room temperature
Summary
Room temperature is the ambient lab temperature, roughly 20–25 °C. When no number is given, the MCAT usually means 25 °C (about 298 K), the temperature at which standard data are commonly tabulated. Convert to kelvin before any gas-law or thermodynamic calculation.
Detail
Meaning: Room temperature is the ambient temperature of a laboratory, roughly 20–25 °C. When a passage gives no number, the MCAT usually means 25 °C, or about 298 K, the temperature at which standard thermodynamic data and equilibrium constants (Ka, Ksp, Kw) are commonly tabulated. It is an approximation, not a strict definition.
Equation: T = t + 273.15, where T is absolute temperature in kelvins (K) and t is the temperature in degrees Celsius (°C). So 25 °C ≈ 298 K.
On the MCAT: Use about 298 K in PV = nRT and in ΔG = ΔH − TΔS. Kw = 1.0 × 10⁻¹⁴ and pH + pOH = 14 at 25 °C. A passage may ask what changes when a reaction is run above or below room temperature. Rates generally rise as temperature rises, and K shifts in the direction set by the sign of ΔH (K increases with T for an endothermic reaction and decreases for an exothermic one).
Common confusion: Room temperature is not STP. STP is 0 °C (273 K) and 1 atm, where one mole of ideal gas occupies 22.4 L. Standard conditions for thermodynamic tables use 25 °C. Neutral pH is 7 only near 25 °C, because Kw changes with temperature. Gas-law and thermodynamic calculations need kelvin, not Celsius.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.