# What is the mass, in grams, of 1.24 moles of water, "H"_2"O" ?

Mar 29, 2018

$\text{22.3 g}$

#### Explanation:

Your tool of choice here is the molar mass of water. The molar mass of a compound tells you the mass of exactly $1$ mole of that compound.

$\textcolor{b l u e}{\underline{\textcolor{b l a c k}{\text{the molar mass of H"_2"O" = "the mass of 1 mole of H"_2"O}}}}$

Now, water has a molar mass of ${\text{18.015 g mol}}^{- 1}$. This tells you that $1$ mole of water has a mass of $\text{18.015 g}$. So every time your sample of water has a mass of $\text{18.015 g}$, you can say for a fact that it contains $1$ mole of water.

In your case, the sample contains $1.24$ moles of water. To find the mass of the sample, use the molar mass of water as a conversion factor.

You need to go from moles to grams, so rearrange the molar mass like this

"18.015 g mol"^(-1) => "18.015 g"/("1 mole H"_2"O") " "color(white)((color(blue)(larr \ "what you need"))/(color(blue)(larr \ "what you have"))

So now all you have to do is to multiply the number of moles given to you by this conversion factor.

$1.24 \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{moles H"_2"O"))) * "18.015 g"/(1 color(red)(cancel(color(black)("mole H"_2"O")))) = color(darkgreen)(ul(color(black)("22.3 g}}}}$

The answer is rounded to three sig figs, the number of sig figs you have for the number of moles of water.