What unit of temperature should be used when applying the ideal gas law?

1. Analyze the question The question asks which temperature unit must be used in the ideal gas law, PV = nRT.

2. Explain the concept The ideal gas law relates pressure (P), volume (V), amount of gas in moles (n), and absolute temperature (T):

PV = nRT

Here R is the gas constant (8.314 J/mol·K, or 0.0821 L·atm/mol·K). Its units contain kelvin, so T must be in kelvin. Kelvin is an absolute scale: 0 K is absolute zero, where a gas would have zero thermal energy. The volume of an ideal gas is directly proportional to absolute temperature (Charles's law), so temperature must start from true zero.

3. Step-by-step reasoning

• Celsius and Fahrenheit can be zero or negative (for example, 0 °C is the freezing point of water, not zero energy). Using them could give zero or negative volumes or pressures, which makes no physical sense.
• Rankine is also an absolute scale, but it is not the SI unit and does not match the standard values of R used in exams.
• Kelvin is the SI unit of temperature and matches the units of R.

To convert from Celsius: T(K) = T(°C) + 273

Example: 27 °C = 27 + 273 = 300 K

4. Answer The correct option is B. Kelvin (K).

5. Exam tip Before substituting into any gas law (Boyle's, Charles's, the combined gas law, or PV = nRT), convert temperature to kelvin first by adding 273. Many students lose marks by forgetting this step, and JAMB often includes the Celsius value as a distractor.