very simple i have model 
and basically i want the entire model to have its on custom properties such as mass and density 
but i dont know how to have an entire model have the same mass and/or density as all the similar models 
also how do i control the center of mass of model 
any help is super appreciated 
Is it a model, or a mesh? Because if it is a model with different parts inside of it, just go into each individual part and change the mass, density, etc.
As to your issue with the center of mass, create a primary part inside of your model where you want the center to be, then set the mass of that part to be larger than all the other parts around it.
yes but each model has different shape and amount of parts, so i need one full model lets say to be at a density of like 50, how would i make the model by density of just 50 when it has so many parts and stuff and i need to do it by script so i can do that mutiple times over many models
You would have to mention the different parts inside of your model, and make the density total 50 by applying different density amount to each part.