How to calculate the enthalpy of vaporisation?

According to Wiki, the heat of vaporisation of methanol is +35.28 kJ/mol. Let’s say I put some methanol in a flask, and measured the pressure at various temperatures. For example, at 19.9ºC, the total pressure is 113.49 kPa and the air pressure is 104.49 kPa, so the vapour pressure of the methanol is 9.07 kPa. At 52.8ºC, the vapour pressure is 40.79 kPa.

How do I approach the problem of finding the enthalpy of vaporisation? That is, how do I find out the value of ∆H[sub]vap[/sub]?

By using the Clausius-Clapeyron equation. This sounds a lot like a homework question.

The Clausius-Clapeyron equation has not been covered. I gather that:

∆H[sub]vap[/sub] = -RT(ln§ - C)

But in order to use it, I need to find the Y-intercept using an Excel graph. In that case, I’m SOL because I have no idea how to make the required graph in Excel. Two sets of data, for methanol and ethanol, need to be put on a single graph. I can only make two separate graphs. (And forget about putting axis labels on them!) Then the combined graph needs to be converted to a linear graph using the C-C equation, and I don’t know how to do that. I use Excel every (working) day, but never for graphing. Nobody I know makes graphs.

So I’m faced with using a function I don’t know how to use, using an equation we haven’t been taught. I’m trying to find a way to do it without having to graph it.

Try graphing lnP versus 1/T.

Yes, that’s the plan. ln§ would be the y-axis, and 1/T would be the x-axis. But I don’t know how to do that. First, I have to get both sets of data onto one graph. Don’t know how to do that. Then I have to enter formulas for the axes. Don’t know how to do that either. I don’t even know how to label them. As I said, nobody I know makes graphs in Excel.

it is always good to practice your solution methods and to check them. this is what you did in grade school and high school. this is still useful and necessary.

take a simple algebra equation. solve it and get some numbers. plot it on paper. then do that in your spreadsheet.