# How many water molecules are in 4.0 moles of water?

##### 1 Answer

#### Explanation:

All that you need to know here is that in order for a given sample of water to contain exactly **mole** of water, it must contain **molecules** of water.

This is known as **Avogadro's constant** and essentially acts as the definition of a mole. If you have **mole** of water.

#color(white)(overbrace(color(blue)(ul(color(black)("1 mole H"_2"O" = 6.022 * 10^(23) quad "molecules H"_2"O"))))^(color(red)(ul("Avogadro's constant")))#

Now, you know that your sample contains **moles** of water. In order to find the number of molecules of water it contains, you can use Avogadro's constant as a **conversion factor**.

You start with *moles* and you want to find the *number of molecules*, so set up the conversion factor like this.

#(6.022 * 10^(23) quad "molecules H"_2"O")/("1 mole H"_2"O")" "color(white)( (color(blue)(larr " what you need"))/(color(blue)(larr " what you have"))#

Finally, multiply the number of moles of water by the conversion factor to get

#4.0 color(red)(cancel(color(black)("moles H"_2"O"))) * (6.022 * 10^(23) quad "molecules H"_2"O")/(1color(red)(cancel(color(black)("mole H"_2"O")))) = color(darkgreen)(ul(color(black)(2.4 * 10^(24) quad "molecules H"_2"O")))#

The answer is rounded to two **sig figs**, the number of sig figs you have for the number of moles present in your sample.