Sorry for posting this again, I don't know if the question was skipped or not, but I was just wondering if I could get some help on this... thanks.

There are 3 known isotopes of Hydrogen and 3 stable isotopes of oxygen. How many types of water molecules would these produce?

Order wouldn't matter then.. right? So then it would be a combination instead of permutation?

This problem is one for statistics. Permutations and combinations.
But look at it this way. It won't get the answer but there are a LOT of different combinations. I don't know the formula but someone surely will post it for you.
H H H O O O H H H
1 2 3 1 2 3 1 2 3

Since there are two H atoms, we can have something like this,
1,1,1 which means H1O16H1

1,1,2
1,1,3
1,2,1
1,2,2
1,2,3
1,3,1
1,3,2
1,3,3
etc. You get the idea.

H2O16H1 is the same as H1O16H2 because the H atoms are equivalent.

To find the number of types of water molecules that can be formed from the 3 known isotopes of hydrogen and 3 stable isotopes of oxygen, we need to consider combinations rather than permutations.

The reason we use combinations is that the order of the isotopes doesn't matter in this case. In other words, H2O16H1 is the same as H1O16H2 because the hydrogen atoms are equivalent.

To calculate the number of combinations, we can use the combination formula:

C(n, r) = n! / (r! * (n - r)!)

In this case, we want to find the number of combinations of 3 hydrogen isotopes taken 2 at a time, multiplied by the number of combinations of 3 oxygen isotopes taken 1 at a time.

Number of combinations = C(3, 2) * C(3, 1) = (3! / (2! * (3 - 2)!)) * (3! / (1! * (3 - 1)!)) = 3 * 3 = 9

Therefore, there are 9 types of water molecules that can be formed from the given isotopes of hydrogen and oxygen.