I have a test tomorrow!! Stoichiometric!!?

Question

How many liters of nitrogen gas are needed to react with hydrogen gas to produce 5.40 liters of ammonia at STP?
N2 + 3H2 --> 2NH3

Limiting Reactants
What is the limiting reactant when 24g H2 and 35g O2 react to produce water? 2H2 + O2 --> 2H2O

Answer 1

Q 1: Remember; 1 mole of any gas is 22.4 liters@ STP!

You can get the answer by inspection; 2.7 liters!

If you want to do it the long way; here's how its done:

From stoichiometry; one nitrogen molecule > 2 ammonia:

5.4/22.4 = 0.241mole ammonia; 0.241 X 22.4/2 = 2.7 liters

You need to divide by 2 because you get 2 nitrogen atoms from each nitrogen molecule, so you only need 1/2 the expected volume! [See the warning at end]

For the other question;

You can easily tell by looking at it, the the oxygen will run out first (limiting reagent), but if you want to do it the "hard" way;

Moles hydrogen 24/1 = 24moles

Moles oxygen 35/16 = 2.1875 moles

The stoiciometry from the equation is 2 H2 per 1 O2:

If all the oxygen reacts; it can only use up 2 x 2.1875 moles of hydrogen! There will be 24-[2 X 2.1875] moles of H left.

Note, they didn't ask for how much was left, only which would run out first, so you can do it by inspection and save your self some work. (However, if you teacher insists you show work, listen to him, he is grading your work, not me)!

The most important thing to remember; 1 mole of any gas @STP is 22.4 moles.

If this gas is diatomic, like N2, O2, H2, etc this mole of gas molecules will actually contain 2 moles of gas atoms!

Be careful not to get mixed up between gas molecules and gas atoms!