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Physical Chemistry Revision: Mole Concept, Redox Reactions & Solutions Notes

Physical Chemistry Revision: Mole Concept, Redox Reactions & Solutions

A comprehensive lecture covering foundational physical chemistry concepts including subatomic counts, stoichiometry, limiting reagents, balancing redox reactions, and solution concentration terms. Detailed methods for computing n-factors, equivalent mass, and solubility behaviors of solids and gases are explained.

JEE
NEET
Class 11 Science
Chemistry
English · 12 topics
Checked against the video on 21 Oct 2026

Made by AI from the YouTube video NEET 2027 Backlog Marathon🔥 | Physical Chemistry | Amit Mahajan Sir (opens YouTube in a new tab) by Competition Wallah, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Physical Chemistry Revision: Mole Concept, Redox Reactions & Solutions

English12 topics · 4 MCQs · 8 flashcardsWatch the video (opens YouTube in a new tab)

Checked against the video

In short

A comprehensive lecture covering foundational physical chemistry concepts including subatomic counts, stoichiometry, limiting reagents, balancing redox reactions, and solution concentration terms. Detailed methods for computing n-factors, equivalent mass, and solubility behaviors of solids and gases are explained.

Introduction and Scope of the Class

The session is designed to cover fundamental concepts of physical chemistry from basic to advanced levels.
Focus is heavily placed on master-level theory alongside essential numerical problems.
PYQ-specific solving sessions are planned separately to consolidate application skills.

Particles, Symbols, and Subatomic Algorithms

Chemistry fundamentally deals with three classes of particles: atoms, molecules, and ions.
An atom is the smallest indivisible unit of an element that retains its properties and is represented by its elemental symbol.
A molecule represents the smallest independent entity formed by two or more atoms.
Ions are charged species produced by the loss of electrons (cations) or gain of electrons (anions).
In a standard nuclear notation (A, Z)X, Z represents the atomic number (protons) and A represents the mass number (protons + neutrons).
Must knowIn any neutral atom, the number of electrons is equal to the number of protons, which is equal to Z.
For molecules, total protons are computed as the sum of (Number of atoms × Z) for each element.
Neutrons are calculated using n = (A - Z) × (Number of atoms).

Charges and Mass Calculations in Subatomic Entities

Protons remain unaffected during ionization; only electrons undergo loss or gain.
In a cation, electrons equal p - |charge|; in an anion, electrons equal p + |charge|.
Must knowThe mass of an electron is approximately 9.11 × 10⁻³¹ kg, which is considered negligible compared to nucleons.
The mass of an atom is defined entirely by the nucleons in its nucleus: Mass = (Number of Nucleons) × 1 a.m.u..
1 a.m.u. equals 1.67 × 10⁻²⁷ kg (or 1.67 × 10⁻²⁴ g).

Atomic and Molecular Mass Scales

Absolute Atomic Mass (A.A.M.) is the physical mass of a single atom, expressed in atomic mass units (u, a.m.u., or Da).
Relative Atomic Mass (R.A.M.) is a unitless ratio comparing the mass of an atom to 1/12th the mass of a carbon-12 atom.
Gram Atomic Mass (G.A.M.) represents the mass of one mole (N_A atoms) expressed in grams.
Must knowThe numerical value across R.A.M., A.A.M., and G.A.M. remains identical; only units vary (dimensionless, u, and grams respectively).

Scale	Definition	Helium Example
A.A.M.	Physical mass of 1 atom	4 u
R.A.M.	Mass relative to (1/12) of C-12	4
G.A.M.	Mass of 1 mole (N_A) of atoms	4 g

Isotopic Classifications and Average Atomic Mass

Isotopes have identical atomic numbers (Z) but different mass numbers (A).
Isobars have the same mass number (A) but different atomic numbers (Z).
Isotones possess an identical number of neutrons (A - Z).
Isodiaphers share the same isotopic excess value, calculated as A - 2Z.
Isoelectronic species contain an identical number of total electrons.
Must knowAverage Atomic Mass (A.A.M.) is calculated as the sum of (% abundance × isotopic mass) / 100.

The Mole Concept and Gas Molar Volumes

A mole represents an exact quantity of particles: N_A = 6.022 × 10²³.
Moles (n) from mass is given by mass / molar mass.
Moles (n) from particle count is given by molecules / N_A.
At NTP (1 atm, 273 K), 1 mole of an ideal gas occupies 22.4 L.
At STP (1 bar, 273 K), 1 mole of an ideal gas occupies 22.7 L.
The ideal gas equation PV = nRT handles non-standard conditions, where R is approximately 1/12 L atm K⁻¹ mol⁻¹ or 25/3 J K⁻¹ mol⁻¹.
Gas density is calculated via d = PM / RT.

Stoichiometric Calculations and Combustion

In a balanced reaction, stoichiometric coefficients (S.C.) represent relative ratios of moles, molecules, or volumes (for gases at constant T and P).
Must knowTo find unknown moles: n_find = [(S.C.)_find / (S.C.)_given] × n_given.
Combustion of a hydrocarbon C_xH_y follows: C_xH_y + (x + y/4)O_2 -> x CO_2 + (y/2) H_2O(l).
Liquid water does not contribute to gaseous volume considerations at standard temperature.
In percentage purity problems, only the pure fraction participates in chemical reactions.

Limiting Reagent and Empirical Formula

The limiting reagent (L.R.) is the reactant completely consumed first and controls product yield.
Must knowIdentify L.R. by finding the lowest value of n / (S.C.) among reactants.
Empirical formula represents the simplest whole-number atomic ratio in a compound.
Divide the mass percentage of each element by its atomic mass, then divide by the smallest resulting value to obtain the empirical formula.
The molecular formula is obtained via (Empirical Formula)_x, where x = Molecular Mass / Empirical Mass.

Laws of Chemical Combination

Law of Conservation of Mass (Lavoisier): Total mass of reactants equals total mass of products in a closed system (fails in nuclear reactions).
Law of Definite Proportions (Proust): A compound always contains exactly the same proportion of elements by mass regardless of source.
Law of Multiple Proportions (Dalton): When two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios.
Gay-Lussac's Law: Gases react in simple whole-number ratios by volume at identical temperature and pressure.
Avogadro's Law: Equal volumes of all gases under identical temperature and pressure contain equal numbers of molecules (V ∝ n).

Oxidation Numbers and Rules

Oxidation number is the arbitrary charge on an atom if all bonds are considered completely ionic.
An element in its free/uncombined state has an oxidation number of 0 (e.g., O_2, S_8, P_4).
Group 1 metals rigidly exhibit +1, and Group 2 metals exhibit +2 in compounds.
Fluorine always has an oxidation state of -1 in compounds.
Hydrogen shows +1 with non-metals and -1 with metals (hydrides).
Oxygen exhibits -2 in standard oxides, -1 in peroxides, and -1/2 in superoxides.
In fractional oxidation states (e.g., Fe_3O_4, C_3O_2, Br_3O_8), values represent average states across different individual atoms.

n-Factor Calculations and Redox Balancing

For an oxidizing or reducing agent, n-factor = (atoms per molecule) × |Initial O.N. - Final O.N.|.
In disproportionation reactions, n_total = (n_ox × n_red) / (n_ox + n_red).
Must knowEquivalent mass is calculated as Molar Mass / n-factor.
In acidic medium redox balancing: balance atoms, add H_2O to oxygen-deficient sides, and balance H with H^+.
In basic medium redox balancing: add H_2O to hydrogen-deficient sides and add equal OH^- to the opposite side.
For KMnO_4, n-factor is 5 in acidic medium (Mn²+), 3 in neutral/weakly basic medium (MnO_2), and 1 in strongly basic medium (MnO_4²-).
For K_2Cr_2O_7 in acidic medium, the n-factor is 6 (Cr³+).

Solutions: Concentrations and Equivalence

Molarity (M): Moles of solute dissolved per liter of solution (M = n_B / V(L)). Temperature dependent.
Molality (m): Moles of solute per kilogram of solvent (m = n_B / W_A(kg)). Temperature independent.
Mole Fraction (X): Ratio of moles of a component to total moles (X_A + X_B = 1).
Normality (N): Gram equivalents of solute per liter of solution (N = M × n-factor).
Dilution law: M_1V_1 = M_2V_2 or N_1V_1 = N_2V_2.
According to the Law of Equivalence, reacting substances always react in equal numbers of gram equivalents: g.eq(A) = g.eq(B) = g.eq(Product).
Dissolution of gas in liquid is exothermic; hence, solubility of gases decreases with increasing temperature.

Key terms

Limiting Reagent: The reactant present in the least stoichiometric amount that is completely consumed first.
Gram Equivalent: The ratio of the mass of a substance to its equivalent weight (mass / E).
Normality: Number of gram equivalents of solute dissolved in 1 liter of solution (N = M × n-factor).
Molality: Concentration expressed as moles of solute per kilogram of solvent; independent of temperature.
Disproportionation: A redox reaction in which the same substance is simultaneously oxidized and reduced.
Molar Volume: Volume occupied by one mole of a gas at specified temperature and pressure (22.4 L at NTP, 22.7 L at STP).

Formulas

Mass of an Atom = (Number of Nucleons) × 1 a.m.u.: Calculating single atom mass from nucleons
Average Atomic Mass = Sum of (% abundance × isotopic mass) / 100: Finding average atomic mass of isotopic mixtures
PV = nRT: Ideal gas equation relating pressure, volume, temperature, and moles
d = PM / RT: Calculating gas density from molecular mass, pressure, and temperature
n_find = [(S.C.)_find / (S.C.)_given] × n_given: Determining unknown moles from stoichiometric ratios
C_xH_y + (x + y/4)O_2 -> x CO_2 + (y/2)H_2O: General combustion equation for hydrocarbons
n-factor = (atoms per molecule) *: Initial O.N. - Final O.N.| | Computing redox n-factor for simple changes
n_total = (n_ox × n_red) / (n_ox + n_red): Computing total n-factor for disproportionation reactions
Equivalent Mass = Molar Mass / n-factor: Calculating equivalent weight
N = M × n-factor: Interconversion between normality and molarity
m = (X_B × 1000) / (X_A × M_A): Relating molality to mole fractions of solute and solvent

Exam tips and traps

Do not include the volume of liquid H2O in gaseous volume calculations for hydrocarbon combustion at room temperature.
Watch out for the word "strongly basic" when calculating the n-factor of KMnO4; weakly basic defaults to the neutral pathway (n = 3), whereas strongly basic gives n = 1.

Quick revision

Read these the night before the exam.

An electron's mass is considered negligible when computing the total mass of an atom.
The mass number (A) represents the sum of protons and neutrons in a nucleus.
1 mole of an ideal gas occupies 22.4 liters at NTP (1 atm, 273 K) and 22.7 liters at STP (1 bar, 273 K).
In the combustion of hydrocarbons, the required O2 coefficient is always x + y/4.
To identify the limiting reagent, calculate moles / stoichiometric coefficient for all reactants; the lowest is limiting.
Oxygen exhibits an oxidation state of -1 in peroxides and -1/2 in superoxides.
The n-factor of K2Cr2O7 in acidic medium is strictly equal to 6.
In disproportionation of H2O2 into H2O and O2, the total n-factor of H2O2 is 1.
Molality and mole fraction are independent of temperature because they rely strictly on mass.
The solubility of gases in liquids decreases as temperature rises due to the exothermic nature of gas dissolution.

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