Convert 2.5 grams of water to molecules of water.

A mole of water is 18 g. This is the molar mass of H2O (16 for O plus 2 for 2 H).

So In 2.5 g you have 2.5/18 Moles of water. 2.5/18 * Avogadro constant gives you the number of molecules. This should be 8.37 x 10^22 molecules. Good luck!