Some Basic Concepts of ChemistryNEET Chemistry · Class 11 · NCERT Chapter 1

You can expect 2 to 3 questions from this chapter in NEET 2027. The most commonly tested areas are the mole concept (moles to mass to particles conversions), stoichiometry (mole ratios in reactions), empirical and molecular formula determination, and occasionally the laws of chemical combination or Dalton's atomic theory.

A mole is simply a counting unit for very large numbers of particles. One mole of any substance contains 6.022 × 10²³ particles (Avogadro's number). Its importance in NEET is that it links the microscopic world (individual atoms, molecules) to the macroscopic world (grams you can weigh, litres you can measure). Almost every numerical problem in physical and inorganic chemistry uses the mole as the bridge.

The empirical formula gives the simplest whole-number ratio of atoms in a compound (e.g., CH₂O for glucose). The molecular formula gives the actual number of atoms per molecule (e.g., C₆H₁₂O₆ for glucose). The molecular formula is always a whole-number multiple of the empirical formula. To find the molecular formula, you need the molar mass of the compound in addition to the empirical formula.

First, convert the given masses of all reactants to moles. Then divide each reactant's moles by its stoichiometric coefficient from the balanced equation. The reactant with the smallest value is the limiting reagent. Use the moles of the limiting reagent and the mole ratio to calculate how much product forms (theoretical yield).

The five laws are: (1) Law of Conservation of Mass (Lavoisier) (2) Law of Definite Proportions (Proust) (3) Law of Multiple Proportions (Dalton) (4) Gay-Lussac's Law of Gaseous Volumes (5) Avogadro's Law. For NEET, the law of conservation of mass and law of multiple proportions are the most frequently tested. The law of multiple proportions generates ratio-type questions: given two compounds of the same elements, find the simple whole-number ratio in which masses of one element combine with a fixed mass of the other.

At STP (Standard Temperature and Pressure, defined as 0°C and 1 atm), one mole of any ideal gas occupies 22.4 litres. This is called the molar volume. Use it to convert between volume of a gas at STP and moles: n = V(in litres) / 22.4. Note: some newer definitions of STP use 0°C and 1 bar, giving a molar volume of 22.7 L. NCERT Class 11 uses the older definition of 22.4 L.

Molarity (M) is moles of solute per litre of solution. It depends on temperature because volume changes with temperature. Molality (m) is moles of solute per kilogram of solvent. It does not change with temperature. For NEET, molarity is tested more frequently in this chapter. Molality appears more in Chapter 2 of Class 12 (Solutions).