Question

What is the number of moles in `"36 g"` of `"H"_2"O"` ?

Answer 1

`"2.0 moles"`

Explanation:

The relationship between the number of moles of a given substance present in a sample and the mass of the sample is given by the molar mass of the substance.

For any chemical species, you have

`color(blue)(ul(color(black)("molar mass" \ = \ "the mass of exactly 1 mole")))`

Now, water has a molar mass of `"18.015 g mol"^(-1)`, which means that `1` mole of water has a mass of `"18.015 g"`.

`"18.015 g mol"^(-1) = "18.015 g"/("1 mole H"_2"O")`

So for every `"18.015 g"` of water present in your sample, you have `1` mole of water.

This means that your `"36-g"` sample will contain

`36 color(red)(cancel(color(black)("g"))) * overbrace(("1 mole H"_2"O")/(18.015color(red)(cancel(color(black)("g")))))^(color(blue)("given by the molar mass of H"_2"O")) = color(darkgreen)(ul(color(black)("2.0 moles H"_2"O")))`

The answer must be rounded to two sig figs, the number of sig figs you have for the mass of water.