Asha V.

asked • 08/18/17

chemistry problem

The natural abundance of 2H is 0.015% and that of 18O is 0.20%. How many 2H218O molecules are there in 1.0 moles of water?

Arturo O.

With the information given in the problem statement, you can compute the abundances of 18O and 2H in the water.  However, you can only compute the probability of the number of 2H218O molecules, as Matthew explained in his solution.  You do not know how the isotopes will be distributed among several possible combinations.  You could have combinations like:
 
1H216O
1H2H16O
2H216O
2H218O
.
.
.
etc.
 
As you can see, there are several possibilities, and there is no guarantee that the heavy isotope of H will necessarily combine only with the heavy isotope of O to form 2H18O. 
 
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08/18/17

Arturo O.

Allow me to fix a typo.  The last sentence above should end with 2H218O.
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08/18/17

1 Expert Answer

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Matthew S. answered • 08/18/17

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It's sort of like a probability problem. There's 0.015% chance each hydrogen is deuterium and 0.2% that the oxygen is 18O. 
 
So the probability of getting 2H18O is 0.015%*0.015%*0.2% or (0.00015)*(0.00015)*(0.002)
 
Then multiply the abundance of 2H18O  by Avogadro's number to get the number for one mole.
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