I can't remember any of the weights of the individual elements but here is how you solve it:
Molecular weight of copper + nitrogen + 3 oxygens = molecular weight of the compound.
M = moles / liter
.350 moles / 1 liter
Do .350 moles / liter x the molecular weight (g / mole) of the compound = the answer in g / L
Water has a density of around 1 g/mL. The density of HDPE varies depending on exact formulation and processing, but it typically is within the range of 0.93 to 0.97 g/mL. This means that HDPE is less dense compared to water.
