Below are five properties, with an example of the units in which that property could be expressed. Which units are wrong for the property?
A. Heat capacity, in bar•m3/mol•K
B. Enthalpy, in kJ/mol
C. Specific internal energy, in kJ/kg
D. Velocity, in cm/day
E. The parameter b in the van der Waals equation, in m3/mol
A. Incorrect. Heat capacity has the same units as the gas constant (remember that for ideal monatomic gases CV* = (3/2)R), and R can be expressed as pressure-volume per amount of substance-absolute temperature.
B. Correct. Enthalpy takes units of energy. MOLAR enthalpy is expressed as energy per amount of substance, shown here.
C. Incorrect. Internal energy takes units of energy, so specific internal energy must have units of energy per mass. Recall that “specific” quantities are expressed as being per unit mass.
D. Incorrect. Velocity is distance per time. While very slow, a snail’s velocity could be correctly given in cm/day.
E. Incorrect. Excluded space must have units of volume per amount of substance. See Example 2-6 to review the van der Waals equation and parameters.
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