Solve Block Problem: Undefined Variable

I realize this is a rather worn out question and problem, but for the life of me I cannot get it to work. I am trying to find the molar volume using the guess-find model using an equation where all the other variables have already been defined. I have guessed a value, then written the equation out using a Boolean equals sign, and then told Mathcad to find my variable. However, no matter how I have tried rearranging, redefining units, etc. it keeps saying that my variable is undefined. If anyone can help me fix this it would be much appreciated.