Mass/Mass Stoichiometry and Percent Yield

Why Use Conversion Factors?

Sometimes it is easy to solve problems without using conversion factors. (Although this is probably not the case in stoichiometry problems!) Below are three real life situations in which conversion factors could be used. Solve the problem, being certain to show your work for each step. After you have solved the problems, click on the word Answer, and see how the problem could be solved using conversion factors and without conversion factors.

1. You are making brownies for a bake sale. Each batch of brownies requires one box of brownie mix, 1/4 cup oil, 1/2 cup water, and 2 eggs. You have 12 boxes of brownie mix, 12 cups of oil, 12 cups of water, and 12 eggs. How many batches of brownies can you make?

Answer:

Using either method, the most batches that can be made depends on which ingredient will make the least number of batches – which ingredient will run out first.  As determined by the brownie mix, you can make 12 batches.  As determined by the oil, you can make 48 batches.  As determined by the water, you can make 24 batches.  As determined by the eggs, you can make 6 batches.  Therefore, you can make 6 batches of brownies.

 

2. You are making goody bags for your little sister’s birthday party. Each goody bag has 6 pieces of candy, 1 noise maker, 3 stickers, 1 bouncy ball, 5 tiny containers of play dough, and 2 tiny containers of bubbles. Your mom has purchased 48 pieces of candy, 6 noise makers, 54 stickers, 12 bouncy balls, 35 tiny containers of play dough, and 35 tiny containers of bubbles. How many goody bags can you make?

Answer:

As determined by the candy, you can make 8 goody bags.  As determined by the noisemakers, you can make 6 goody bags.  As determined by the stickers, you can make 18 goody bags.  As determined by the bouncy balls, you can make 12 goody bags.  As determined by the play dough, you can make 7 goody bags.  As determined by the bubbles, you can make 17.5 goody bags.  Therefore, you can make 6 goody bags.

 

  

3. You are going into the business of making pizzas. The following is the list of ingredients to make ONE pizza.

400 g flour

50 mL water

10 g yeast

120 g sauce

250 g cheese

5 g oregano

5 g basil

Your business has purchased 500 pizza pans, 60 kg of sauce, 100 kg of cheese, 2.5 kg of basil, 2.5 kg of oregano, 5 kg of yeast, and 200 kg of flour. You have as much water as you need. How many pizzas can you make?

Answer:

As determined by all of the ingredients except the cheese, you can make 500 pizzas.  As determined by the cheese, you can only make 400 pizzas.  Therefore, you can make 400 pizzas.

 Notice that in all three of these problems, the ingredient that runs out first is the one that determines how much product can be formed. In the next lesson, you are going to be taught about limiting reactants. The same thing is true for a chemical reaction. The reactant (or ingredient) that runs out first is the one that determines how much product can be formed — it limits the formation of product – which is why it is called a limiting reactant. Once the limiting reactant or ingredient runs out, the process must stop, regardless of how much of the other ingredients is left.

Adapted from a GAVL course:
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