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

1 Answer
Jul 31, 2018

Answer:

#"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.