Answer :
The formula for calculating density is expressed as
Density = mass/volume
From the information given,
mass of empty pycnometer = 10.2
mass of the same pycnometer with an unknown liquid is 21.8.
Mass of unkown liquid = 21.8 - 10.2 = 11.6
volume of unknown liquid = 7
Thus,
Density = 11.6/7
Density = 1.7 g/ml