Limiting Reagents

[CookieZine]

I am having trouble with comparing ratios in limiting reagent problems. I will use BR Example 1.32 as an example.

What is the limiting reagent when 22.0 grams C3H8 are reacted with 48.0 grams O2?

C3H8 (l) + O2(g) → CO2(g) + H2O(g)

A. Oxygen is the limiting reagent
B. Propane is the limiting reagent
C. Water is the limiting reagent
D. There is no limiting reagent

The actual moles are:
0.5 mol C3H8

1.5 mol O2


Balancing the equation:

C3H8 (l) + 5O2(g) → 3CO2(g) + 4H2O(g)

Actual moles of O2 : Needed moles of O2
1.5 : 5

Actual moles of C3H8 : Needed moles of C3H8
0.5: 1


Since the ratio of Actual moles of O2 : Needed moles of O2 is smaller than the Actual moles of C3H8 : Needed moles of C3H8 O2 is the limiting reagent.

Is my thinking on this correct? More specifically, can I always compare the ratio of the actual moles of Reagent A to the needed moles of Reagent A against the ratio of the actual moles of Reagent B to the needed moles of Reagent B? The wording in TBR is a little confusing and this method is easier for me to understand.

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[DrknoSDN]

Conceptually that is exactly how you determine limiting reagents.

How much you should need vs how much you actually have. What runs out first is the limiting reagent.

 

[Czarcasm]

Yeah, that's what I usually . But you should also know how to relate amount of each reagent to the amount of product produced. There's all sorts of different questions that can be asked. For instance, how many moles/grams/molecules of CO2 is produced? How many moles/grams/molecules of H2O is produced? Which reagant is in excess (in moles/grams)? Which reagant is limiting (in moles/grams). How much excess reagant is there (in grams/moles) provided there's a limited amount of both? What is the theoretical yield? or percent yield? Those are usually the questions they ask.

If you can understand the basics, how to inter-convert grams to moles to molecules and vice versa, and also how to relate this info to the stoichiometric coefficients (making sure it's balanced too), you should be alright.