Chemistry - Science Track
What the paper tests
The chemistry paper is set in Khmer and tests atomic structure and periodic trends, mole calculations, solution concentration, stoichiometry, acid–base reactions, redox, and the functional groups of organic compounds. Calculations are short but are marked step by step.
Core rules
- n=m/M; for solutions, n=cV with V in litres.
- Balance the equation before using mole ratios; coefficients give the ratio of moles, not masses.
- In titration, moles of acid and base are related by the balanced equation, not automatically 1:1.
- Oxidation number increases on oxidation; the oxidising agent is itself reduced.
Worked example
25.0 mL of hydrochloric acid is neutralised by 20.0 mL of 0.100 M sodium hydroxide. Find [HCl]. The equation is HCl+NaOH→NaCl+H₂O, so the ratio is 1:1. n(NaOH)=cV=0.100×0.0200=2.00×10⁻³ mol. Therefore n(HCl)=2.00×10⁻³ mol. c(HCl)=n/V=2.00×10⁻³/0.0250=0.0800 M, or 8.00×10⁻² mol L⁻¹.
Traps
Leaving volumes in mL; choosing the wrong indicator for a weak acid–strong base titration; treating a 2:1 equation as 1:1; and confusing “oxidised” with “oxidising agent”.