In calculations involving the ideal gas law, when must temperature values be expressed in kelvin units?

• A. Never; Celsius can be used in all cases.
• B. Always, because the gas law equations require absolute temperature.
• C. Only when the temperature is below freezing.
• D. Only when calculating pressure.

Answer: (B)

Verified step by step guidance

1. Understand that the ideal gas law is given by the equation \(P V = n R T\), where \(P\) is pressure, \(V\) is volume, \(n\) is the number of moles, \(R\) is the ideal gas constant, and \(T\) is the temperature.
2. Recognize that the temperature \(T\) in the ideal gas law must be in absolute units because the law is derived based on the absolute scale of temperature, which starts at absolute zero.
3. Recall that the Kelvin scale is an absolute temperature scale where 0 K represents absolute zero, the point at which molecular motion theoretically stops.
4. Note that Celsius is a relative temperature scale and can have negative values, which would not make physical sense in the context of the ideal gas law calculations.
5. Therefore, always convert temperature from Celsius to Kelvin using the formula \(T(K) = T(^\circ C) + 273.15\) before using it in the ideal gas law.