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Midterm Comments General comments: 1) A standard cubic foot (SCF) is NOT a measure of volume. It is a measure of mass. 379 SCF is equal to one lb-mol. It is NEVER correct to convert the volume of gas in some control volume at an arbitrary temperature and pressure into moles using the 379 SCF/lbmol conversion. Example: An annulus of 11.76 ft 3 contains methane at 135 F and 100 psia. What is the mass of methane? WRONG ANSWER: 11.76 ft 3 × 1 lb-mol/379 SCF = 0.031 lb-mol RIGHT ANSWER: 11.76 ft 3 / . One could evaluate the molar volume from the ideal gas law: 2) A pound mass (lb or lbm) is a unit of mass. A pound force (lb-f) is a unit of force. It is NEVER correct to write 1 psi/day = 1 lb of gas per day per in 2 of annulus. HINT: if you have trouble remembering the conversion between lb and lb-f, go straight to SI units. There is never any confusion there. A kg is a unit of mass, and a Newton (kg-m/s 2 ) is a unit of force. Problem 1. Realizing that (d) was true was a bit subtle. Thus failing to circle (d) did not result in a deduction of points. If you did circle (d) and missed other parts of this
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