Synthesis of Lead Chromate
Purpose
Calculate the percent yield of lead (II) chromate obtained from the reaction of a
solution of potassium chromate and a solution of lead (II) nitrate.
Background Information
When two chemical systems interact, sometimes the result is the formation
of a precipitate. Thus, the reaction that occurs goes essentially to completion.
The stoichiometry of such a reaction can be considered if solutions of known
concentration are allowed to react. For example, if solutions of potassium chromate
and of lead (II) nitrate are allowed to react, we can calculate the percent yield of
product by finding the weight of dry lead (II) chromate obtained.
The reaction involved in the experiment is shown below
K2CrO4(aq) + Pb(NO3)2(aq)
__________>
PbCrO4(s) + 2 KNO3(aq)
Inspection of the reaction reveals that one mole of potassium chromate reacts with
one mole of lead (II) nitrate to form one mole of lead (II) chromate. In all
experiments, the number of moles of a product formed will depend on the number
of moles of the limiting reactant. The stoichiometry of this reaction is such that
potassium chromate and lead (II) nitrate must react in the ratio of one mole of
potassium chromate to one mole of lead (II) nitrate. If the quantity of either
reactant is such that the ratio is not 1:1, the excess quantity of reactant will not
react. For example, if 2 mol of potassium chromate were reacted with 1 mol of lead
(II) nitrate, the potassium chromate would be present in excess, so that the number
of moles of lead (II) chromate formed would depend on the number moles of lead
(II) nitrate present. In this case, 1 mol of lead (II) chromate would be formed. Thus,
lead (II) nitrate would be the limiting reactant for this experiment.
Equations for Calculations
Number of moles of reactant=(molarity of solution) x (volume of solution in liters)
The percent yield of product can be found using the following equation
Percent yield =(Number of grams of product formed)
(Theoretical number of grams of product)
x 100
Procedure
Measure 50 mL of 0.10 M potassium chromate solution in a clean 100 mL
graduated cylinder. Transfer this solution to a clean 250 mL beaker. Thoroughly
wash the graduated cylinder with tap water.. Rinse the graduated cylinder with a 20
mL portion of distilled water so that the water contacts the entire inner surface of
the cylinder. Discard the rinse water. Repeat this procedure with two additional 20
mL portions of distilled water. Discard the rinse water.
Measure 50 mL of 0.11 M lead (ii) nitrate solution in a clean 100 mL
graduated cylinder. With continuous stirring, using a glass rod, slowly pour the lead
(II) nitrate solution into the potassium chromate solution. Continue to stir the
mixture for 5 minutes.
Weigh a clean, dry watch glass and a piece of filter paper. Record the total
mass of the watch glass and paper on the Data Sheet.
Place a clean filtering funnel in a funnel stand, or in a utility clamp attached
to a ring stand. Fold a piece of filter paper as shown in figure 1. Fold the circle of
filter paper in half. Make a second fold as shown so that the edges of the filter paper
do not quite match, as shown, and so that the angle formed by the two edges of the
paper should be about 5 to 10 degrees. Tear off a corner of the smallest section of
the filter paper. Place the torn corner of the filter paper in the paper cone, since the
mass of this piece of filter paper was included when the watch glass and filter paper
were weighed.
Place a 150 mL beaker under the funnel so that the stem of the funnel
touches the side of the beaker. Moisten the filter paper in the funnel with distilled
water from a wash bottle. Firmly press the edges of the filter paper against the
funnel. Add sufficient distilled water to the funnel to fill the stem of the funnel.
Using a medicine dropper, add a few drops of lead (II) nitrate solution to the
reaction mixture. If some precipitate forms as the additional precipitant (the lead
(II) nitrate solution) is added, slowly add an additional 10 mL portion of lead (II)
nitrate solution to the reactant mixture while stirring the reaction mixture
constantly with a glass stirring rod. Continue to stir the reaction mixture
continuously for 5 minutes. Allow the precipitate to settle to the bottom of the
beaker. Retest the reaction mixture with a few drops of the lead (II) nitrate solution
to determine if the precipitation is complete.
Decant as much of the supernatant (the liquid portion of the reaction
mixture) as possible from the precipitate into the funnel. Guide the supernatant
with a glass stirring rod from the beaker into the funnel as shown in figure 2.
After most of the of the supernatant has been filtered, add about 20 mL of
distilled water to the precipitate in the beaker by directing a stream of distilled
water from a wash bottle onto the remaining precipitate in the beaker. Allow the
precipitate to settle and decant the wash solution through the funnel, using the
procedure previously described. Wash the precipitate with two additional portions
of distilled water and each time transfer the wash solution to the funnel.
After the precipitate is washed thoroughly, transfer it from the beaker to the
funnel with the aid of a stream of distilled water from the wash bottle, as shown in
figure 3. After washing as much of the precipitate as possible out of the beaker and
into the funnel, transfer the remaining portions of the precipitate from the beaker to
the funnel by using a stirring rod fitted with a rubber policeman. After the transfer
of the precipitate is completed, direct a stream of distilled water from the wash
bottle onto the rubber policeman, holding the attached stirring rod so that the
washings go into the funnel.
Using a gentle stream of distilled water from the wash bottle, wash the
precipitate down the sides of the filter paper cone into the bottom of the cone.
Collect 1 mL of filtrate in a test tube. To the filtrate in the test tube, add 1 mL of 0.10
M potassium chromate solution to test for lead (II) ion in the filtrate. If a yellow
precipitate of lead (II) chromate forms, wash the precipitate on the filter paper by
carefully adding distilled water from the wash bottle to the funnel. Retest a second
sample of filtrate for lead (II) ion. If lead (II) ion is still present in the filtrate, wash
the precipitate again. Allow the funnel assembly to stand until all of the liquid has
passed through the filter paper.
Carefully remove the filter paper and precipitate from the funnel and place
the paper and precipitate on the preweighed watch glass. Carefully open the filter
paper out flat on the watch glass.
Label the watch glass and contents with an identifying mark. Place the watch
glass and contents in an oven at 80oC for 30 minutes. Using crucible tongs, remove
the watch glass and contents from the oven and allow them to cool in the air. When
cool, weigh the watch glass, filter paper, and precipitate. Enter this mass on the
Data Sheet. Repeat this procedure until the mass of precipitate remains constant.
Calculations
1. Calculate the number of moles of potassium chromate and lead (II) nitrate
used in this experiment.
2. Calculate the number of moles of lead (II) chromate formed in this
experiment.
3. On the basis of the limiting reactant, calculate the theoretical number of
moles of lead chromate that should have been formed in the experiment.
4. Calculate the theoretical mass of lead (II) chromate that should have been
formed in the experiment.
5. Calculate the percent yield of lead (II) chromate obtained in this experiment.