1.48g of H2 is allowed to react with 9.660g N2. This reaction yields 2.40g NH3.?

Question

What is the theoretical yield for this reaction?

Answer 1

Write your reaction
3H2 + N2 => 2NH3

Find the moles of NH3 = 2.4/17 (about .14)
Find the "moles" of H2 = 1.48/2 (about .75)
Find the "moles" of N2= 9.66/28 (about .35)
From this, H2 is limiting. For all the available N2 to react, there would be about 1.05 mole H2 required (see reaction). We don't have enough. From the reaction, for all the available H2 to react, 0.25 moles of N2 are needed. So the theoretical yield is based on the moles of NH3 formed to the moles of NH3 that could be formed from 0.25 moles of N2. From the reaction, that is 0.50 moles of ammonia. However, since we only have 0.14 moles, the yield is about 100*0.14/0.50 or 28 percent.

Answer 2

Calculating a theoretical yield starts with determining which of the reagents are limiting. The reaction will only go as far as the reagent that is in the smallest supply. The balanced equation for this reaction is:
3H2 + N2 -----------> 2NH3


1.48 grams of hydrogen gas requires 6.85555 grams of Nitrogen gas (using the balanced equation for the reaction, I converted to moles of hydrogen, then to moles of nitrogen, then to grams of nitrogen). We have 9.660 grams of nitrogen gas available for this reaction, thus we have more than enough of that. So, hydrogen is the limiting reagent here. So, the amount of ammonia (NH3) that is going to be formed is going to depend on the hydrogen gas that is available.

So,

1.48 g H2 x 1 mole H2/2.01588 g H2 x 2 moles NH3/3 moles H2 x 17.03056 g NH3/1 mole NH3 = 8.3355 grams NH3

Thus, theoretically, using the given amount of hydrogen gas we expect 8.34 grams of Ammonia to form in this reaction.

So, the theorectical yield is 8.34 grams.

Now, we didn't actually form that much ammonia gas. We only formed 2.40 grams of it. So, the actual yield is 2.40 grams, and the percentage yield is 2.40/8.34 x 100 = 28.8 %
That means that we only formed 28% of the product that we expected to form if conditions were optimal.

But, you were only asked for the theoretical yield, which is 8.34 grams (or 0.489 moles) of ammonia.

Hope that helped you out.