Determine # of molecules in an unknown sample How might one determine the # of molecules in an unknown sample? If the sample is a gas,  we might use $PV=nRT$ to estimate # of molecules. If the sample is a solid or liquid and the chemical formula is known, we simply need to measure the mass. But can we determine the # of molecules without first determining the chemical formula?
There are attempts to redefine the mol through measuring spheres of monocrystalline silicon.  If you have the macroscopic dimension of your sample and the dimensions of your unit cell (from x-ray diffraction) you know how many atoms if silicon are there.