chemistry
posted by Navroz .
there are 3 naturally occurring stable isotopes of oxygen:O16,O17, O18. there are 2 naturally occurring stable isotopes of hydrogen: H1, H2.how many different water molecules can be made using the various combination's of all the isotopes of hydrogen and oxygen?

I'm not good at counting these things. You really need a math man to answer this; however, I count 9. Maybe Mathmate, Reiny, or Bob Pursley will do this right.

You need two H, one O
ways: 3*2*2=12 Three choices on O, two choices each H. 
I hate to be dense about this BUT, I can't count but 9.
1 1 16
1 1 17
1 1 18
1 2 16
1 2 17
1 2 18
2 2 16
2 2 17
2 2 18
2 1 16
2 1 17
2 1 18
Which gives 12; however, the last three, considering that they are H2O molecules, are the same as set #2 but reversed. It doesn't make a new molecule of H2O, so I still count only 9.