In finding limiting reactants and yields, we need to study the specific chemical reaction equation.

I'd like to start with an analogy. Imagine we are builders and we've got supplies to make houses. A house is built according to our blueprint that says: 1 Walls + 1 Roof --> 1 House. Easy enough, right? We know that if we have 2 sets of walls and 2 sets of roofs, we can build 2 houses. If we have 1 set of walls and 2 sets of roofs though, we know that we can only build 1 house. In that case, the number of walls that we have limited our ability to build more houses because we can't build a house without walls. Our walls were our "limiting reactant" and 1 house was our "theoretical yield." Now imagine that instead of telling you I have 1 set of walls and 1 roof, I tell you that I have 800 lbs of wall and 1200 lbs of roof. Let's assume that 1 full set of walls weighs 400 lbs and that 1 full roof weighs 500 lbs. In that case, I know that I have enough supplies to build 2 sets of walls and 2.4 roofs. Once again, my walls are my limiting reactant and 2 houses is my theoretical yield.

For a chemical reaction, we take the same approach. The walls and roofs that we discussed previously are our reactants while our house is the product of the reaction. We can't build an incomplete house just like we can't build an incomplete molecule.

Consider the following equation:
2LiCl + Na₂CO₃ --> 2NaCl + Li₂CO₃. If I have 20.0 g of LiCl and 20.0 g of Na₂CO₃, that doesn't actually tell me much because I don't know how many molecules of each reactant I have. If I want to find that out, I need to convert the masses I was given into a number of moles, where one mole represents 6.02 x 10²³ molecules, by using the molar mass in the following way:
# of moles = (# of grams)/(# of grams in one mole).

The molar mass of LiCl is 42.39 g/mol so we will have 0.472 moles of LiCl. The molar mass of Na₂CO₃ is 105.99 g/mol so we will have 0.189 moles of Na₂CO₃.

Now, we look back at our chemical reaction equation and notice that for every one mole of Na₂CO₃ I use in my reaction, I use 2 moles of LiCl. That means that for X number of moles of Na₂CO₃, I need 2X moles of LiCl, otherwise LiCl is the limiting reactant. If I have more than 2X, Na₂CO₃ will be my limiting reactant. Ultimately, the limiting reactant is just the reactant I have that will run out first.

With 0.189 moles of Na₂CO₃ and 0.472 moles of LiCl, I can see that I will only need 0.378 moles of LiCl to consume all of the Na₂CO₃, which leaves me with 0.094 moles of unreacted LiCl. Therefore, the Na₂CO₃ ran out first and is our limiting reactant.

To then get to the yield, I once again look at the chemical reaction equation. For every mole of Na₂CO₃ I consume, I make 2 moles of NaCl and 1 mole of Li₂CO₃. Given that I consumed all 0.189 moles of Na₂CO₃ that I had, that means I was able to make 0.378 moles of NaCl and 0.189 moles of Li₂CO₃. If you want to convert that value to grams, multiply the value in moles by the molar mass of the molecule and you will get its theoretical yield in grams.

One more extra point: we call this the theoretical yield because we assume that absolutely all of the input molecules react and make the product. In reality, however, we will tend to lose a little bit of our reactants as they get consumed by other chemical reactions that we did not account for. This will result in a lower actual yield than the theoretical yield. The yield percent, therefore, is just the percent of the theoretical yield that the actual yield represents.