Chemistry
Chemistry
Chemistry questions target stoichiometry, atomic structure and bonding, equilibrium, acid–base and redox chemistry, and basic organic functional groups. The exam rewards clear mole bookkeeping and correct state and charge labels.
Core rules
- Balance atoms and charge; only then use mole ratios.
- Moles n=m/M, and for solutions n=cV with V in litres.
- The limiting reagent determines theoretical yield; excess reagent does not.
- Equilibrium shifts oppose a change in concentration, pressure, or temperature; K changes only with temperature.
- Acid–base neutralization: moles H⁺ equals moles OH⁻ at equivalence for monoprotic species.
- Redox: oxidation number increases on oxidation; balance electrons in half-reactions.
Worked example
25.0 mL of 0.100 mol/L HCl is neutralized by 20.0 mL NaOH. Find [NaOH]. Moles HCl=0.100×0.0250=2.50×10⁻³ mol. HCl and NaOH react 1:1, so moles NaOH=2.50×10⁻³ mol. Concentration=2.50×10⁻³/0.0200=0.125 mol/L. Check: the acid volume is larger but less concentrated; the base is more concentrated, consistent with the smaller volume.
Traps that cost marks
- Using mL directly in cV without converting to litres.
- Assuming 1:1 stoichiometry for H₂SO₄ or Ca(OH)₂.
- Forgetting spectator ions when writing net ionic equations.
- Confusing strong/weak acids with concentrated/dilute solutions.
- Reporting more significant figures than the data justify.