Percent Yield Lab. This experiment has proven that KI is the limiting reagent in this chemical reaction.

by honeybooboo (student)

Percent Yield Lab

Introduction

When a chemical reaction occurs and two reactants mix, we should in theory be able to determine the amount of each product formed using stoichiometric calculations. The maximum value of the final product is limited to the limiting reagent not the reagent in excess. From determining the limiting reagent we are able to calculate the theoretical mass of the product formed.

First step to find the percentage yield is to balance. The number of atoms in the reactants must be equal to the atoms in the products. This is due to the law of conservation of mass. Matter can’t be destroyed or created but transferred so therefore the chemical equation has to be balance so that the number of atoms on both side and the exact ratio of how the reactants is displayed.

Second step is to find the limiting reagent. Limiting reagent is the reactant that is depended upon to determine how much of product is made. In order to find the limiting reagent the equation moles= mass/molar mass. Now that the moles are figured out, multiply the number of moles of the reactants by the ratio of the reactant and the product. This calculation is performed twice once for the first reactant and one for the second. Then the Theoretical Yield is calculated, it is how much product will be synthesized with the reactants. Multiply the lowest number of moles (limiting reagent’s mole) by the molar mass of the product. This will give you the mass of the product.

In order to find the percent yield, a ratio between the actual yield and the theoretical yield is used. This indicates the percent of how much of the theoretical yield was obtained in the experiment. This equation can be used: Percentage yield= mass of actual yield/ mass of theoretical yield * 100%.

Purpose:

To quantitatively and qualitatively determine the limiting reagent in the chemical reaction. To also compare the theoretical and actual values in order to find the percentage yield of the reaction.

Hypothesis

Pb (NO3)2(aq) + 2KI (aq) PbI2 (s) + 2KNO3

Test #1 (Pb (NO3)2)

xPbI20.00906mol=1/1

X= 0.00906mol

Test #2 (KI)

xPbI20.0120mol=1/2

X= 0.00602mol

Theoretical Yield

0.00602*(461) = 2.78 g

The limiting reagent should be KI. KI has the least amount of moles therefore you do not have ...

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Hypothesis

Pb (NO3)2(aq) + 2KI (aq) PbI2 (s) + 2KNO3

Test #1 (Pb (NO3)2)

xPbI20.00906mol=1/1

X= 0.00906mol

Test #2 (KI)

xPbI20.0120mol=1/2

X= 0.00602mol

Theoretical Yield

0.00602*(461) = 2.78 g

The limiting reagent should be KI. KI has the least amount of moles therefore you do not have enough to react with the excess Pb (NO3)2. Whereas PbI2has a higher amount of moles and will result to be the excess reagent. The theoretical mole is 0.00602 moles and the theoretical mass is 2078g.

Equipment

Potassium Iodide(2.00g)

Lead Nitrate (3.00g)

Distilled water (200ml)

Beakers (300ml *5)

Goggles

Glass Stirring Rod

Filter Paper (small x2 large x1)

Funnel

Retort Stand

Ring Clamp

Scale

Procedure:

1. The filter paper was placed on top of the laboratory digital scale and zeroed. Then the Potassium Iodide was weighted with a laboratory digital scale for 3.00g.

2. The Lead Nitrate was weighted with a laboratory digital scale and filter paper for 2.00g.

3. In one beaker the Potassium Iodide was mixed with 100mL of distilled water.

4. In another beaker the Lead Nitrate was mixed with 100mL of distilled water.

5. The mixture of Potassium Iodide and the Mixture of Lead Nitrate were mixed into one beaker.

6. The large filter paper was folded and place in the funnel in a retort stand and ring clamp.

7. A clean beaker was placed under the funnel and the mixture of the two substances was then filtered twice.

7. The solid in the funnel was placed into the incubator over night.

8. The dry solid was weighed.

9. The solution left was poured into a two beakers and KI was added.

Observations

Testing for Limiting Reagent:

Mass of dry filter paper: 1.51g

Mass of PbI2: 4.16

Percentage Yield

Mass with filter paper- mass of filter paper= mass of PbI2

4.16-1.51 = 2.65

Actual mass/ theoretical mass * 100% = Percentage Yield

2.65/2.78 =0.95323741 *100% = 95.3%

Analysis

The yellow precipitant produced was the reaction between the lead and iodine. When those two reactants reacted it undergone a double displacement and a yellow precipitant was produce in a clear solution. PbI2 was the solid produce and KNO3 was the solution left behind. There was extra solution left behind because not enough of one reactant was present for more PbI2 to form. This is the limiting reagent. When testing for the limiting reagent Pb (NO3)2 was placed into KNO3 and no reaction occurred. Whereas when KI was placed into KNO3 yellow precipitant started to form again. This proves and shows that KI was the limiting reagent and prevented more precipitant to form since there was not enough to react with Pb(NO3)2. With the percent yield it has be determined that our experiment has captured 95.3% of the PbI2 compared to the theoretical mass.

Result

This experiment has proven that KI is the limiting reagent in this chemical reaction. This was tested by adding more KI to a beaker full of KNO3 and adding more Pb(NO3)2 to a different beaker full of KNO3. With this simple experiment it was shown that when KI was added more precipitant formed where was Pb(NO3)2 no reaction occurred. This is because KI is the limiting reagent and Pb(NO3)2 is the excess. When this chemical reaction occurred and two reactants mix, theory was able to determine the amount of each product formed using stoichiometric calculations. The maximum value of the final product is limited to the limiting reagent not the reagent in excess. So in this case the maximum value of final product was limited by the amount of KI. From determining the limiting reagent the theoretical mass of the product was calculated.

Discussion:

The results compared to the previously stated hypothesis and expected outcome was perfect. The hypothesis states that KI is limiting with Pb(NO3)2 being excess and the experiment proved this true. However the percentage yield was not 100% because there may have been some experimental errors that may have impacted our results. The PbI2 was stuck to the side of the beaker and the stirring rod as it was being filtered and this may have caused the results to be a little off and under the expected results. If this experiment was to be repeated or any future study was to be done with limiting reagent there is not much that can be done to ensure that the percent yield is 100%. A suggest of improvement is to be more careful while doing this experiment to ensure that all the solid particles are captured. Also adding more distilled water to the beaker and stirring rod and filtering the solution more times will help increase the percentage yield and make the results more accurate and precise. If the solution is filtered enough all the yellow particles will be captured and the percent yield will be closer to 100%. Other than filtering the solution more, using something else to scrap the solid into the funnel may work however the solid is very stubborn and transfers from one object to another.

Conclusion:

In conclusion this experiment led students to find the limiting and excess reagent after their prediction of Pb (NO3)2(aq) +2KI (aq) PbI2 (s) + 2KNO3. The limiting reagent in the lab was KI because it had the least moles. Therefore KI stopped the reaction from further progressing and carrying on whereas Pb(NO3)2 was the excess reagent. Pb(NO3)2 was the excess reagent because it had the most moles. It was plentiful and was not fully used in this chemical reaction because there was not enough KI to react with. The most important finding was that with theory anything can be predicted in a chemical reaction, the limiting reagent and excess reagent can be found. Through this experiment we were able to quantitatively and qualitatively determine the limiting reagent in the chemical reaction. Also compare the theoretical and actual values in order to find the percentage yield of the reaction and achieve a result that was 95% accurate to the predicted mass. A trend that occurred amongst many groups is not being able to obtain all solid and this is one of the major challenges that everyone had with this lab. Also all groups were able to predict the result that KI is limiting.

Applications

Limiting reagents, excess reagents, theoretical yield and percentage yield may be used in the real world. Some chemicals sold in stores may have to be experimented on to find out if there are enough of both reactants to produce a functioning chemical and efficiency of the chemicals being combined. However this may also be used in the food industry to test for how much of each ingredient is in a product or how much of an active ingredient is needed to react with the other ingredients. Experiments on limiting reagents may be used frequently in the science world however there is not much information on where and how these experiments are performed.

References

Clancy, C. (2001). McGraw-Hill Ryerson chemistry 11. Whitby, Ont.: McGraw-Hill Ryerson.

Hathaway, J. (2012). Chemistry 11U Notes. Bradford: Bradford District High School. J. Hathway