Some Basic Concepts in Chemistry formulas for JEE Main & Advanced

22 formula entries for Some Basic Concepts in Chemistry, in the order the chapter builds them. Each shows the formula, when to use it and the first trap to avoid.

Mole Concept

n = \frac{N}{N_{A}} = \frac{m}{M} = \frac{V}{22.4 L (STP)}

When to use. Use to convert between mass, moles, number of particles, and gas volume at STP.

Trap. Confusing molar mass (g/mol) with molecular mass (amu); they are numerically equal but units differ.

Atomic Mass and Molecular Mass

Atomic mass = \sum (isotopic mass \times abundance)

When to use. Use to calculate molar mass, percentage composition, and empirical/molecular formulas.

Trap. Using atomic number instead of atomic mass in calculations.

Empirical and Molecular Formula

Molecular formula = (Empirical formula)_{n}

, where

n = \frac{Molecular mass}{Empirical formula mass}

When to use. Use when given percentage composition or combustion analysis data to find the formula of a compound.

Trap. Assuming empirical formula is same as molecular formula without checking molecular mass.

Limiting Reagent

Moles of reactant \div stoichiometric coefficient \to smallest value is limiting

When to use. Use in any reaction where amounts of two or more reactants are given.

Trap. Comparing masses directly instead of converting to moles.

Percentage Yield and Purity

% yield = \frac{actual yield}{theoretical yield} \times 100

;

% purity = \frac{pure mass}{impure mass} \times 100

When to use. Use when a reaction gives less product than expected or when reactant is impure.

Trap. Using actual yield as theoretical yield in calculations.

Concentration Terms: Molarity, Molality, Normality

M = \frac{n}{V (L)}

;

m = \frac{n}{mass of solvent (kg)}

;

N = \frac{equivalents}{V (L)}

When to use. Use to express concentration in solutions; molarity for reactions in solution, molality for colligative properties.

Trap. Confusing molarity with molality; molarity depends on temperature, molality does not.

Mole Fraction and ppm

χ_{A} = \frac{n_{A}}{n_{A} + n_{B}}

;

ppm = \frac{m_{solute}}{m_{solution}} \times 10^{6}

When to use. Use for dilute solutions or when dealing with gas mixtures.

Trap. Forgetting that sum of mole fractions of all components equals 1.

Equivalent Mass and n-Factor

Equivalent mass = \frac{M}{n}

;

Number of equivalents = \frac{m}{Equivalent mass} = n \times moles

When to use. Use in titration problems and for calculating normality.

Trap. Using same n-factor for different reactions (e.g., H2SO4 in acid-base vs redox).

Titration and Law of Equivalence

N_{1} V_{1} = N_{2} V_{2}

(for same reaction);

n_{1} M_{1} V_{1} = n_{2} M_{2} V_{2}

(using n-factor)

When to use. Use in acid-base, redox, or precipitation titration problems.

Trap. Using molarity instead of normality in $N_{1} V_{1} = N_{2} V_{2}$ without adjusting for n-factor.

Stoichiometry and Balanced Equations

a A + b B \to c C + d D

implies

\frac{n_{A}}{a} = \frac{n_{B}}{b} = \frac{n_{C}}{c} = \frac{n_{D}}{d}

When to use. Use for any quantitative chemical reaction problem.

Trap. Using unbalanced equations for mole ratios.

Gas Laws and Ideal Gas Equation

P V = n R T

;

R = 0.0821 {L atm mol}^{- 1} K^{- 1}

8.314 {J mol}^{- 1} K^{- 1}

When to use. Use for gases when pressure, volume, temperature, or moles are involved.

Trap. Using wrong units for R (e.g., using L atm with J value).

Dalton's Law of Partial Pressures

P_{total} = P_{1} + P_{2} + . . .

;

P_{i} = χ_{i} P_{total}

When to use. Use for gas mixtures, especially when collecting gas over water.

Trap. Confusing partial pressure with vapor pressure.

Empirical Formula from Combustion Data

n_{C} = \frac{m_{C O_{2}}}{44}

;

n_{H} = 2 \times \frac{m_{H_{2} O}}{18}

;

n_{O} = \frac{m_{sample} - m_{C} - m_{H}}{16}

When to use. Use when given combustion analysis data (masses of CO2, H2O, etc.) for an organic compound.

Trap. Forgetting to multiply by 2 for hydrogen atoms from water.

Significant Figures and Scientific Notation

Number = a \times 10^{b}

, where

1 \leq a < 10

When to use. Use in all calculations to report answers with correct precision.

Trap. Counting leading zeros as significant (e.g., 0.0012 has 2 sig figs, not 4).

Percentage Composition

% element = \frac{atomic mass \times number of atoms}{molar mass} \times 100

When to use. Use to find mass of element in a given mass of compound or to verify formula.

Trap. Using atomic number instead of atomic mass.

Law of Conservation of Mass and Definite Proportions

\sum m_{reactants} = \sum m_{products}

When to use. Use to check consistency of reaction data or to find unknown masses.

Trap. Forgetting that mass is conserved but moles may not be.

Redox Titration Calculations

N_{1} V_{1} = N_{2} V_{2}

;

n_{1} M_{1} V_{1} = n_{2} M_{2} V_{2}

When to use. Use for problems involving KMnO4, K2Cr2O7, oxalic acid, etc.

Trap. Using wrong n-factor for the redox reaction (e.g., KMnO4 in acidic vs neutral medium).

Average Atomic Mass

\overset{ˉ}{M} = \frac{\sum (m_{i} \times {abundance}_{i})}{100}

When to use. Use when given isotopic masses and abundances to find atomic mass of an element.

Trap. Using arithmetic mean instead of weighted average.

Volume Strength of H2O2

2 H_{2} O_{2} \to 2 H_{2} O + O_{2}

;

Volume strength = 11.2 \times M

(where M is molarity)

When to use. Use for problems involving hydrogen peroxide solutions.

Trap. Confusing volume strength with molarity directly.

Hardness of Water

Hardness (ppm) = \frac{{mass of CaCO}_{3} equivalent}{mass of water} \times 10^{6}

When to use. Use in water analysis problems.

Trap. Forgetting to convert all hardness-causing ions to CaCO3 equivalents.

Eudiometry (Gas Analysis)

V_{reacted} = V_{initial} - V_{final}

;

Volume of gas \propto moles

When to use. Use for problems involving combustion of hydrocarbons or gas mixtures with volume changes.

Trap. Forgetting that water vapor condenses at room temperature, so its volume is not counted.

Raoult's Law and Colligative Properties

\frac{P^{0} - P}{P^{0}} = χ_{solute}

;

Δ T_{f} = i K_{f} m

;

Δ T_{b} = i K_{b} m

;

π = i C R T

When to use. Use for solutions, especially to find molar mass or degree of dissociation.

Trap. Forgetting van't Hoff factor (i) for electrolytes.

Values to remember

$6.022 \times 10^{23}$ Mole Concept
$22.4 L/mol$ Mole Concept
$1 amu = 1.6605 \times 10^{- 24} g$ Atomic Mass and Molecular Mass
$0.0821$ Gas Laws and Ideal Gas Equation
$8.314$ Gas Laws and Ideal Gas Equation
$22.4 L$ Gas Laws and Ideal Gas Equation
$P_{water vapor}$ at given temperature Dalton's Law of Partial Pressures
$44 g/mol$ Empirical Formula from Combustion Data
$18 g/mol$ Empirical Formula from Combustion Data
$16 g/mol$ Empirical Formula from Combustion Data
$5$ (n-factor for KMnO4 in acidic medium) Redox Titration Calculations
$6$ (n-factor for K2Cr2O7 in acidic medium) Redox Titration Calculations
$11.2$ Volume Strength of H2O2
$100 g/mol$ (molar mass of CaCO3) Hardness of Water
$K_{f}$ and $K_{b}$ for water: $1.86 K kg/mol$ and $0.512 K kg/mol$ Raoult's Law and Colligative Properties

Read the full chapter: Some Basic Concepts in Chemistry for JEE, with what JEE asks and the common traps.