First, find the molar mass of H2O. It may vary depending on your source. Use the source your teacher gave you.
I got 18.015 g/mol as the molar mass of water.
Then you basically do some math to figure out the amount of moles...
9.15 grams × (1 mole H2O)/(18.015 grams H2O) = .5079
There is approximately .51 moles present.
