Explain what a limiting reactant is and describe the process to determine the limiting reactant in the example listed below:

Question

Given 3.4 grams of x compound with a molar mass of 85g and 4.2 grams of y compound with a molar mass of 48g

How much of compound  xy can be generated 2x + Y2 = 2xy

another question

5: explain where you would find the actual yield and the theoretical yield needed to calculate the percent yield of a reaction. Is the answer listed above the theoretical or actual yield? 

Answer 1

Hi Emily,

Think of a limiting reactant like baking. If a recipe calls for 2 eggs and 3 cups of flour for each batch of cookies and you have 4 eggs alongside 500 cups of flour, you can only bake 2 batches of cookies. This is because the eggs in this case act as a limiting reactant. In a similar way, the formula for different molecules contains different atoms that make it up, like ingredients. Let's look at your example.

In your example, you have a recipe for creating 2xy. Let's try to calculate how many moles of each compound you have, analogous to the amount of ingredients you have. There are 3.4/85 moles of compound x, or .04 moles total, and 4.2/48, or .0875, moles total of compound y. There is the same amount of x and y in the reaction, so the smaller of the two (x) will be our limiting reactant, or our eggs in the baking analogy.

Given this, you should be able to finish off the rest of it and find the number of moles of compound xy generated in the end. As for whether the yield is theoretical or not, the question is most simply asking if this is from running an experiment or just from math/calculation. A theoretical yield is what we predict the yield will be, an actual yield is the final result. Some major reasons these can be different include that theoretical yields don't include waste or residue, alongside sources of error and outside contaminants.

If you have any further questions, please feel free to reach out or comment below.

Alim Merchant