 /12 x 1.9927 x 10-23 g

               = 1.66 x 10-24g


  • One mole of NaCl contains6.02×1023
    1. Ions
    2. Atoms
    3. Molecules
    4. Formula Unit

Answer ) Formula Unit

Brief Explanation :

  • One mole of Sodium Chloride contains 6.023 x 1023 NaCl Formula units

  • Mass of one mole of oxygen is
    1. 32 g
    2. 16 x NA g
    3. 32 x NA g
    4. 32 / NA g

Answer ) 32 / NA g

Brief Explanation :

Mass of NA (6.023 x 1023) O2 – molecules = 32g

Mass of one O2 – molecule = 32/NA g


  • Number of atoms in 64 a. m. u. of SO2 is
    1. 3
    2. 64
    3. 3 x NA
    4. 64 x NA

Answer ) 3

Brief Explanation :

64 a.m.u. is the mass of one molecule of SO2 and thus contains three (3) atoms. However 64 g is the mass of one mole of SO2 molecule and contains 3 x 6.023 x 1023atoms.


  • A balanced chemical equation may not obey
    1. Law of conservation of mass
    2. Law of conservation of volume
    3. Law of constant composition
    4. All of these

Answer ) Law of conservation of volume

Brief Explanation :

In a balanced chemical equation, each compound has definite composition with correct chemical formula. The total mass of reactants is always equal to total mass of products in balanced equation. However, volumes of reactants and products may not be equal. Thus balanced equation obeys the law of conservation of mass and law of definite proportion but may or may not obey the law of conservation of volume.


  • The limiting reactant in a chemical reaction is
    1. Is used up earlier and completely
    2. Controls the yield of reaction
    3. Gives the least amount of product
    4. All of these

Answer ) All of these

Brief Explanation :

A limiting reactant in a chemical reaction is always used up earlier, gives the least amount of product and controls the yield of reaction. It is present in lesser amount than the required stoichiometric amount.


  • When equal moles of the reactant react together, the limiting reactant is
  1. K2Cr2O7
  2. HCl
  3. Cl2
  4. None

Answer ) HCl

Brief Explanation :

K2Cr2O7 + 14 HCl to form 3Cl2 + other products. Let’s one mole of each reactant takes part in a chemical reaction.

One mole of K2Cr2O7 = 3 mole Cl2

Similarly, 14 moles of HCl = 3 mole Cl2

1 mole of HCl = 3/14 mole of Cl2

As 1 mole of HCl produces the least amount of Cl2 product so HCl is a limiting reactant.


  • A yield of reaction that is independent of their reaction conditions is

1.Percent

2.Actual

3.Theoretical

4.All of them

Answer ) Theoretical

Brief Explanation :

A theoretical yield of reaction is independent of the reaction conditions because it involves no practical work


  • The yield of reaction is becomes 100% when
    1. Actual yield < Theoretical yield
    2. Actual yield > Theoretical yield
    3. Actual yield ≤ Theoretical yield
    4. Actual yield  = Theoretical yield

Answer ) Actual yield  = Theoretical yield

Brief Explanation :

The yield of reaction is  become 100% when actual yield equal to theoretical yield of reaction


  • Actual yield is always lesser than theoretical yield due to
    1. Lack of optimum reaction conditions
    2. Mechanical loss of reactants and / or products
    3. Reversibility of reaction
    4. All of these

Answer ) All of these

Brief Explanation :

Actual yield is always lesser than theoretical yield due to one or more reason like not availability of reagent and the most suitable reaction conditions; loss of reactants / products during experimental work, reversibility of reaction of by-products formation etc.



  • Mass of Cl-1 ions in a sample of AlCl3 that contains 6.023 x 1023 Al+3 ions
    1. 3
    2. 3 x 1021
    3. 6.023 x 1023 / 3
    4. 3 x 6.023 x 1023

Answer ) 3

Brief Explanation :

From the formula AlCl3 we have

1 Al+3 ions = 3Cl-1 ion

6.023 x 1023 Al+3ion = 3 x 6.023 x 1023 Cl-1 ion

                                         = 3 mole of Cl-1 ion


  • The simplest ration b/w atoms (Pb:O) of a binary compound of oxygen containing 90.66% lead 82Pb207, is
    1. 1:1
    2. 1:2
    3. 3:4
    4. 9:1

Answer ) 3:4

Brief Explanation :

We have that

Lead (Pb) = 90.66% = 90.66/207 mol = 0.4379 mol

Oxygen (O2) = 100 – 90.66 = 9.34% = 9.34/16 mol = 0.5838 mol

Relative number of atoms of Pb = 0.4379/0.4379 = 1

Relative number of atoms of Oxygen = 0.5838/0.4379 = 1.333

To change number of atoms into whole number, multiplying both the values with 3 i.e.

Atoms of Pb = 1 x 3 = 3

Atoms of Oxygen = 1.333 x 3 = 3.999 ~ 4 Hence the simple ration between Pb and Oxygen is (3:4)


  • When 60cm3 each of H2 and O2 reacts together, the theoretical decrease in volume of the reacting gasses will be
    1. 120 cm3
    2. 90 cm3
    3. 60 cm3
    4. 30 cm3

Answer ) 90 cm3

Brief Explanation :

From the balanced equation  2H2 +O2 to form 2H2O ,

 We have that 2cm3 of H2 reacts with = 1 cm3 of O2

60 cm3 of H2 reacts with = 60/2 = 30 cm3 of O2

Total volume of reacting gases = 60+60= 120 cm3

Volume of gases used = 60 cm3 of H2 + 30 cm3 of O2 = 90 cm3


  • Moles of fluorine required to reach with 0.6 mole of metal M to form46.8 grams of MF2
    1. 0.12 mole
    2. 1.00 mole
    3. 1.20 mole
    4. 2.00 mole

Answer ) 1.20 mole

Brief Explanation :

  • From the formula  MF2 we have that

1 mole of metal M reacts with = 2 mole Fluorine

 0.6 mole of metal M will reacts with= 2 x 0.6 = 1.2 mole of Fluorine

Notes of all the Topics of Stoichiometry with Important Explanation in detail:

Mole and Avogadro’s Number | Important Questions

How do you calculate the percent composition of an element in a compound

Mole and Chemical Equation

Law of conservation of mass

Law of Constant Composition(law of definite proportions)

How to find Excess Reactant and Limiting Reactant

How do you calculate the yield of a reaction


Related Important Questions for Exams !!

Q1.What is the mass of 0.5 mole of CaCO3 ?

Moles of CaCO3 = 0.5 mol

Mass of CaCO3 =?

SOLUTION:

Molar mass of CaCO3 = 100 g/ mol

 FORMULA:

No of moles= mass / molar mass

Mass of CaCO3 = moles. X   molar mass

Mass of CaCO3 =  0.5   X100

                       = 50g CaCO3


Q2.Calculate the Formula mass of the following:

(i) MgSO4 (ii) C3H6O (iii) C3H8 (iv) C2H5OH (v) Al2O3

Solution:

(i) formula mass of MgSO4 = 24 + 32 + 16 x 4 = 24 + 32 + 64 = 120

(ii) formula mass of C3H6O = 12 x 3 + 1 x 6 + 16 = 36 + 6 + 16 = 58

(iii) formula mass of C3H8 = 12 x 3 + 1 x 8 = 36 + 8 = 44

(iv) formula mass of C2H5OH = 12 x 2 + 1 x 6 + 16 = 24 + 6 + 16 = 46

 (v) formula mass of Al2O3 = 27 x 2 + 16 x 3 = 54 + 48 = 102

  (vi) formula mass of K2Cr2O7 = 39 x 2 + 52 x 2 + 16 x 7 = 78 + 104 + 112 = 294

Q3.What is the mass of one mole Carbon atom?

Ans. The mass of one mole of carbon atom is 12g.


Q4.How many moles are there in 60g of NaCl  ?

Solution:

Given:

Mass of NaCl= 60g

SOLUTION:

Moles of NaCl = ?

SOLUTION:

Molar mass of NaCl= 23+35.5= 58.5g/mol

 FORMULA:

No of moles= mass / molar mass

No of moles= 60 / 58.5

No of moles(n) =1.02 mol



Q5.What is formula mass?

Ans) Sum of relative atomic masses of atoms in a formula unit of a compound is called formula mass.



Q6.Calculate the number of molecules in 12g of ice?

Given:

Mass of ice (water) = 12g

Molar mass of water = 18g/mol

Solution:

Number of molecules of ice (water) = Mass of ice (water) x NA

Molar mass of water

Number of molecules of ice (water) = 12g x 6.022 x 1023

18g/mol

= 4.01 x 1023 molecules


Q7.Define molar volume of a gas. What will be the volume of 2.5 moles of H2 gas and 60 g of NH3 at STP.

  • Volume of one mole of a gas at a given temperature and pressure is called molar volume of the gas at the given condition e.g. molar volume of a gas at STP 22.4 dm3 while molar volume of the gas at RTP is 24dm3.

Volume of H2 gas at STP = moles x 22.4dm3 = 2.5 x 22.4dm3 = 56dm3

Volume of NH3 gas at STP = ?

Moles of NH3 = mass / molar mass = 60g / 17g mol-1 = 3.53 mol

Volume of NH3 gas at STP = moles x 22.4 = 3.53 x 22.4 = 79.07 dm3


Given the equation

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Heat

How can this chemical equation be read in terms of particles , moles and masses?

Ans) CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Heat

The equation shows 1 molecule of methane reacts with 2 molecules of oxygen to produce 1 molecule of carbon dioxide and 2 molecules of steam along with liberation of heat.           

Or        the equation shows 1 mole of methane reacts with 2 moles of oxygen to produce 1 mole of carbon dioxide and 2 moles of steam along with liberation of heat.

Or        the equation shows 1 dm3 of methane reacts with 2 dm3 of oxygen to produce 1 dm3 of carbon dioxide and 2 dm3 of steam along with liberation of heat.

Or        the equation shows 16 g of methane reacts with 64 g of oxygen to produce 44 g  of carbon dioxide and 36 g of steam along with liberation of heat.


Q8.Calculate the percentage composition of a compound. ( MgSO4 )

Molar mass of MgSO4 = 1 x 24 + 1 x 32 + 4 x 16 = 24 + 32 + 64 = 120

               % age of Mg =         Mass of Mg             x 100       =  24  x 100    = 20 %

                                           Molar mass of MgSO4     120 

               % age of S =         Mass of S            x 100       =  32  x 100    = 26.67 %

                                           Molar mass of MgSO4     120 

               % age of O =         Mass of O           x 100       =  64  x 100    = 53.33 %

                                           Molar mass of MgSO4     120 


Q9.Aluminium sulphate hydrate [Al2(SO4)3.x H2O] contains 8.20% of Al by mass, calculate x.

Solution:

            Formula mass of Al2(SO4)3.x H2O      = 27×2 + 32×2 + 16×12 + x(1×2 + 16)

or         Formula mass of Al2(SO4)3.x H2O      = 54 + 96 + 192 + x(18)

or         Formula mass of Al2(SO4)3.x H2O      = 342 + 18x

Percentage of Al in Al2(SO4)3.x H2O = 8.20%

     But      Percentage of Al in Al2(SO4)3.x H2O =     Amount of Al in Al2(SO4)3.x H2O x 100

                                                                                     Formula mass of Al2(SO4)3.x H2O

8.20     =          54                  x 100

                                                                                     342 + 18x

Or                                            8.20 (342 + 18x)    =   5400

Or                                            2804.40 + 147.60x = 5400

Or                                            147.60x = 5400- 2804.40

Or                                             147.60x = 2595.60

Or                                            x = 2595.60 / 147.60 =   17.59   ≈ 18

Also Read !!

Stoichiometry Important mcq with answers and Explanation

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