Thermo help?

Question

1. Assume air behaves as an ideal gas. A mass of air enters a compressor at p1=1 bar and t1=200k, and leaves at p2=6bar and t2=600K. The ratio V2/V1 is nearest:

the answer is 1:3

2. Nitrogen gas at 151 K has a pressure=102 bar. For nitrogen assume pc=34 bar and tc=126.2K. Determine Z from the compressibility chart.

3. A mass of H2O is at 100 bar and a saturation temperature of 311 deg C. Assume pc=220.55 bar and Tc=647.15K. Use either steam tbales or the compressibility chart to determine values of the compressibility factor (z) for (a) saturated liquid, (b) saturated vapor. The molecular weight of H2O is 18.015 kg/kmol.

4. An automobile tire has a volume of 0.05m^3 when inflated to a pressure of 125 kPa(abs) at 25 deg C. Calculate the additional mass of air at 25 deg C needed to increase the pressure in the tire to 250 kPa (abs), assuming the volume of the tire is unchanged.

thanks so much! this is for my final and i missed these..if you could just show me how to set them up.

Answer 1

When you took chemistry, you had Boyle's Law, Charles' Law, and g*y-Lussac's Law.  These are: P1V1 = P2VT; V1/T1 = V2/T3; and P1/T1 = P2/T2 respectively.

These are combined together with Avagardro's law to make the combined gas law dor ideal gasses:

PV = mRT

Using it twice and dividing through, this can be written as P1V1/T1 = P2V2/T2

Problem 1: Use this directly, solve for V2/V1. Use it again for Problem 4, which means you are doubling the air in the tire. So, use PV=mRT for air (look up R) to determine mass of air.

For gases, PV = ZmRT, where Z=1 for ideal gases, and less than 1 for non-ideal. This is used for problems 2&3, but I don't have the compressibility charts/steam tables.