To a first approximation we have a single molecule of #""^(1)H_2^(16)O#. (The left hand superscripts, as always, refer to the nuclear mass, the isotope. Of course there are some different (so-called) isotopomers, but we really don't have to worry about them given their abundance.)

So for #""^16O# there are #"8 neutrons"#, and for #""^1H# there are NO neutrons. If you don't know how I knew this I would review your knowledge of isotopes.......

So we know the number of neutrons of in one molecule molecule, we just multiply this number out by the molar quantity:

#"Number of neutrons"# #=# #"8 neutrons"xxN_Axx5#, where #N_A="the Avocado Number........"#

And when you do this problem, can you calculate how neutrons in the same quantity of #D_2O#, where #D=""^2H#? #D_2O# is often used in a laboratory.