# Homework Help: Ideal gas law numerically

1. Nov 4, 2007

### coolnessitself

[SOLVED] ideal gas law numerically

If I calculate the pressure of water vapor at stp (in mmHg),
P= 1000*7.502e-3*R*(0+273)*.804/18;
where the 7.502e-3 factor converts from Pa to mmHg, 1000 is because my density (.804) is in grams/cm^3, 18 is the molar mass of water, etc etc. and this comes out to be 760mm (1atm), as expected. Now, here's the problem. I've got carbon tetrachloride at it's critical point. Literature shows its T_c = 283.15, density_c=.5576, and pressure_c = 34181 mmHg. However, if I just do
P= 1000*7.502e-3*R*(283.15+273)*.5576/154.01;
I get 125.6 mmHg, not 34181 mmHg. Where did I go wrong? Is it just that the dieal gas law fails, because I've also tried data points that aren't near the critical region and at room temp and neither work..

Last edited: Nov 4, 2007
2. Nov 4, 2007

### learningphysics

Is your density in g/cm^3 or g/L ?

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