Question

What is the approximate mass of one water molecule?

Answer 1

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")`