This question is incomplete so because of unavailability of proper information i will simply let you understand this by an example:
lets say you have 4.2 moles of methanol in 5.67 L.
Firstly convert the moles into mass
mass = moles x molar mass of methanol
mass = 4.2 moles x 32.05 g/mol = 134.61 g
Density = mass / vol
convert 5.67L into mL which is 5670mL
putting values
134.61 / 5670 mL = 0.0237 g/mL
hope you get it!