limiting reactant question [ENDORSED]

[sandraabdelmalak1D]

For the following equation, determine the limiting reagent if 21.4 g NH3 is reacted with 42.5 g of O2.
4NH3(g) + 5O2(g) ---> 4NO(g) + 6H2O(g)

I understand how to do this problem until I get to the part where I compare the calculated moles with the required moles in the balanced equation. How do I use the ratio of moles in the balanced equation to determine whether something is in excess or is limiting?

[Sarah_Wilen]

Look at my attached image for a worked out solution. I will show you the dance step by step:

1) Balance--yay
2) Choose a quantity (either NH3 or O2). You can choose whichever you want because this is America
3) I decided to go with ammonia, but you can choose oxygen if your heart so desires
4) I use stoichiometry to determine how many grams of O2 will react with NH3 pretending that NH3 is the limiting reactant.
5) I found that 50.4 grams of O2 would be needed to react with NH3 if NH3 was the limiting reactant. Oh no! I'm fretting because we are only given 42.5 grams of O2. Does that mean that the NH3 is actually the excess reactant? Indeed! Now we know that O2 is the limiting reactant.

Rejoice!

[Chem_Mod]

A budding chemist and stand-up comic! :-)

[Sarahi Cota-Felix 1C]

Ohhhh, so the limiting reactant does not mean the number that is the least in amount rather the number that controls how much will be made?

[Sarah_Wilen]

Ohhhh, so the limiting reactant does not mean the number that is the least in amount rather the number that controls how much will be made?

Exactly! The smaller of the initial amount of masses of reactants given may not be the limiting reactant. Sometimes, questions will give you the same amount of masses for reactants, then you solve for the limiting reactant. Like this problem:

A reaction container holds 5.77 g of P4 and 5.77 g of O2. The following reaction occurs: P4 + O2 P4O6. If enough oxygen is available then the P4O6 reacts further: P4O6 + O2 P4O10.

The same amount of initial reactant is given.

[805070989]

Is there a good way to see which is the limiting reagent because I'm still a bit confused on how to compare. I understand how to do the calculations until I get to the part where I compare the calculated moles with the moles from the balanced equation. As I am a bit confused can someone clarify how to use the ratio of moles in a balanced equation to find the excess or limiting reagent?

[Sarah_Wilen]

Is there a good way to see which is the limiting reagent because I'm still a bit confused on how to compare. I understand how to do the calculations until I get to the part where I compare the calculated moles with the moles from the balanced equation. As I am a bit confused can someone clarify how to use the ratio of moles in a balanced equation to find the excess or limiting reagent?

I explained a pretty simple way to determine the limiting reactant in this post: https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=13&t=21202&sid=42018388ccee1c99865b57c20f6721e9. LMK if it needs clarifying.