What is the approximate mass of one water molecule?

1 Answer
Oct 29, 2015

You can determine that by using the molar mass in #"g/mol"# of the water molecule and converting so that you get the absolute mass in #"g"#.

The molar mass is #"MM"_("H"_2"O")" = 18.015 g/mol"#.

If you don't remember that, you can also look it up on Wikipedia, or refer to a periodic table and add up the molar masses of two hydrogen atoms and one oxygen atom.

#"MM"_("H"_2"O") = 2*"1.0079 g/mol H" + "15.999 g/mol O"#

#= "18.015 g/mol H"_2"O"#

Now we just need the relationship between the units #"mol"# and #"number of things in a mol"#.

You may have been taught Avogadro's number, which is #6.0221413xx10^(23) "things/mol"#, and you can use that for the stoichiometric conversion. You multiply by the reciprocal to cancel out the units because the goal for the final units is #"absolute mass"/("molecule H"_2"O")#.

#("18.015 g H"_2"O")/cancel("mol H"_2"O") xx cancel("1 mol H"_2"O")/(6.0221413xx10^(23) "molecules of H"_2"O")#

#= color(blue)(2.9915xx10^(-23) "g/molecule H"_2"O")#