General Science: Units, Concentration and Basic Principles
General Science provides the physics, chemistry, biology and mathematics underpinning the technical domains. Questions are usually straightforward calculations, unit conversions and definitions.
Core rules
- SI units: length metre, mass kilogram, time second, amount mole, temperature kelvin; derived units include newton, pascal, joule, watt and coulomb.
- Concentration: molarity = moles/litre; % w/v = g/100 mL; ppm = mg/L; dilution C1V1 = C2V2.
- pH: pH = −log[H⁺]; strong acid and base calculations; buffers and Henderson–Hasselbalch.
- Gas laws: Boyle, Charles, Avogadro and the ideal gas equation PV = nRT.
- Biology: cell structure, photosynthesis, respiration, blood groups, hormones and basic genetics.
- Instruments: spectrophotometer, pH meter, centrifuge, autoclave and microscope principles.
Worked example
5.85 g of NaCl (molar mass 58.5 g/mol) is dissolved to 500 mL. Moles = 5.85/58.5 = 0.1 mol; molarity = 0.1/0.5 = 0.2 M. To prepare 100 mL of 0.05 M from this stock: C1V1 = C2V2 gives V1 = (0.05 × 100)/0.2 = 25 mL, made up to 100 mL.
Traps
Do not confuse molarity with molality; check whether percentage is w/v, w/w or v/v; remember that pH is logarithmic, so a one-unit fall means a tenfold rise in [H⁺]; and convert mL to L before calculating molarity.