<p>In order to calculate the vapor pressure of a substance using the ideal gas equation, you must know the volume the vapor takes up correct?</p>
<p>pv=nrt</p>
<p>p=nrt/v</p>
<p>R is a constant, so you need moles, volume, and absolute temperature to find the vapor pressure. Don't forget to use Raoult's law if collected with other substances.</p>
<p>You would just multiply whatever P you got from ideal gas law by the mole fraction of the substance right?</p>
<p>Yes. And if it asks for moles of X given the other values (including total vapor pressure), then you take the total vapor pressure and subtract the vapor pressure of the other substances from it (usually only water), then plug them in the equation.</p>
<p>Wait, how do you know how many moles of gas are above the solution?</p>
<p>If I have a 5 mol of NaCl and I want to calculate the vapor pressure at 1 atm and 278 K, you would need to use the equation relating enthalpy of vaporization and temperature to vapor pressure.</p>
<p>I think your first response was explaining how to find vapor pressure based on knowing how many moles are in the vapor and the temp/volume. However, my actual question was how to find the vapor P based solely on Temp/volume of substance and mol of substance. Now that I think about it, I think it needs Clausius-Clapeyron equation in which moles of substance/volume have no influence.</p>
<p>And so I guess what you explained to calculated based on knowing mol in vapor and temp/volume is total vap. Pressure. So then subtracting vapor p of solvent and plugging the partial P of the volatile solute into PV= nRT would only solve for moles of solute in the vapor, not moles of solute in the solution. To do that you would need the vapor pressure of the pure solute at the temperature of the problem.</p>
<p>I thought you were talking about vapor pressure in context of collection over a solution. If you just meant a container full of gas. Ideal gas law works fine. It is not 100$ accurate though unless it is an ideal gas (no such thing exists). On a test, you'd use pv=nrt and that's it.</p>
<p>Be careful with the term "vapor pressure". Going back to the original question, you can NOT calculate "vapor pressure" from the ideal gas law. "Vapor Pressure" is the maximum pressure a given substance can produce in a closed container at a given temperature. It varies for every substance and depends on the strength of the interparticle forces. It must be experimentally determined. (Tables of vapor pressures for various substances are available.) If liquid is placed in a container, it may or may not all evaporate. When you plug V,n and T into the ideal gas law and solve for P, the liquid all evaporates if calculated P is less than or equal to Vapor Pressure. If calculated P is greater than Vapor Pressure, the actual pressure in the container will be the Vapor Pressure, and the excess moles will stay in liquid form.</p>