By definition a mole of stuff SPECIFIES #6.022xx10^23# individual items of that stuff. Why should we use such an absurdly large number? Well, it turns out that #6.022xx10^23# individual #""^1H# atoms have a mass of #1.00*g# precisely. The mole is thus the link between the submicro world of atoms and molecules, to the macro world of grams, kilograms, and litres.

We also conveniently use the symbol #N_A#, where #N_A-=6.022xx10^23#. And we can use the #"mole"# as we would any other collective number, i.e. #"dozen"#, #"Bakers' dozen"#, #"score"#, #"gross......"#

And so we have #18*mol# of #H_2O#, which is equivalent to #3xx18xx6.022xx10^23# individual atoms. What is the mass of this quantity of given atoms?