Number of water molecules to contribute one molecule to every milliliter on earth How many millilitres of water would it take so that, when they are thoroughly dispersed around the world, it would yield an average of one molecule of the original sample of water in each millilitre of water in the world?  For the sake of argument, assume that the H and O atoms do not dissociate, and the water molecules remain intact.


I calculated it once and got something around 40 ml, but I forgot how I made that calculation.
A very quick Googling tells me that the volume of the oceans is 1.3 billion $\mathrm{km}^3 = 1.3\times10^9\;\mathrm{km}^3 = 1.3\times10^{18}\;\mathrm{m}^3 = 1.3\times10^{24}\;\mathrm{cm}^3 = 1.3\times10^{24}\;\mathrm{mL}$.

To get one molecule into each of those mL, we'd need $1.3\times10^{24}$ molecules of water, or $\frac{1.3\times10^{24}}{N_A} = \frac{1.3\times10^{24}}{6.02\times 10^{23}}$ moles of water, about 2.2 mols.  Since water is 18 g/mol, that comes out to 39 grams of water or about 39 mL.