the government would probably distribute geometry files (maybe shapefiles) which would contain the geometry data ... and have attribute files containing the attributes you need. you join them together, and do the calculations if required in the attribute table. If they had population density available, then your task would be one step closer to being complete. Often, you need to take what is available and piece it together to get what you want. All information is available in the help files and on basic tutorials within esri depending on your comfort level in using ArcMap