You are trying to find the number of moles (n) of air that occupy a volume of 500.0 ml at a temperature of 25ºC and pressure of 102.0 kPa. This can be solved using the ideal gas law:

PV = nRT

P = pressure = 102.0 kPa

V = volume = 500.0 ml = X (1 L/1000 ml) = 0.5000 L

T = (25ºC + 273) = 298 K

R = 8.3145 L-kPa/mol-K

n =PV/RT=(102.0 kPa)(0.5000L)/(8.3145 L-kPa/mol-K)(298 K)=**0.02058
moles**

As you can see from the units of the ideal gas constant, it was necessary to convert volume to liters and temperature to Kelvins. Temperature must always be in Kelvins for gas law calculations because the Kelvin temperature scale is a proportional scale with a true zero point and no negative values. In addition, a value of R must be used that's consistent with the pressure units given in the problem.

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