1.1 What is Chemistry?

http://www.ck12.org Chapter 12. Stoichiometry

TABLE12.7:NH3 Table Part I

N 2 3H 2 2NH 3

Molar Mass 28.02 g/mol 2.02 g/mol 17.04 g/mol

Initial Mass 7.16 g 3.40 g 0 g

Initial Moles 0.256 mol 1.68 mol 0 mol

Change in Moles -x -3x +2x

Final Moles 0 mol

Final Mass 0 g

Looking at the difference between the initial and final moles for N 2 , it is clear that x must have a value of 0.256
moles. Using this value, we can replace the "Change in Moles" line with specific values, and the "Final Moles" line
can be filled in by simple addition or subtraction.

TABLE12.8:NH3 Table Part II

N 2 3H 2 2NH 3

Molar Mass 28.02 g/mol 2.02 g/mol 17.04 g/mol

Initial Mass 7.16 g 3.40 g 0 g

Initial Moles 0.256 mol 1.68 mol 0 mol

Change in Moles -0.256 mol -0.768 mol +0.512 mol

Final Moles 0 mol 0.91 mol 0.512 mol

Final Mass 0 g

Finally, we can convert moles to grams using the molar masses of each reaction component:

0 .91 mol H 2 ×(

2 .02 g H 2

1 mol H 2

) = 1 .84 g H 2

0 .512 mol NH 3 ×(

17 .04 g NH 3

1 mol NH 3

) = 8 .72 g NH 3

TABLE12.9:NH3 Table Part III

N 2 3H 2 2NH 3

Molar Mass 28.02 g/mol 2.02 g/mol 17.04 g/mol

Initial Mass 7.16 g 3.40 g 0 g

Initial Moles 0.256 mol 1.68 mol 0 mol

Change in Moles -0.256 mol -0.768 mol +0.512 mol

Final Moles 0 mol 0.91 mol 0.512 mol

Final Mass 0 g 1.84 g 8.72 g

Note that if we calculate the total mass of all components, it is the same before and after the reaction (10.56 g total).
This is consistent with the law of conservation of mass.

Now, we can answer specific questions about this reaction. The limiting reactant is N 2 , because it is completely used
up before any of the other reactants. We would know that we chose the wrong limiting reactant if any of the other
final amounts turned out to be negative numbers. The theoretical yield of ammonia is 8.72 g, and there is 1.84 g of
H 2 left over after the reaction is complete.