EXPERIMENT 6

Limiting Reagent

 

PRELAB NOTES

 

 

  1. Read Experiment 6 and review the material on limiting reagent stoichiometry in your lecture textbook.

  2. When completing the prelaboratory exercise, the following explanations should be helpful.

    1. The values for the column titled Òmole ratio Ó are to be calculated from the actual number of moles of KIO3 and Pb(NO3)2 for that particular letter.

 

 

Example:

 

If moles KIO3 = .01402

 

and if moles Pb(NO3)2 = .009063

 

then the mole ratio



    1. To determine the limiting reagent, compare your calculated mole ratio to the theoretical mole ratio obtained from the balanced reaction equation.

 

Example:

 

Suppose the theoretical mole ratio is

 

then the actual mol ratio of 1.547 is too low, which means that the numerator of the ratio, namely Òmol KIO3Ó is too low, and so KIO3 is the limiting reagent.

 

 

 

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PRELABORATORY EXERCISE

 

LIMITING REAGENT

 

Read the prelab notes and the experiment, then complete the following as directed.

 

  1. Balance the equation:

 

_______ Pb(NO3)2 + _______ KIO3 ¨ _______ Pb(IO3)2 + _______ KNO3



  1. Complete the table for the seven different combinations of reactants. For each of the seven combinations you must first determine which reactant is limiting and then calculate the expected amount of product on this basis. You will be assigned one of these combinations to use during your laboratory period. Be sure to use the correct number of significant figures.

 

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EXPERIMENT 6

LIMITING REAGENT STOICHIOMETRY

 

Purpose:   The purpose of this experiment is to deal with the concept of limiting reactant in a chemical equation.

 

Theory:    When two or more reactants react chemically they do so in a certain mole ratio (the stoichiometric ratio). This can be seen from the equation:

 

2NaOH + H2SO4 ¨ Na2SO4 + 2H2O

 

where two moles of sodium hydroxide react with one mole of sulfuric acid (forming one mole of sodium sulfate and two moles of water).

 

If one of the reactants is present in an amount less than the stoichiometric proportion to that of the other (or those of the others), it will limit the amount of products produced. Thus, if two moles of sodium hydroxide and one-half mole of sulfuric acid are allowed to react, only one mole of the sodium hydroxide will be consumed with the half mole of acid, and only one-half mole of sodium sulfate and one mole of water will be produced. In this example, the sodium hydroxide is said to be present in excess, and sulfuric acid is called the limiting reactant (or limiting reagent).

 

Method:  For this experiment you will work with the metathetical reaction between lead (II) nitrate and potassium iodate, which produces insoluble lead (II) iodate. From the balanced reaction equation you will determine the correct stoichiometric ratio of reactants. You will then combine different weights of reactants, collect and weigh the insoluble product, and compare the results to those predicted by the equation.

 

Caution:  Remember that lead salts are extremely toxic.

 

Procedure:  A letter, corresponding to a certain amount of each of the two reactants, will be assigned to you. Obtain from the reagent shelf—in separate 50 ml beakers—quantities of those reagents approximating the values assigned. (Sample sizes will be on display on the instructorÕs desk.) Weigh out the proper amounts to the nearest milligram. Try to get with +/- .050 grams of your assigned quantity. Remember it is more important to know exactly how much of each reactant you have than to get exactly the amount assigned. Do this by transferring reagent with your spatula from your beaker to a piece of pre-weighed (or tared) weighing paper, until the amount is reached. Then, for each reagent, carefully transfer the weighed amount from the weighing paper to a separate, properly labeled, 250-ml beaker. Make sure all of each reagent is transferred to its respective beaker. To each 250-ml beaker add 100 ml of D1 water and dissolve both reagents.

 

With a stirring rod, gently pour and mix the lead (II) nitrate solution into the potassium iodate solution. Rinse the beaker with DI water to assure a quantitative transfer of lead (II) nitrate. While gently stirring the mixture, heat it up until it is almost to the boiling point, then let it settle and cool for about ten minutes. While this is cooling set up a BŸchner funnel and vacuum train. The equipment may be checked out from the stockroom and the set-up will be demonstrated by the instructor.

Weigh a piece of fiberglass 7.5 cm filter paper to the nearest milligram and record the weight. Place the filter paper into the BŸchner funnel and dampen it to make it stick. Slowly turn on the vacuum line. (Too great a vacuum will cause the paper to tear.) Pour the contents of the beaker into the funnel so that the suspended precipitate goes onto the center part of the filter. If you can keep the precipitate back from the edge, it will be much easier to remove the filter paper, dry it and weigh it without loss of any solid. Remove the remaining precipitate from the beaker by using three 25 ml portions of water and your rubber policeman to scrub the solid Pb(IO3)2 from the sides of the beaker. Allow the suction to dry the precipitate for a minute or so; then turn off the vacuum and carefully remove the filter paper and precipitate, losing as little as possible. Dry the paper and reaction product on a labeled watch glass for 30 minutes in an oven set at 110¡C.

 

Weigh the cooled filter paper, watch glass and precipitate to the nearest milligram and record. If at all possible, reheat your filter paper and precipitate for another 30 minutes. Cool and reweigh. The second weighing should agree with the first one to within .004 g.

 

Calculations: Determine the expected weight of product from the reaction equation and your actual initial weights. Compare the expected weight with your result and calculate your percent yield.

 

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                                                                                                Section                                                                       

 

                                                                                                Date                                                               

 

EXPERIMENT 6

LIMITING REAGENT STOICHIOMETRY

REPORT SHEET

 

DATA                        Assigned Letter                                              

 

Pb(NO3)2        Calculated moles to use                                 

 

KIO3               Calculated moles to use                                 

 

Actual weight of Pb(NO3)2 used.

 

            mass Pb(NO3)2 + weighing paper       =                                               Actual moles Pb(NO3)2 used

 

                                    mass weighing paper   =                                             

 

                                    mass Pb(NO3)2           =                                                                                               

 

Actual weight of KIO3 used.

 

            mass KIO3 + weighing paper              =                                                Actual moles KIO3 used:

 

                                    mass weighing paper   =                                             

 

                                    mass KIO3                  =                                                                                             

 

Weight of filter paper                                                                                      g

 

Weight of watch glass                                                                                      g

 

Weight of filter paper, watch glass                                                                  g

and precipitate 1st drying

 

Weight of filter paper, watch glass                                                                  g

and precipitate 2nd drying

 

RESULTS

Reagent present in excess                                             Limiting reactant                                          

                                                                                                            (based on actual weights used)

Expected number of moles of Pb(103)2                                              

 

Expected weight of PB(103)2                                                              g

 

Actual weight of Pb(103)2 collected                                                   g

 

Percent Yield                                                                                      

 

LIMITING REACTANT

STOICHIOMETRY

 

Problems

 

The following four questions are based on the reaction

 

            2 KC1 + 2 MnCl2 + 5 PbO2 + 4 HC1 ¨ 2 KMnO4 + 5 PbC12 + 2H2O

 

in which 25.00 g of MnC12 is reacted with 100.0 g of PbO2.

 

  1. How many grams of KMnO4 can be produced by this reaction?

 

 

 

 

 

 

 

 

 

 

  1. If 21.42 g of KMnO4 is actually produced, what is the percent yield?

 

 

 

 

 

  1. How many grams of what starting substances will be left over after the reaction?

 

 

 

 

 

 

  1. How many grams of what starting substance (i.e., MnCl2 or PbO2) must be added to the original quantities of reactants so that there will be no PbO2 nor MnC12 left over after the reaction?