# Determination of constants

• Nov 13th 2006, 06:56 PM
jturn14
Determination of constants
Hi, i'm having this problem with an engineering formula I have to do something with.

First, let me explain what it is. the experiment is to experimentally determine the latent heat of vaporization for water between 50 C and 100 C in a depressurized vapor (so only the vapor pressure affects it). part of the analysis is to find constant relating the heat of vaporization (h(fg)) to the temperature.

To do this, I use an integrated form of the clapeyron equation with h(fg) represented as A+BT, where T is temperature and A and B are the constants. Also, I use a linear regression giving ln(P) = f(T), (the integrated form of the Clapeyron equation.

Basically, what I have is this:

ln(P) = (1/R)*(B*ln(T) - (A/T)), where R is the universal gas constant, P is the pressure, T is the temperature, and A and B are the constants in question.

Also, i have, from the regression, ln(P) =0.0403*T - 10.414. I can also do a logarithmic regression, giving me ln(P) = 14.032*ln(T) -78.508. Somehow, I need to use these two (or one of these) equations to find A and B in terms of R, P, and T.

Any help would be GREAT (the TA wasn't very useful).
• Nov 14th 2006, 04:14 AM
topsquark
Quote:

Originally Posted by jturn14
Hi, i'm having this problem with an engineering formula I have to do something with.

First, let me explain what it is. the experiment is to experimentally determine the latent heat of vaporization for water between 50 C and 100 C in a depressurized vapor (so only the vapor pressure affects it). part of the analysis is to find constant relating the heat of vaporization (h(fg)) to the temperature.

To do this, I use an integrated form of the clapeyron equation with h(fg) represented as A+BT, where T is temperature and A and B are the constants. Also, I use a linear regression giving ln(P) = f(T), (the integrated form of the Clapeyron equation.

Basically, what I have is this:

ln(P) = (1/R)*(B*ln(T) - (A/T)), where R is the universal gas constant, P is the pressure, T is the temperature, and A and B are the constants in question.

Also, i have, from the regression, ln(P) =0.0403*T - 10.414. I can also do a logarithmic regression, giving me ln(P) = 14.032*ln(T) -78.508. Somehow, I need to use these two (or one of these) equations to find A and B in terms of R, P, and T.

Any help would be GREAT (the TA wasn't very useful).

Using your ln-ln regression and comparing coefficients with the Clapeyron equation:
$\displaystyle \frac{1}{R}B = 14.032$
$\displaystyle -\frac{1}{R} \frac{A}{T} = -78.508$ (For some specific T. See below.)

You know the value for R, so you can get B from the first equation. For the second part, what is your intercept value for T? That is at what temperature T on the ln-ln plot is ln(P) = -78.508? It's T = 1 of course. So the second equation will read:
$\displaystyle -\frac{1}{R} \frac{A}{1} = -78.508$

You can now use this to find A.

-Dan