You had two mistakes in the expression for Temp.t: a missing bracket at the end, and an incorrect expression involving the H2O term in the top line. These are fixed in the attached.
However, I have to say that you have chosen a ridiculous way to construct Temp.t! It's no wonder there were some mistakes. Break this sort of expression into smaller, bite-size chunks, using more function definitions before the solve block, so that it is easier to see what is happening and easier to debug.
Alan
Found a third error (in the denominator of Temp.t). Corrected file attached.
Version c is a simplified (though still pretty complicated!) version that is equivalent to version b, but is easier to debug (I think).
However, I think there must be something wrong with your basic equations for dTemp/dt, as it produces some silly temperatures.
Sometimes when I run either of these worksheets I get a "repeated singularity" error, but simply re-calculating the worksheet usually gets rid of it!!!
