A sample of water has a mass of 10.0 grams. What is the number of molecules of water in the sample? (H = 1 u, O = 16 u, and 1 u = 1.66 × 10−27 kg.)

To find the number of molecules in the 10.0 grams of water, we need to follow these steps:

1. First, we need to find the molar mass of water (H2O). The molar mass of an element is its atomic weight in grams per mole (g/mol). For hydrogen (H), it's approximately 1 gram per mole (g/mol), and for oxygen (O), it's 16 g/mol. Since water consists of 2 hydrogen atoms and 1 oxygen atom, its molar mass is (2*1) + 16 = 18 g/mol.

2. Next, we need to find the number of moles in our 10.0 gram sample. We do this by dividing the mass of the sample by the molar mass of water. So, 10.0 grams / 18 g/mol = 0.56 moles.

3. Finally, we use Avogadro's number, which states that there are approximately 6.022 x 10^23 molecules in one mole. So, 0.56 moles * 6.022 x 10^23 molecules/mole = 3.36 x 10^23 molecules.

So, there are approximately 3.36 x 10^23 molecules of water in the 10.0 gram sample.