[xoxcalliexox]

Ok I have my chemistry 11 exam TOMORROW and as I was looking through my notes, I realized that I don't (and never really did) understand how to find the limiting/excess reactants in a reaction. I'm kind of panicking, because I really really need to get a good mark on this, and I want to be sure about everything before I go in to write it. Any help on this would be SO appreciated.

(Ex of a question I was looking at):

How many moles of sodium fluoride is produced when 24.0 moles of sodium combines with 15 moles of flourine gas?

the work our teacher showed us...

2Na + F2 --> 2NaF
24molNa x 1F2/2Na = 12mol F2

therefore, Na is limiting and F2 is excess

WHY?????
(there's more to this one question of course, but I'm just confused about the limiting/excess part of the chapter in general)

[spock]

Compare the amount of product produced by the two reactants.  The reactant that produces the SMALLER amount of product is the limiting reagent and will determine the maximum amount of product  that can be produced.  This is because once one reactant runs out, the reaction stops, even though there is enough of the other reactant to make more product.

If you were going to make cheese sandwiches according to the formula:
        2 Bread   +   1 Cheese  -->  1 Sandwich
and you have 16 slices of bread and 10 slices of cheese.  How many sandwiches could you make?

The answer is 8 because you only have enough bread to make 8 sandwiches, even though you have enough cheese to make 10 sandwiches.  The bread will run out and thus limit the number of sandwiches (which makes the bread the limiting reagent) while the cheese is in excess (there's more than I can use up).
hey

[cdementor]

The previous helper did explained very clairly giving you a good example. However, you will have to remember fluorine in atmosphere is found as F2 (diatomic) and sodium Na (monoatomic). To make a compound, one sodium will combine to one flurine atom (out of 2).
hey

[xoxcalliexox]

thanks so much! :)
hey