Question #17e2e

2 Answers
Sep 30, 2016

You are to determine the number of molecules of #NH_3# .

One molecule of #NH_3# contains 3 atoms of Hydrogen.So to produce one mole of #NH_3# we require #3/2molH_2# The following equation also supports that

#N_2+3H_2->2NH_3#

So 1 mole #H_2# produces #2/3mol NH_3#
Hydrogen taken #=7.98xx10^-4g#

#=7.98xx10^-4gxx1/(2.016g/"mol")#

So no.of moles of #NH_3# produced
#=2/3xx7.98xx10^-4gxx1/(2.016g/"mol")#

So no.of molecules of #NH_3# produced
#=2/3xx7.98xx10^-4gxx1/(2.016g/"mol")xx6.023xx10^23"molecule"/"mol"#
#=15.9xx10^19#molecules

Sep 30, 2016

Answer:

Here's what I got.

Explanation:

The problem wants you to find the number of molecules of ammonia, #"NH"_3#, not hydrogen gas, #"H"_2#.

Your calculation is set up to find the number of molecules of hydrogen gas present in the #7.98 * 10^(-4)"g"# sample.

#7.98 * 10^(-4)cancel("g H"_2) * (1cancel("mole H"_2))/(2.016cancel("g H"_2)) * (6.022 *10^23"molec H"_2)/(1cancel("mole H"_2))#

# = 2.38 * 10^20"molecules H"_2#

As you can see, you're missing a conversion factor to take you from molecules of hydrogen gas to molecules of ammonia.

This conversion factor comes from the balanced chemical equation that describes the synthesis of ammonia

#color(blue)(3)"H"_ (2 (g)) + "N"_ (2(g)) -> color(purple)(2)"NH"_ (3(g))#

Notice that you have #color(blue)(3)# molecules of hdyrogen gas reacting with #1# molecule of nitrogengas to produce #color(purple)(2)# molecules of ammonia.

You can thus say that if you start with #x# molecules of hydrogen gas, you can get to molecules of ammonia by using

#xcolor(white)(a)cancel("molecules H"_2) * (color(purple)(2)color(white)(a)"molecules NH"_3)/(color(blue)(3)cancel("molecules H"_2)) = (2/3x)" molecules NH"_3#

In your case, you will have

#2.38 * 10^(20)cancel("molec H"_2) * (color(purple)(2)color(white)(a)"molec NH"_3)/(color(blue)(3)cancel("molec H"_2)) = 1.59 * 10^(20)"molec NH"_3#

In fact, you don't need to find the number of molecules of hydrogen gas, all you need to do is use the aforementioned #color(blue)(3):color(purple)(2)# mole ratio.

Since a mole is simply a fixed number of molecules, you can say that

#7.98 * 10^(-4)cancel("g H"_2) * (1cancel("mole H"_2))/(2.016cancel("g H"_2)) * (color(purple)(2)cancel("moles NH"_3))/(color(blue)(3)cancel("moles H"_2)) * (6.022 *10^23"molec NH"_3)/(1cancel("mole NH"_3))#

# = 1.59 * 10^(20)"molec NH"_3#

So remember, don't jump in head-first, make sure you understand what the problem wants you to find first.

Once you have a clear understanding of what you have and waht you need to calculate, organize your conversion factors and plug in your values.