Anonymous

# Mass of One Water Molecule?

The answer says the mass of one water molecule is

2.99 X 10^-23 g.

I do not know how to get that answer. I thought that the mass of one water molecule would be the molecular mass of H2O, which is 18 amu.

Clearification is much appreciated. Thanks in advance =]

Relevance

Alright i've gotten a couple thumbs down on this one and i'm not quite sure why, so i'll add a narrative:

they are both correct. 1 amu = 1 g/mol .....amu and g/mol are interchangable.

and we know there are 6.022 * 10^23 molecules in 1 mole.

1 mol = 6.022 x 10^23 molecule

So to change our units from g/mol to grams per molecule, we multiply g/mol by mol/molecule, there is a mol on top and bottom, and they cancel out, and leave you with the answer in grams/molecule

18.0 g/mol * (1 mol) / (6.022 * 10^23 molecules) = 2.99 * 10^-23 g/molecule

• Login to reply the answers
• Molecular Mass Of Water

• Login to reply the answers
• What is the mass of one molecule of H2O?

Step #1 - the molar mass of water is 18.015 g/mol. This was calculated by multiplying the atomic weight of hydrogen (1.008) by two and adding the result to the weight for one oxygen (15.999).

Please remember that you need the molar mass first when trying to find the mass of one molecule.

Step #2 - divide the substance's molar mass by Avogadro's Number.

18.015 grams/mol ÷ 6.022 x 1023 mol¯1 = 2.992 x 10¯23 grams

Note that the final answer has been rounded to four significant figures (from 2.9915 - note use of rounding with five rule) and that the unit of mole cancels.

• Login to reply the answers
• Molecular Weight Of Water

• Login to reply the answers
• Login to reply the answers
• Login to reply the answers