Final answer:
The average volume of a water molecule is calculated using the mass of water and the molar mass of water to find the number of moles, and then dividing the volume of one mole by Avogadro's number, resulting in approximately 2.99 x 10^-23 mL per molecule.
Step-by-step explanation:
To calculate the average volume of a water molecule, we'll use the given mass of water and the molar mass of H2O to determine the number of moles, and then find the volume of one mole of water molecules. The mass of 18.0 mL of water is given as 18.0 g. According to the molar mass of water, which is (1.0079 + 1.0079 + 15.999) = 18.015 g/mol, we can calculate moles using the following conversion:
1 mole of water / 18.015 g = mass of water in grams / 18.015 g/mol
By substituting the mass of the water, we find that there is 1 mole of water in 18.0 g, since the mass of the water is equal to its molar mass. Since 1 mole of any substance contains Avogadro's number of molecules (approximately 6.022 x 1023 molecules), the volume occupied by these many water molecules (one mole) is 18.0 mL. Therefore, to find the average volume of one water molecule, we divide the volume of one mole by Avogadro's number:
18.0 mL / 6.022 x 1023 molecules = 2.99 x 10-23 mL/molecule
The average volume of a water molecule is thus approximately 2.99 x 10-23 mL.