BCECE Chemistry: Mole Ratios, Equilibrium and Functional Groups
What BCECE Chemistry tests
The Chemistry section covers physical, inorganic and organic chemistry at 10+2 level: mole concept and stoichiometry, atomic structure, chemical bonding, thermodynamics, equilibrium including ionic equilibrium, redox, electrochemistry, kinetics, periodic trends, p-block and d-block chemistry, coordination compounds, hydrocarbons, haloalkanes, alcohols, carbonyl compounds, amines, biomolecules and polymers.
Core rules
- Balance the equation before any mole calculation; the coefficients are the mole ratios.
- Identify the limiting reagent by dividing moles by the stoichiometric coefficient.
- For a weak acid, [H+] = √(Ka·C) only when dissociation is small; otherwise solve the quadratic.
- Hybridisation: count σ bonds plus lone pairs on the central atom (steric number).
- In organic reactions, track the functional group and the electrophile or nucleophile; Markovnikov addition places H on the carbon bearing more H.
Worked example
4 g of H2 reacts with 16 g of O2. Find the mass of water formed. 2H2 + O2 → 2H2O. Moles: H2 = 4/2 = 2 mol; O2 = 16/32 = 0.5 mol. 0.5 mol O2 needs 1 mol H2, so H2 is in excess and O2 is limiting. 0.5 mol O2 gives 1 mol H2O = 1 × 18 = 18 g.
Traps
- Using the reactant with the larger mass as the limiting reagent.
- Forgetting that the molar mass of O2 is 32, not 16.
- Confusing a mole of atoms with a mole of molecules in diatomic gases.
- Writing incorrect oxidation states for transition metals.
- Mixing empirical and molecular formulae when the question gives percentage composition.
- Ignoring stereochemistry when the question specifies cis/trans or R/S.