When heated, lithium reacts with nitrogen to form lithium nitride:

6Li+N2 --> 2Li3N

What is the theoretical yield of Li3N in grams when 12.4 g of Li is heated with 33.9g of N2

3 answers

This is a limiting reagent problem. How do I know that? Because BOTH reactants are given, not just one.
1. You have the equation and it is balanced.
2a. Convert 12.4 g Li to moles. moles = grams/molar mass.
2b. Convert 33.9 g N2 to moles the same way.

3a. Using the coefficients in the balanced equation, convert moles Li in 2a to moles Li3N.
3a. Same process, convert moles N2 in 2b to moles Li3N.
3c. It is more than likely that the two answers for moles Li3N will be different which means one of your values is wrong. In limiting reagent problems the correct answer is ALWAYS the smaller value and the reagent producing that smaller value is the limiting reagent.

4. Now convert moles Li3N from 3c to grams. g = moles x molar mass. This is the theoretical yield.
Post your work if you get stuck.

22.7g