Class 11 Chemistry Chapter Stoichiometry Notes
Stoichiometry is quantitative chemistry. “Calculations based on the law of conservation of mass, law of constant composition and balanced chemical equations are called stoichiometric calculations.”
Science, Society Relationship:
Stoichiometry has many applications in human society.
- Concentrations of different substances in a sample.
- Glucose in the blood of diabetics.
- Steroid and other stamina enhancing drugs. So helps in doping control.
- Cholesterol level in blood.
- Quantity of medicines.
- Metal in ore (metallurgy).
Some basic Chemistry definitions:
Matter: Anything that has volume, as well as mass is called matter.
Mass: The quantity of matter in a body is called mass.
Weight: The force with which a planet attracts a body towards its centre is called weight. It depends upon the gravity of the planet e.g. gravity of the moon is one-sixth (1/6) of earth’s gravity so the weight of a body on the moon is 1/6 of its weight on earth.
Atom: Atomos is a Greek word meaning indivisible. The Greek philosopher Democritus gave the idea of atoms (460-370 BC). Atom is defined as the “smallest particle of matter which possesses all the properties of that matter but which cannot exist independently.” e.g. O, H, N, Cl, Br etc.
Watch -> What is an atom? ( in Urdu)
Molecule: It is defined as the “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.
Watch -> Molecule and its types ( in Urdu)
Ions: Charge carrying species are called ions. Ions bearing positive charge are called cations while ions bearing negative charge are called anions. The formation of cations is an endothermic process while the formation of anions is an exothermic process.
Ions are of three types.
- simple ion:
If a single atom bears +ve or –ve charge then it is called a simple ion e.g. H+, O-2.
- compound ion: When a group of atoms bear +ve or –ve charge then it is called a compound ion e.g. NH4+, SO4-2 etc.
- molecular ion: When a molecule gains or loses electrons, the resulting species is called a molecular ion e.g. CH4+, O2+, CO+, N2+ etc. Cationic molecular ions are more common than anionic molecular ions.
Watch -> What is an ion?
Watch -> Molecular ion
A substance that is made up of only one kind of atom is called an element e.g. O2, O3, Cl2, S8, He etc.
There are 92 natural elements while the total elements are more than 110.
A substance that is made up of more than one kind of atoms chemically combined together, is called a compound e.g. CO2, H2SO4, C6H12O6 etc.
Atoms having the same atomic number but different atomic masses are called isotopes.
e.g. Hydrogen has three isotopes protium 1H1, deuterium 1H2 and 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 including 40 radioactive isotopes.
Often, elements with even atomic numbers have more isotopes and are abundant.
Watch ->Isotope definition and examples
Atomic Number (Z):
“Number of protons in the nucleus of an atom” or “Number of electrons in an atom” is called the 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:
- Mass Number (A):
The Sum of protons and neutrons in an atom is called its mass number or nucleon number.
A = Protons + Neutrons
- Relative atomic mass (Ar):
The atomic mass of an atom is compared to the mass of Carbon-12, is called relative atomic mass e.g. relative atomic mass of Hydrogen (protium i.e. 1H1) is 1.0078 amu.
- Average atomic mass (Ar):
The weighted average of isotopic masses of an element is called the 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
= 12.01 amu
e.g. (ii) Relative isotopic mass Relative abundance
Copper-63 62.93 amu 69.09%
Copper-65 64.9278 amu 30.91%
Average atomic mass = 62.93 x 69.09 + 64.9278 x 30.91
= 63.55 amu
Atomic mass unit (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
The symbolic representation of a compound that shows elements in the compound and also the ratio of their atoms is called the chemical formula.
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 an actual number of their atoms is called the molecular formula.
(iii) Structural formula:
The formula of a compound that shows elements in the compound, the actual number of their atoms and the arrangement of the atoms in the molecule is called the structural formula.
The simplest unit whose repetition builds crystal is called the formula unit.
Usually, for ionic compounds, their empirical formula is their formula unit while for covalent compounds their molecular formula is their formula unit.