Thermo...any help is welcomed?

Question

You come out and find that you have a car tire that is partially flat but with fixed volume of 11.4 ft3. You check the pressure and find it is 20 psi and the temperature is 80 oF. How much air must you add to the tire to raise the pressure to 35 psi? Solve for both mass and volume

Answer 1

Looks like an application of the ideal gas law...

PV=nRT

Pi= 20 psi

Pf= 35 psi

Vi=Vf=11.4 ft3

Ti=Tf=80 F + 459.7=539.7 R

R= 10.73 ft3-psi/ lb-mol- deg R

Calculate ni PiVi/RTi

ni=20 * 11.4 /10.73 * 539.7

ni=0.039 lb-mol

calculate nf= PfVf/RTf= 35 * 11.4 / 10.73 *539.7

nf=0.069 lb mol

amount of air added= 0.069 - 0.039= 0.03 lb mol

formula weight for air is 29 lb/lb-mol

Mass of air added= 0.03 lb-mol * 29 lb/lbmol=0.89 lbs

Assume you want the volume at Standard conditions:

STP 60F and 14.6 psi. Volume= 379.5 ft3/lbmol

0.03 lb mol * 379.5 ft3/lbmol=11.4 ft3 added to the tire