1. Gravimetric Analysis
Assignment #5

2. Announcements
Thursday I will be unavailable for office hours at 3 pm; I will make myself available Thursday 9 – 10 am and Thursday 4-6 pm (assuming thesis committee breaks at 4pm)
Addition: Friday 9 – 10 am office hour

3. Gravimetric Analysis- What is It?
Definition: a precipitation or volatilization method based on the determination of weight of a substance of known composition that is chemically related to the analyte
analyte - chemical element or compound of interest

4. Gravimetric Analysis- What is It?
Reaction: aA + rR > AaRr ppt where:
a is # of moles of analyte A
r is # of moles of reagent R
AaRr is a pure, insoluble precipitate which we can dry and weigh or ignite to convert to something we can weigh
ppt=precipitate

5. T.W.Richards
1914 Nobel Prize to T.W.Richards (Harvard University) for the atomic weights of Ag, Cl, and N
Richards and his group determined atomic weights of 55 of the 92 known elements using gravimetry

6. T.W.Richards
“Every substance must be assumed to be impure, every reaction must be assumed to be incomplete, every method of measurement must be assumed to contain some constant error, until proof to the contrary can be obtained. As little as possible must be taken for granted.”

7. 7 Steps in Gravimetric Analysis
Dry and weigh sample
Dissolve sample
Add precipitating reagent in excess
Coagulate precipitate usually by heating
Filtration-separate ppt from mother liquor
Wash precipitate (peptization)
Dry and weigh to constant weight

8. Suction Filtration Filter flask Buchner funnel Filter paper Glass frit
Filter adapter
Heavy-walled rubber tubing
Water aspirator

9. Suction Filtration
Mother liquor

10. Advantages/Disadvantages
Experimentally simple and elegant
Accurate
Precise ( %)
Macroscopic technique-requires at least 10 mg ppt to collect and weigh properly
Time-consuming (1/2 day?)

11. What Do We Get Out of Gravimetry?
% of analyte, % A
%A = weight of analyte x weight of sample
weight of ppt directly obtained ->?A

12. How Do We Get %A? G.F. = a FW[analyte] b FW[precipitate]
% A = weight of ppt x gravimetric factor (G.F.) x weight of sample
G.F. = a FW[analyte] b FW[precipitate]
G.F. = # gms of analyte per 1 gm ppt

13. Gravimetric Factor X apples + Y sugar = Z apple pies
What is this relationship in chemistry?

14. The Gravimetric Factor
G.F. = a FW[analyte] b FW[precipitate]
Analyte ppt G.F. CaO CaCO3 FeS BaSO4 UO2(NO3)2.6H2O U3O8 Cr2O3 Ag2CrO4

15. Gravimetric Factor
Analyte ppt G.F. CaO CaCO3 CaO/CaCO3 FeS BaSO4 FeS/BaSO4 UO2(NO3)2 U3O8 3UO2(NO3)2/U3O8 Cr2O3 Ag2CrO4 Cr2O3/2Ag2CrO4
Naming is critically important (next class)

16. Why AgCl? Reaction is highly selective - no interferents
2AgCl ----> 2Ag + Cl2(g)
AgCl is insoluble in water, i.e., only slightly soluble in water-losses negligible
1.4 mg/L at 200C 22 mg/L at 1000C

17. Why AgCl is a Good Precipitate?
Small mass of analyte yields large mass of precipitate-sensitive technique
AgCl precipitates in curds/lumps that can be easily collected, dried, and weighed
Precipitate (ppt) is not hygroscopic

18. Problem
Consider a g sample containing 75% potassium sulfate (FW ) and 25% MSO4. The sample is dissolved and the sulfate is precipated as BaSO4 (FW ). If the BaSO4 ppt weighs , what is the atomic weight of M2+ in MSO4?
ANS: Mg2+

19. Answer
The hard part is setting up the correct equation (good stoichiometry skills are essential here!):
Rearranging and solving:

20. Problem
A mixture of mercurous chloride (FW ) and mercurous bromide (FW ) weighs 2.00 g. The mixture is quantitatively reduced to mercury metal (At wt ) which weighs 1.50 g. Calculate the % mercurous chloride and mercurous bromide in the original mixture.
ANS: g

21. Answer Again, important to set up correct equation:
Rearranging and solving: