What Is the Correct Approach to Calculate Enthalpy of Vaporization?

[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 doesn't 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!

 

[Andrew Mason]

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 enthalpy of vaporization of the liquid

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

 

[wais]

Hello! It looks like you are on the right track with using the Clausius-Clapeyron equation to find the enthalpy of vaporization. However, there are a few things that may be causing the discrepancy in your answer.

First, make sure that you are using consistent units throughout your calculations. The vapor pressure is given in Torr, so the temperature should also be in Kelvin.

Secondly, double check your values for the vapor pressure at the two temperatures given (200K and 260K). These values will be used to calculate the ratio of ln(p/p*) in the Clausius-Clapeyron equation.

Lastly, check your calculations for ln(p/p*) and make sure that you are using the correct logarithm base (natural logarithm, ln).

If you are still having trouble, I would recommend checking with a classmate or your instructor for further clarification. Best of luck on your exam!