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How To Find Limiting Reactant, Theoretical Yield And Amount Of Excess Reagent Left (with examples)

Table of Contents:



A limiting reactant is a reactant that limits how much product will be produced. It will run out first and stop the reaction from going any further.


In order to do limiting reactant calculations, we MUST first have a BALANCED equation. Please make sure the equation is balanced before going further when you do a problem.


1. How To Find Liming Reactant When Moles Are Given:


Calculate moles of product from each reactant using the coefficients in the balanced equation. The reactant that gives the LEAST moles of product is limiting. If there are two products, choose either one but make sure to use the same product for both reactants.


How To Find Liming Reactant When Moles Are Given:
How To Find Liming Reactant When Moles Are Given:

Reactant A is limiting because it produced the least moles of product C.


2. How To Find Liming Reactant When Grams Are Given


When grams of each reactant are given, first convert them to moles by using molar mass. Then, convert moles of each reactant to moles of the product by using coefficients in the equation. The reactant that produces the least product is the limiting reactant.


How To Find Liming Reactant When Grams Are Given
How To Find Liming Reactant When Grams Are Given

Zinc is limiting because it produced the least moles of product.


3. How To Find Theoretical Yield

Theoretical yield is the maximum amount of product that can be obtained in the reaction. To find theoretical yield, we simply need to convert moles of the product that the limiting reactant produces to grams using molar mass of the product.


How To Find Theoretical Yield
How To Find Theoretical Yield

4. How To Find Percent Yield

% yield tells us what is the % of the product that was obtained in the lab out of the maximum amount that could be produced.

How To Find Percent Yield
How To Find Percent Yield

5. How To Determine How Much Excess Reagent Will Remain Unreacted(Left)?



Step1: Determine which reagent is limiting as shown in Part 1 and 2.

Step 2: Use the product from the limiting reagent and molar ratio from the equation to find the amount of excess reagent that will be USED UP.

Step 3: Subtract the grams of excess reagents reacted from the initial amount of excess reagent.



How To Determine How Much Excess Reagent Will Remain Unreacted


Example: 2CH3CHO(l) + O2(g)--> 2HC2H3O2(l)

In a laboratory test of this reaction, 20.0 g CH3CHO and 10.0 g O2 were put into a reaction vessel. a. How many grams of acetic acid can be produced by this reaction from these amounts of reactants? b. How many grams of the excess reactant remain after the reaction is complete?


Step 1. First we need to figure out which reagent is limiting (reagents/reactants are on the left side of the equation).




Step 2. Now that we know which reagent is limiting, we are going to use the moles of product it produced to calculate the amount (grams) of the excess reagent that will be used up in the reaction.



Step 3. Now, all the is left is to subtract the amount of excess reagent used from the initial grams of the excess reagent given.



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