Question #a4835

1 Answer
Feb 15, 2018

Answer:

#"0.0395 g"#

Explanation:

As you know, Avogadro's constant tells you the number of molecules of ammonia needed in order to have #1# 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 #1# mole of ammonia, is equal to #"17.031 g mol"^(-1)#. This means that #1# mole of ammonia has a mass of #"17.031 g"#.

#"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 #2.38 * 10^(22)# 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.