The number of water molecules present in a drop of water weighing 0.9 g is

Sure, here are the steps to calculate the number of water molecules in a drop of water weighing 0.9 g:

1. First, we need to know the molar mass of water (H2O). The molar mass of hydrogen (H) is approximately 1 gram/mole and oxygen (O) is approximately 16 grams/mole. Since water consists of 2 hydrogen atoms and 1 oxygen atom, the molar mass of water is (2*1) + 16 = 18 grams/mole.

2. Next, we need to convert the weight of the water drop from grams to moles. We do this by dividing the weight of the water drop by the molar mass of water. So, 0.9 g / 18 g/mole = 0.05 moles.

3. Finally, we use Avogadro's number, which states that there are approximately 6.022 x 10^23 molecules in one mole. So, the number of water molecules in the water drop is 0.05 moles * 6.022 x 10^23 molecules/mole = 3.011 x 10^22 molecules.

So, there are approximately 3.011 x 10^22 water molecules in a drop of water weighing 0.9 g.