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.
