Thermodynamics Work from pressure

[Logan McEntire]

Homework Statement

The tires on a bicycle require an air pressure of 80 psig. When isothermally pumped up the bicycle tires, the volume of the air (that was originally in the atmosphere) is reduced by a factor of 5.7. Please determine the work that must be done on each lbm of air that is pumped into the tires. Please report your answer in lbf-ft/lbm.
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Homework Equations

W=nRT*ln(V2/V1)
Pv=nRT

The Attempt at a Solution

My attempt might be very wrong, but this is what i tried.
I simplified Pv=nRT to T=PV/nR
Plugged that into W=nRT*ln(V2/V1)
W=PV*ln(V2/V1)
I know the standard pressure (14.7psi) and the pressure inside the tire (80psig+14.7psi=94.7psi), but I don't know the volume of to plug in for the V.

 

[Chestermiller]

Use nRT, with T equal to room temperature.

 

[Logan McEntire]

What do I use for n?

 

[malemdk]

For isothermal compression P1/P2 =V2/V2, instead of using volume, you can use pressure ratio

 

[Chestermiller]

What do I use for n?

You are trying to get the work per pound mass. So you don't need to know n. You are solving for W/n.