# Question #0c399

##### 1 Answer

#### Explanation:

A **mole** of molecules is simply a *very large* collection of molecules. In order to have **one mole** of oxygen molecules, you need to have exactly

This very large number, which is known as **Avogadro's number**, allows you to convert from *moles* of a substance to *molecules* of a substance.

In your case, the problem tells you that you have a total of *significantly less* than one mole, since

#1.26 * 10^(19) < 6.022 * 10^(23)#

To determine exactly how many moles of oxygen you have, use Avogadro's number as a *conversion factor*

#1.26 * 10^(19) color(red)(cancel(color(black)("molecules O"_2))) * overbrace("1 mole O"_2/(6.022 * 10^(23)color(red)(cancel(color(black)("molecules O"_2)))))^(color(blue)("Avogadro's number")) = color(green)(2.09 * 10^(-5)"moles O"_2)#

Alternatively, you can express this number in standard notation to get

#n = color(green)("0.0000209 moles O"_2)#

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