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Suppose you have a gaseous mixture of nitrogen and pure steam (no liquid water), at temperature $T$ and net pressure $P$. The mole fraction of steam in the mixture is $x$. You are told that the steam is saturated.

I want to find temperature $T$ of this mixture.

I know I can find saturation temperature of water knowing saturation pressure of water. How do I know what is the saturation pressure? More specifically, is the saturation pressure of the steam equal to the partial pressure of the steam, $xP$, or the net pressure of the mixture, $P$?

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If you know that the mixture is at thermal equilibrium (which is a good bet), calculate the partial pressure of the steam from Raoult's Law (https://en.wikipedia.org/wiki/Raoult%27s_law). Then, go to a saturated steam table and find the temperature that corresponds to this pressure.

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  • $\begingroup$ Thank you! as a quick side question, if I wanted to evaluate the enthalpy of this steam, I would still evaluate it at the saturation pressure and temperature, right? $\endgroup$ – user207526 Aug 12 '19 at 22:39
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    $\begingroup$ Yes, and the saturated steam table should have a value for enthalpy at the temperature and partial pressure that you used. Note that the nitrogen also has an enthalpy at that temperature and its own partial pressure, so if you are doing a heat balance, be sure to include it. $\endgroup$ – David White Aug 12 '19 at 23:21

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