Air (79% N2 and 21% O2 by volume) is compressed isothermally at 500 K from 4 MPa to 8 MPa in a steady-flow compressor at a rate of 5 kg/s. Assuming no irreversibilities, determine the power input to the compressor. Treat air as a mixture of (a) perfect gases, (b) ideal gases and (c) real gases.