This document describes the precipitation of lead chromate from an aqueous solution. Lead(II) ions and chromate ions react to form an insoluble yellow lead chromate precipitate and aqueous potassium nitrate. Lead chromate is commonly known as chrome yellow and has been used as a pigment since the 1800s, though it is toxic. The document provides the net ionic equation for the precipitation reaction and discusses experimental procedures for analyzing precipitates.

1. Precipitation of Lead Chromate
• By Dr. Robert D. Craig, Ph.D.

2. From instructors manual
Pb(NO3)2(aq) + K2CrO4(aq) ==> PbCrO4(s) + 2 KNO3(aq)
• Pb2+(aq) + CrO42-(aq) ==> PbCrO4(s)
• (Ksp = 2.8 x 10-13).
• Lead(II) chromate is also known as "chrome yellow"
and has been used as a yellow pigment since the
1800's. It is, however, fairly toxic. It occurs naturally in
the rare mineral "crocoite".2

3. Chrome yellow
• Pb+2 + CrO4-2 → PbCrO4(s) (yellow)

4. Chrome yellow
• Add ~20 drops of hot distilled water to the
solid precipitate from above.
• Centrifuge while hot, decant, and save both
the solution and the solid.
• Add 2-3 drops of 1 M K2CrO4 to the solution.
A bright yellow precipitate confirms the
presence of Pb2

5. • .

6. • Pb CrO4- lead Chromate is Toxic
• Pb (NO3)2 is Toxic
• K2 CrO4 –oxidizing agent-can burn skin

7. • TODAY!!!! YOU WILL CALCULATE
• THE EXPERIMENTAL YIELD, ACTUAL, AND
THEORECTICAL YEILD IN A PRECIPITATION
REACTION

8. • Gravity filtration is the method of choice to
remove solid impurities from ... they are
located under the main hood or on the back
shelves of each lab room. ...
orgchem.colorado.edu/hndbksupport/filt/filtr
ation.htm

9. Your set up !!!!

10. .

11. A precipitation reaction
• The most common
• AgNO3 + KCl  AgCl(s) + KNO3
• Some ions are spectator ions-are not involved
in the reaction

13. molarity
• The molarity of a solution is calculated by
taking the moles of solute and dividing by the
liters of solution.
• This is probably easiest to explain with
examples.

14. the molarity of this solution
• Suppose we had 1.00 mole of sucrose (it's about
342.3 grams) and proceeded to mix it into some
water. It would dissolve and make sugar water.
We keep adding water, dissolving and stirring
until all the solid was gone. We then made sure
that when everything was well-mixed, there was
exactly 1.00 liter of solution.
• What would be the molarity of this solution?

15. The answer is 1.00 mol/L. Notice that
both the units of mol and L remain.
Neither cancels.

16. What actual happens . . .
• K2CrO4 + Pb(NO3)2  PbCrO4(s) + KNO3
• The net Ionic :
• Pb2+ + CrO4(s) PbCrO4(s)

17. molarity of this solution
• When Ions that have large nuclei, or are
• Just “bulky” molecules-they precipitate out of
solution

18. Can use this web site!!!
• http://orgchem.colorado.edu/hndbksupport/f
ilt/filtration.html

19. Natural precipitation reactions
• Gall stones
• Kidney stones
• High density Lipo proteins
• Low density Lipo proteins-harden arteries

20. • PROCEDURE
• 1. To eache flask add 4 mL of 1000 ppm Pb. Bring
to 20 mL vol.. To the fifth flask add 20 mL
• of one of your soil sample digests. To one of the
first four (lead standard) flasks add 4 mL of
• 1000 ppm Zn.
• 2. If necessary, neutralize 20mL solution with
NaOH to pH 7 (use indicator paper) in a 100 mL
• beaker. Solution will be slightly cloudy. What is
the chemistry that makes it cloudy?

21. From instructors manual
• Comments on this experiment
• Solution concentrations should be known to
two significant digits
• Using volumetric glassware

22. From instructors manual
• In step 4: contiunous stirring is as a
precaution against colloid formation
• In step 6: emphasizing the importance of the
meniscus
• In step 16-19: “if the liquid level rises above
the top edge of the filter paper in the funnel,
the precipitate may be washed down the sides