Question

How many molecules are there in 39 grams of water?

Answer 1

`"39 g H"_2"O"` contains `1.3xx10^24"molecules H"_2"O"`.

Explanation:

One mole of water has a molar mass of `"18.015 g/mol"` and `6.022xx10^23 "molecules H"_2"O"`. Therefore,

`"1 mol H"_2"O"` `=` `"18.015 g H"_2"O"` `=` `6.022xx10^23"molecules H"_2"1O"`

Multiply the given mass of water times the conversion factors that cancel grams and moles of water, leaving molecules of water, as demonstrated below.

`39cancel("g H"_2"O")xx(1cancel("mol H"_2"O"))/(18.015cancel("g H"_2"O"))xx(6.022xx10^23"molecules H"_2"O")/(1cancel("mol H"_2"O"))=1.3xx10^24"molecules H"_2"O"`