# Homework Help: Help with finding enthalpy

1. Dec 10, 2004

### stunner5000pt

Vapour pressure of a liquid in the temperature range of 200Kto 260K is given by this expression

ln (p/Torr) = 16.255 - 2501.8(T/K)

Calculate the enthaly of vaporization of hte liquid

since this is the liquid vapour boundary

$$p = p^* e^-\chi$$

and $$\chi = \frac{\Delta H}{R} (1/T - 1/T^*)$$

so then the ratio of Ln p to ln p* would yield the expression for chi which i cna then solve for delta H but it doesnt yield that same answer

what am i doing wrog here can you push (or shove) me iin the right direction

i have an exam today thus i need to answer this once and for all

thank you for help!

2. Dec 10, 2004

### Andrew Mason

First of all it is important to get the question right if you want people to help you. Your expression is unintelligible as it is. The expression must be:

$$ln(P) = 16.255 - \frac{2501.8}{T}$$ where P is the ambient pressure in Torr (mm/hg) and T is in Kelvins. Now it makes sense.

The expression for vapour pressure is given by:

$$ln(P) = constant - \frac{\Delta H_{vap}}{RT}$$ where $\Delta H_{vap}$ is the Heat or Enthalpy of vaporization in J/mol.

From the expression for this gas, it is apparent that:
$$\frac{\Delta H_{vap}}{R} = 2501.8$$

$$\Delta H_{vap} = 20801.1 \text{ J/mol.}$$

AM

Last edited: Dec 10, 2004