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

Jul 31, 2018

$\text{2.0 moles}$

#### 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

$\textcolor{b l u e}{\underline{\textcolor{b l a c k}{\text{molar mass" \ = \ "the mass of exactly 1 mole}}}}$

Now, water has a molar mass of ${\text{18.015 g mol}}^{- 1}$, which means that $1$ mole of water has a mass of $\text{18.015 g}$.

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

So for every $\text{18.015 g}$ of water present in your sample, you have $1$ mole of water.

This means that your $\text{36-g}$ sample will contain

$36 \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{g"))) * overbrace(("1 mole H"_2"O")/(18.015color(red)(cancel(color(black)("g")))))^(color(blue)("given by the molar mass of H"_2"O")) = color(darkgreen)(ul(color(black)("2.0 moles H"_2"O}}}}$

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