Basic Principles
Scope
Basic principles questions are the cheapest points on the exam and the easiest to lose. They test unit consistency, gas-law behavior, stoichiometry, vapor pressure and humidity, and elementary statistics. You must be fluent converting among mass, molar, and volumetric bases; between gauge and absolute pressure; and between SI and US customary units.
Concepts to own
- Ideal gas: PV = nRT, with T always absolute and R matched to the chosen units (8.314 kPa·m³/kmol·K; 10.73 psia·ft³/lbmol·R).
- Dalton's law, partial pressure, and the fact that mole fraction equals volume fraction for ideal gases.
- Vapor pressure (Antoine, Cox chart), relative humidity, dew point, molal humidity versus relative saturation.
- Stoichiometry, limiting reactant, percent excess, percent conversion.
- Mean, standard deviation, and sensible rounding.
Traps
- 100 psig is not 100 psi. Add 14.7 psi before any gas-law or relief calculation.
- Degrees Celsius cannot be used in PV = nRT or in ratio equations.
- "Standard" cubic feet is usually defined at 60 °F and 1 atm in US practice, while normal conditions in SI are 0 °C and 1 atm; the 5–6% difference changes answers.
- lbm versus lbf requires dividing by gc = 32.174 lbm·ft/(lbf·s²).
Worked example
Convert 100 psig saturated steam to kPa absolute: (100 + 14.7) psia × 6.895 kPa/psi = 791 kPa.
Rule of thumb: convert everything to one coherent set (kg, m, s, K, kmol, Pa) before substituting, then convert the answer back to the units offered.