Gravimetric Analysis Assignment #5

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

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

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

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

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 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

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

Suction Filtration Mother liquor

Advantages/Disadvantages Experimentally simple and elegant Accurate Precise (0.1-0.3 %) Macroscopic technique-requires at least 10 mg ppt to collect and weigh properly Time-consuming (1/2 day?)

What Do We Get Out of Gravimetry? % of analyte, % A %A = weight of analyte x 100 weight of sample weight of ppt directly obtained ->?A

How Do We Get %A? G.F. = a FW[analyte] b FW[precipitate] % A = weight of ppt x gravimetric factor (G.F.) x 100 weight of sample G.F. = a FW[analyte] b FW[precipitate] G.F. = # gms of analyte per 1 gm ppt

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

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

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)

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

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

Problem Consider a 1.0000 g sample containing 75% potassium sulfate (FW 174.25) and 25% MSO4. The sample is dissolved and the sulfate is precipated as BaSO4 (FW 233.39). If the BaSO4 ppt weighs 1.4900, what is the atomic weight of M2+ in MSO4? ANS: Mg2+

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

Problem A mixture of mercurous chloride (FW 472.09) and mercurous bromide (FW 560.99) weighs 2.00 g. The mixture is quantitatively reduced to mercury metal (At wt 200.59) which weighs 1.50 g. Calculate the % mercurous chloride and mercurous bromide in the original mixture. ANS: 0.5182 g

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