How many grams of nitrogen are needed to produce 364.5 grams of ammonia gas?

Question

Select one:

a. 300

b. 200

c. 193

d. 138

Answer 1

N₂ + 3H₂ ==> 2NH₃ ... balanced equation

moles NH3 desired = 364.5 g NH3 x 1 mole/17 g = 21.44 moles

mass N₂ required = 21.44 moles NH_{3 x} 1 mole N₂/2 moles NH₃ x 28 g N2/mole N₂ = 300 g (answer a)

Answer 2

This is a simple conversion/stoichiometry problem.

As with most chemistry problems, we need a reaction first. But what do you do with all reactions:

That's right, you balance them.

Now ammonia has a formula of NH3 and Nitrogen gas has a formula of N2. So you have the N for ammonia, but not the H. As you can guess, we are going to combine N2 to hydrogen (H2) to make NH3.

So balance it:

x N2 + y H2 --> z NH3

x=1 y=3 and z = 2

While you CAN use grams, using moles is much easier. So how do you get from grams to moles?

With molecular weight. So, use the molecular weight of ammonia to convert the mass to moles of ammonia. For every 2 moles of ammonia, you have 1 mole of nitrogen. If you set up your ratios, you'll see you have "half" the moles. With moles of nitrogen, you use its molecular weight to convert it to grams and you'll have your answer.

Ball parking the answer, probably a, but try it yourself.