# Question #2d08d

##### 1 Answer

#### Explanation:

The key here is the definition of the *mole* because if you know how many particles are needed to have exactly **mole** of ammonia, you can use the number of particles present in your sample to calculate how many *moles* it contains.

You should know that the definition of the *mole* is given by **Avogadro's constant**, which tells you the number of particles needed to have exactly **mole** of a given substance.

In your case, in order to have **mole** of ammonia, you need to have **molecules** of ammonia.

#"1 mole NH"_3 " " stackrel(color(white)(acolor(blue)("Avogadro's constant")aaaa))(->) " " 6.022 * 10^(23)color(white)(.)"molecules NH"_3#

Now, to calculate the number of moles present in your sample, set up Avogadro's constant as a *conversion factor* that has the number of molecules in the **denominator** and **numerator**.

You will end up with

#1.253 * 10^(24) color(red)(cancel(color(black)("molecules NH"_3))) * "1 mole NH"_3/(6.022 * 10^(23)color(red)(cancel(color(black)("molecules NH"_3))))#

# = color(darkgreen)(ul(color(black)("2.081 moles NH"_3)))#

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