What is the density of one gram of pure water?

The density of 1 gram of pure water is the same as the density of 1 ton of pure water, or of a gallon, or of a reservoir full of it!

The fact that you haven't framed a sensible questions shows you are not thinking.

You probably are trying to find the volume of 1 gram of pure water? Or are you just after the density of pure water?

The density of pure water is 1g/cc at 4 deg. C, hence 1 gram of pure water has a volume of one cubic centimeter at that temp.

Geddit?