Stoichiometry

I've been getting a lot of tutoring inquiries lately. The first thing I ask is "what is your son or daughter having issues with?" Predominantly, it's been some variation on the word "stoichiometry."

Stoichiometry is a matter of translating amounts of one substance into another.

Example: If I have 6 pounds of ripe bananas, how many loaves of banana bread can I produce?

Observe that you're translating the amount of bananas into banana bread. Stoichiometry is the same concept. What you do need is a conversion factor. Say that per one loaf of banana bread, you need 2 lbs of bananas.

Here's how that example would play out:

BB stands for banana bread. The main thing I want you to observe is the set up of the conversion factor. The lbs of bananas are on the bottom so they can cancel units with the original given. On the top of the conversion factor is the loaf of BB because that is what we want left, and that is what we want to figure out. 

Now lets move on to some harder problems

I'm going to bypass mole to mole conversions for the moment because they're easier to pick up after you learn the entire process. Most of the time, you learn them just to be able to get to the longer process anyway.

Most stoichiometry problems tend to fall into one of two categories:

1.) Solve it through

ex. How much product can I get with x amount of reactant?

      How much reactant is needed to make x amount of product? 

For these problems you just write your given, convert to moles if you need to, use your mole ratio conversion factor, then convert that answer back to grams if you need to. 

Now, the procedure I've outlined above is not what most students would be use to seeing. It explains each step, but once you get the rhythm down for conversions it becomes more efficient to set up all your information in a single train.

This is what I've seen a lot of teachers teach straight off. Recognize that the steps are the same. Multiply across the rows and divide the top by the bottom. Also, cancel your units to check if your work is set up 

properly.

The units that cancel are color coded. 

2.) Limiting Reactant

ex. If I have x amount reactant A and y amount reactant B, how much product could I make?

For these problems, you have to essentially do the process for problem one, but with both of the reactants and convert them to products, then you have to analyze which reaction is feasible.

For instance, how many loaves of banana bread can I make with 4 lbs of bananas and 6 cups of flour? If we assume you need 1 cup of flour per loaf, I can make six loaves based off of the flour, but only two loaves based off of the pounds of bananas. I can only make two loaves of bread with the ingredients I have and I will have excess flour. Bananas are the limiting ingredient. This is the general idea to follow.  

The second step is where you have the real limiting reagent analysis. 

Look at what you have: 

0.0755 moles Pb(NO3)2

0.100 moles NaI

Calculate what you need of the other reactant to fully use what you have:

0.0755 moles Pb(NO3)2 would need 0.151 moles of NaI

0.100 moles NaI would need 0.050 moles of Pb(NO3)2 to work.

So which scenario works?

For the first scenario, we don't have enough NaI, while in the second we have more than enough Pb(NO3)2. Since all of the NaI gets used up, it is the limiting reactant. You'll have extra of the other reactant. 

Here's a practice problem with the answer from chemfiesta. Give it a try and post any questions in the comments section.