By definition, there are
6.022
×
10
23
such molecules, or
N
A
such molecules in ONE mole of water. And thus in such a quantity there are
N
A
oxygen atoms, and
2
×
N
A
hydrogen atoms...and the mass associated with this numerical quantity of water molecules is approx.
18
⋅
g
...
And so we simply take the quotient....
Moles of water
=
2.60
×
10
23
⋅
water molecules
6.022
×
10
23
⋅
water molecules
⋅
m
o
l
−
1
=
0.432
⋅
m
o
l
...and thus a mass of
0.432
⋅
m
o
l
×
18.01
⋅
g
⋅
m
o
l
−
1
=
7.78
⋅
g
...
Instead of the mole, we could use the dozen, or the gross, or some other numerical quantity ... it just happens that one mole of hydrogen ATOMS have a mass of
1
⋅
g
more or less precisely
HOPE IT HELPS