Does anyone know if we have to calculate the values using significant digits?

If you're looking for buoyancy formulas, then here is a good source:
http://hyperphysics.phy-astr.gsu.edu/hbase/pbuoy.html

Considering the vagueness of the rules, I believe that the best method is to do what the person above said. There are hundreds of books in the library or online for density, buoyancy, concentration, etc.

What would be the best way to differentiate silk from polyester when looking at them under a microscope? Should I just rely on the burn test for these two fibers?

I have a question '~'
To 2.00 L of 0.445 M HCl, you add 3.88 L of a second HCl solution of an unknown concentration. The resulting solution is 0.974 M. Assuming the volumes are additive, calculate the molarity of the second HCl solution.