HVAC · Thermodynamics · Problem 2

Problem & Solution

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Problem: Thermo 2, what is the enthalpy of one pound mole of 300 degrees steam with a quality of 60%.
Given: 16 pounds per pound mole of oxygen and there's only one of them; 18 pounds per pound mole and we have one pound mole;...
Approach: So first of all, what is a pound mole?
Key formula: formula is written like this
Calc: I think we tend to think of a mole as we remember it from chemistry as six times 10 to the 23rd, almost like a dozen, right?
Calc: There are six times 10 to the 23rd items in a mole.
Result: H equals mass times specific enthalpy.

Office Hours 2

Student questions asked in live office hours about this problem

OH 92: HVAC: Thermo Module #2

Q: When pulling atomic masses from the periodic table (like 16 for oxygen), shouldn't those values be expressed in grams rather than atomic mass units?

A: The periodic table values work in multiple unit systems: they're atomic mass units, but also numerically equal to grams per mole (SI) or pounds per pound mole (US customary)—the key insight is that 'mole' implicitly means 'gram mole' in SI, so you can use the same periodic table numbers directly in whatever unit system your problem requires.

OH 108: HVAC: Thermo Module #2

Q: Can you clarify the difference between pound-mole, pound-mass, and gram-mole, and what problem statement indicators tell me when to use molar versus mass-based approaches?

A: Good question—most people are rusty on this because chemistry class uses SI units and gram-moles implicitly. A mole is just 6.022×10²³ particles; in chemistry we convert moles to grams, but in US customary units used on the PE exam, we need to convert moles to pounds instead, which is where pound-moles come in. The key is recognizing whether a problem gives you information in molar terms or mass terms—that tells you which unit system and approach to use.