Stoichiometry Notes ( Full Chapter ) | Important Questions- Class 11 Chemistry Notes

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.” 

Basic Concepts of Chemistry Important Definitions:

Matter:

Any thing that has volume as well as mass is called matter.

What is an 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)

Characteristic Properties of Fundamental sub-atomic particles:

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.

Avogadro’s Number and Mole:

In Chemistry , mole is defined as the amount of substance that exactly contains 6.022 x 1023 particles.

Mole is a counting device or a number like:

12 things   =  Dozen

144 things  =  Gross

2 things = Pair

1000 g    =   1k g

similarly:

6.022 x 1023 particles =1 mole

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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