Chemistry help?

Question

Here is my recipe for s’mores: 1/4 chocolate candy bar, 1 marshmallow, and 2 halves of a gram cracker. After hiking into the back woods of Maine today, my kids and I are definitely ready to eat as many s’mores as we can make. Here is the inventory of supplies I have in my backpack – 6 chocolate bars, 1 bag of 30 marshmallows, and 40 whole gram crackers. 

Backpack inventory: 

40 whole graham crackers + 6 chocolate bars + 30 marshmallows 

Determine how many s’mores can be made from the supplies in the backpack. Use models if necessary. 

Perform calculations to show how many s’mores could have been made from each ingredient. Show work. 

Chocolate bars 

Marshmallows 

Graham crackers 

How many s’mores can be made from the supplies in the backpack? 

What do you run out of first? 

Do you think this ingredient is considered a limiting reagent or an excess reagent? Why? 

How much of the other ingredients are left over? 

Do you think these ingredients are considered limiting reagents or excess reagents? Explain. 

What happens to the left-overs? That is, will they be used to make s’mores tonight (Remember, I had to hike for hours into the woods of Maine to arrive at my campsite, so it is impossible to resupply)?

Answer 1

Here you have a real-world problem using ratios: recipes. You will use the same amount of each of the 3 ingredients, and you will repeat it until you run out of something and can no longer adhere to the recipe, this will identify what your "limiting reagent" is. If you run out of chocolate first, that is your limiting reagent. Then you can't keep making smores with the original recipe once you've run out of one ingredient. All the leftover ingredients are excess, extra, can't make any more smores with just 2 out of 3 ingredients.

You can solve this many ways. You can look at each ingredient one by one:

Chocolate. You have 6 bars. The recipe uses 1/4 (0.25) of a bar for each smore. How many smores can you make with 6 bars? (6 divided by 0.25)

Marshmallows. You have 30 mallows. The recipe uses 1 mallow per smore. How many smores can you make with 30 mallows?

Graham crackers: you have 40 crackers. The recipe uses 2 halves of a cracker per smore (2 halves = 1 whole). How many smores can you make with 40 whole crackers?

Now look at your three answers. The smallest number will tell you your limiting reagent, the ingredient you will run out of first, and therefore the total number of smores that you can possibly make with this recipe.