How can I fix this error?

Hello, I want to find a ksi value for the K values I obtained from the T values, but when I run the code even though my K values are there vpasolve gives me this message: "Empty sym: 0-by-1" I would really appreciate if you can guide me. Thanks.

clc;

clear;

nN2 = 1; %starting mol

nH2 = 3; %starting mol

nNH3 = 0; %starting mol

R = 8.314;%J/molK

deltaGstd = -33; %kJ/mol

deltaHstd = -92.2; %kJ/mol

deltaSstd = -0.1987; %(deltaGstd - deltaHstd)/(-T)

syms ksi

T = 298:1:434; %T values

K = exp(((-1).*deltaHstd.*1000 + T.*deltaSstd.*1000)./(R.*T)) %K values

eqn =((4.*ksi.^2)./((4-2.*ksi).^2))./(((1-ksi)./(4-2.*ksi)).*(((3-3.*ksi)./(4-2.*ksi)).^3))==K

S = vpasolve(eqn,ksi)

[Rik]

Are you sure there is a solution and you didn't make a typo?

[Houssem]

I think you can make a loop on T

Pleas try this code

clc;

clear;

nN2 = 1; %starting mol

nH2 = 3; %starting mol

nNH3 = 0; %starting mol

R = 8.314;%J/molK

deltaGstd = -33; %kJ/mol

deltaHstd = -92.2; %kJ/mol

deltaSstd = -0.1987; %(deltaGstd - deltaHstd)/(-T)

syms ksi

for T = 298:1:434; %T values

K = exp(((-1).*deltaHstd.*1000 + T.*deltaSstd.*1000)./(R.*T)); %K values

eqn =((4.*ksi.^2)./((4-2.*ksi).^2))./(((1-ksi)./(4-2.*ksi)).*(((3-3.*ksi)./(4-2.*ksi)).^3))==K;

S = vpasolve(eqn,ksi)

end

[asli eylul sert]

When I run this code it gives me K and S values only for my last T value. It evaluates it in the Command Window but I only see one value for K and S in the Workspace. Any hint on how to correct it?

Answer 1

[Walter Roberson]

Your T is a vector which leads to eqn being a vector. When you vpasolve a vector, it tries to find a combination of variables that solves all of the elements simultaneously. solve() and vpasolve() are simultaneous equation solvers, not solvers of each equation independently.

Houssem is correct that one way of solving this problem is to loop over the T values. The particular way that they looped does not store the individual results, but it is certainly possible to modify the code slightly to store the values.

Another approach is not to loop, to allow the vector eqn to be created, but then to use

S = arrayfun(@vpasolve, eqn, 'uniform', 0)

This will produce a cell array of outputs. I recommend a cell array output unless you are certain that mathematically there will definitely be a solution for each entry.

[asli eylul sert]

It seems to be working out, thank you. Although, I have a follow up question. I want my solutions to be in the [0 1] range. I was able to do it in the vpasolve but how am I gonna arrange now, am I able to do that in arrayfun?

[Walter Roberson]

S = arrayfun(@(X)vpasolve(X,[0 1]), eqn, 'uniform', 0)

[asli eylul sert]

Thank you so much.