Question

relative density of mercury is 13.8 what is its density

Answer 1

It is defined as the ratio of the density of the
substance to the density of water at 4°C.
The density of mercury can be obtained from the expression
RD = density of mercury/density of water , hence
Density of mercury = RD x density of water = 13.6 x 1000 = 13.6 x 103 kg m-3