= Water mass to molecular count
Choose approximate <atomic weights> $A_{\rm r}(\mathrm H)=1.008$ and $A_{\rm r}(\mathrm O)=15.999$, and use $M_{\rm u}\approx1\,\mathrm{g\,mol^{-1}}$. The <chemical formula> $\mathrm{H_2O}$ then gives <water> <molar mass>
$$
M(\mathrm{H_2O})\approx\bigl(2(1.008)+15.999\bigr)\,\mathrm{g\,mol^{-1}}=18.015\,\mathrm{g\,mol^{-1}}.
$$
These are deliberately chosen rounded inputs, not exact <atomic weights> or a universal value for every <water> sample. Different <isotopic abundances>, notably those in <heavy water>, change the mean <molar mass>; <chemical formula> alone does not specify them. Adding <atomic weights> also neglects the tiny <mass> change associated with <chemical bond> <energies>, immaterial at this rounding precision.
For an illustrative pure <water> sample of <mass> $4.50375\,\mathrm g$, this convention gives
$$
n\approx\frac{4.50375\,\mathrm g}{18.015\,\mathrm{g\,mol^{-1}}}=0.250000\,\mathrm{mol},\qquad N\approx1.50553519\times10^{23}.
$$
The count refers to <water> <molecules>. Their <hydrogen> <chemical atom> count is approximately $2N=3.01107038\times10^{23}$, and their <oxygen> <chemical atom> count is approximately $N$. The mean <mass> per <water> <molecule> is $M/N_{\mathrm A}\approx2.99146\times10^{-26}\,\mathrm{kg}$. Extra displayed digits expose the arithmetic; they do not imply that the sample composition or <mass> was measured that precisely.
\Image[/molar-mass-water-particle-map.png]
{title=Mapping <water> <mass> through <amount of substance> to <molecule> and <chemical atom> counts using chosen approximate <atomic weights>}
{id=molar-mass-water-particle-map}
The same conversion makes <stoichiometry> a <mass> calculation. For the <chemical equation> $2\mathrm{H_2}+\mathrm{O_2}\to2\mathrm{H_2O}$, complete conversion of $0.250000\,\mathrm{mol}$ of <molecular hydrogen> and $0.125000\,\mathrm{mol}$ of <molecular oxygen> gives $0.250000\,\mathrm{mol}$ of <water>. With the chosen approximate <atomic weights>, the reactant <masses> are $0.50400\,\mathrm g$ and $3.99975\,\mathrm g$, summing to $4.50375\,\mathrm g$. The $2:1:2$ ratio belongs to <molecule> counts and <amount of substance>, not to <masses>; this bookkeeping neglects the much smaller reaction <energy> contribution to <mass>.
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