# Question #52fdb

##### 1 Answer

Just randomly take one trillion water molecules. (These molecules will weigh as few as 30 picograms)

Water molecules with Oxygen-18 atoms will be present in the 0.20% of one trillion, that is 2 billions water molecules, with 4 billions hydrogen atoms.

The probability that there is a deuterium atom ** D** is 0,015% that is 0.015/100 = 0.00015. Four billions multiplied by 0.00015 gives only six hundred thousands of water molecules with one

**atom on average (a few molecules may have two, and some others none).**

*D*Now, in those six hundred thousands molecules, having on average one ** D** atom, the probability that there is a second

**is independent by the presence or not of a "twin**

*D**D*" in the molecule and it is - once again - 0.00015. Therefore, by multiplying 600,000 molecules by this probability we get 90 molecules of

Finally, let's scale this number to a mole, that is one ** Avogadro's number** of molecules, roughly

Given that there were 90