Not quite. There are a couple of ways to do it, but to get the mass in grams, this is a good one.
You have 12 molecules, you need to see how many moles that it (it is much much less than 1 mole), so you DIVIDE by avogradro's number:
12 molecules * (1 mole/6.022x10^23 molecules)= 1.99x10^-23 moles
Now multiply by the molar mass
1.99x10^-23 moles * (17g/1 mole)= 3.38x10^-22 grams
