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:
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 :
Characteristic Properties of Fundamental sub-atomic particles:
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.
Charge carrying species is called ion.
Types of Ions:
Ions bearing positive charge are called cations
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.
(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.
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.
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.
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.
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 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
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
6.022 x 1023 particles =1 mole
Notes of all the Topics of Stoichiometry with Important Explanation in detail:
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 =?
Molar mass of CaCO3 = 100 g/ mol
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
(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 ?
Mass of NaCl= 60g
Moles of NaCl = ?
Molar mass of NaCl= 23+35.5= 58.5g/mol
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?
Mass of ice (water) = 12g
Molar mass of water = 18g/mol
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
= 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.
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
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