# Question #a4835

##### 1 Answer

#### Explanation:

As you know, **Avogadro's constant** tells you the number of molecules of ammonia needed in order to have **mole** of ammonia.

#6.022 * 10^(23) quad "molecules NH"_3 = "1 mole NH"_3#

Now, the **molar mass** of ammonia, which tells you the mass of exactly **mole** of ammonia, is equal to **mole** of ammonia has a mass of

#"1 mole NH"_3 = "17.031 g"#

You can thus say that, for ammonia, you have

#6.022 * 10^(23) quad "molecules NH"_3 = "1 mole NH"_3 = "17.031 g"#

which, of course, implies that

#6.022 * 10^(23) quad "molecules NH"_3 = "17.031 g"#

Your sample contains **molecules** of ammonia, so you can say the mass of the sample will be equal to

#2.38 * 10^(22) color(red)(cancel(color(black)("molecules NH"_3))) * "17.031 g"/(6.022 * 10^(23)color(red)(cancel(color(black)("molecules NH"_3)))) = color(darkgreen)(ul(color(black)("0.0395 g")))#

The answer is rounded to three **sig figs**, the number of significant figures you have for the number of molecules of ammonia.