How much does a cubic foot of air weigh in pounds?

About 0.075 lbs. Caculated using the ideal gas law: PV=nRT. Assuming the T=68F (528R) and P=1atm with the MW of air approx. 28.8 lb/lb-mole. The gas constant R, using these units is 0.7302. Rearranging the formula to give: mass = MW*(PV/RT) Hope this helps.
If measured at sea level it's 25,401.6 lbs per square foot. That is using the number of square inches in a cube 1,728.