Chemistry
Chemistry in the Bihar Polytechnic entrance concentrates on atomic structure, periodic trends, chemical bonding, the mole concept, acids and bases, and simple organic chemistry. Many questions pair a definition with a short calculation.
Atomic structure and periodicity
- Atomic number Z equals the number of protons; mass number A equals protons + neutrons. Isotopes have the same Z but different A.
- Electrons fill shells in the order 2, 8, 8. Valence electrons determine the group; the outermost shell number determines the period.
- Across a period, atomic radius decreases and electronegativity increases. Down a group, radius increases and ionisation energy decreases.
Bonding and reactions
- Ionic bonding transfers electrons, usually metal + non-metal. Covalent bonding shares electrons, usually non-metal + non-metal.
- A balanced equation conserves atoms and charge. Acids donate H⁺; bases accept H⁺. At 25°C, pH 7 is neutral, below 7 acidic, above 7 basic.
Mole concept
- One mole contains 6.022×10²³ particles and has a mass equal to the molar mass in grams. n = m/M.
- At STP, one mole of an ideal gas occupies 22.4 L.
Worked example
How many molecules are in 11 g of CO₂? M(CO₂) = 12 + 2×16 = 44 g/mol. n = 11/44 = 0.25 mol. Number of molecules = 0.25 × 6.022×10²³ = 1.5055×10²³, about 1.51×10²³.
Traps
- CO₂ has molar mass 44 g/mol, not 28 g/mol (that is CO or N₂).
- Atomic number is not the same as mass number.
- pH 3 means [H⁺] = 10⁻³ mol/L, not 3 mol/L.
- In ionic equations, balance charge as well as atoms.