The formula of density is:
Density = mass (in kg) / volume (in m^3)
So we need to convert 8 mg to kg.
8 mg = 0.000008 kg
So using the density formula
Density = mass / volume
Density = 0.000008 kg / 5.5 m^3
Density = 0.0000014545 kg/m^3
Density = 1.4545 x 10^-6 kg/m^3