Find the Mass of Air in the Room and the Air's Weight

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SUMMARY

The discussion focuses on calculating the mass and weight of air in a room measuring 8 ft x 9 ft x 12 ft at 80°F and 1 atm. The initial calculation incorrectly assumes the mass of air equals the volume of the room, leading to a mass of 864 ft³. The correct approach involves using the ideal gas law, where the density of air is derived from the equation dair = 1 atm / (53.35 ft lb/lbm °R)(540.67°R), resulting in a density of 3.4668x10-5 atm lbm/ft lbf. The participants highlight the importance of unit consistency and the correct application of the gas constant R.

PREREQUISITES
  • Understanding of the ideal gas law
  • Familiarity with unit conversions, particularly between Fahrenheit and Rankine
  • Knowledge of density calculations in fluid mechanics
  • Basic principles of thermodynamics
NEXT STEPS
  • Study the ideal gas law and its applications in real-world scenarios
  • Learn about unit conversions between different temperature scales, specifically Fahrenheit to Rankine
  • Explore density calculations for gases under varying conditions
  • Investigate the significance of the gas constant R in thermodynamic equations
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This discussion is beneficial for students in physics or engineering, particularly those studying thermodynamics and fluid mechanics, as well as educators looking for practical examples of gas law applications.

Northbysouth
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Homework Statement



The air in a room measuring 8 ft x 9 ft x 12 ft is at 80°F and 1 atm. Determine the mass of the air, in lb, and its weight, in lbf, if g = 32.0 ft/s2

Homework Equations


1 °F = 460.86°R

dair = mair/vair


The Attempt at a Solution



The air has the same mass as the room, thus mair = 8x9x12

mair = 864 ft3

dair = mair/vair

dair = ρ/RT

dair = 1 atm/(53.35 ft lb/lbm °R)(540.67°R)

dair = 3.4668x10-5 atm lbm/ft lbf

My density seems kinda low. What am I missing? Is it the gc?
 
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You should check your units, especially for R.

A density unit of atm lbm/ ft-lbf makes no sense.
 
Wrong question.
 
Last edited:

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