I have temperature- and pressure data from equilibrum between liquid and gas for an unknown compound.
I have approximated the gas to be ideal and now that i have identified the compound, cyclohexane, i want to check if my approximation was justified(if it wasnt i might be wrong in my identification).
I am planning on calculating the molar volume using van der Waals equation and again using ideal gas law for each pair of temperature and pressure, taking the mean, calculating the percentile difference between the two and claiming that the deviation from ideality has the same percentile difference.
Can i do this? Is the quotient between ideal molar volumne and the van der Waals molar volume the same as the deviation from ideality for the gas?
Best Answer
The Van der Waals equation of state contains two constants $a$ and $b$, and for an ideal gas these are zero. So the question you need to answer is whether your experimental data shows these constants to be significantly different from zero.
So you need to perform some form of curve fitting to calculate $a$ and $b$, then determine the statistical error in the calculation. Comparing the statistical error with the calculated values of $a$ and $b$ will tell you if the deviation from ideality is significant.
A convenient double check is provided by this Wikipedia page that lists the values of $a$ and $b$ for a range of gases including cyclohexane.