And I use #"US gallon"# because even here in the UK we have moved away (thankfully) from the gallon as a unit of volume......(NB #"1 Imperial Gallon"-=4546*mL#). Maybe now that we are leaving the EU, there will be a move to reinstate pounds, shillings, and pence, and tons, and pounds, and gallons, and pints and gills, as the units of measure.
And if there are #2xx10^6*"hydrogen atoms"#, there are #1xx10^6# individual water molecules.....
And thus there are........
#(1xx10^6*"water molecules")/(6.022xx10^23*"water molecules"*mol^-1)xx18.01*g*mol^-1=#
#3.00xx10^-17*g-=3.00xx10^-17*mL-=7.90xx10^-21*"US gallons."#
This measurement is almost meaningless, and it is really only an exercise.
