Gravimetric Analysis Assignment 5 Announcements Thursday I will
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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 4 pm) n Addition: Friday 9 – 10 am office hour n
Gravimetric Analysis- What is It? n 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 n analyte - chemical element or compound of interest
Gravimetric Analysis- What is It? n Reaction: a. A + r. R -----> Aa. Rr ppt where: – a is # of moles of analyte A – r is # of moles of reagent R – Aa. Rr 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 n 1914 Nobel Prize to T. W. Richards (Harvard University) for the atomic weights of Ag, Cl, and N n Richards and his group determined atomic weights of 55 of the 92 known elements using gravimetry
T. W. Richards n “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 n n n n 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 n n n Filter flask Buchner funnel Filter paper Glass frit Filter adapter Heavy-walled rubber tubing Water aspirator
Suction Filtration n Mother liquor
Advantages/Disadvantages n Experimentally simple and elegant n Accurate n Precise (0. 1 -0. 3 %) n Macroscopic technique-requires at least 10 mg ppt to collect and weigh properly n Time-consuming (1/2 day? )
What Do We Get Out of Gravimetry? n% of analyte, % A n %A = weight of analyte weight of sample n weight x 100 of ppt directly obtained ->? A
How Do We Get %A? n n n % 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 n X apples + Y sugar = Z apple pies n What is this relationship in chemistry?
The Gravimetric Factor n G. F. = a FW[analyte] b FW[precipitate] n Analyte Ca. O Fe. S UO 2(NO 3)2. 6 H 2 O Cr 2 O 3 ppt G. F. Ca. CO 3 Ba. SO 4 U 3 O 8 Ag 2 Cr. O 4
Gravimetric Factor n Analyte Ca. O Fe. S UO 2(NO 3)2 Cr 2 O 3 ppt Ca. CO 3 Ba. SO 4 U 3 O 8 Ag 2 Cr. O 4 G. F. Ca. O/Ca. CO 3 Fe. S/Ba. SO 4 3 UO 2(NO 3)2/U 3 O 8 Cr 2 O 3/2 Ag 2 Cr. O 4 n Naming is critically important (next class)
Why Ag. Cl? n Reaction is highly selective - no interferents – 2 Ag. Cl ----> 2 Ag + Cl 2(g) n Ag. Cl is insoluble in water, i. e. , only slightly soluble in water-losses negligible – 1. 4 mg/L at 200 C 22 mg/L at 1000 C
Why Ag. Cl is a Good Precipitate? n Small mass of analyte yields large mass of precipitate-sensitive technique n Ag. Cl precipitates in curds/lumps that can be easily collected, dried, and weighed n Precipitate (ppt) is not hygroscopic
Problem Consider a 1. 0000 g sample containing 75% potassium sulfate (FW 174. 25) and 25% MSO 4. The sample is dissolved and the sulfate is precipated as Ba. SO 4 (FW 233. 39). If the Ba. SO 4 ppt weighs 1. 4900, what is the atomic weight of M 2+ in MSO 4? n ANS: Mg 2+ n
Answer n The hard part is setting up the correct equation (good stoichiometry skills are essential here!): n Rearranging and solving:
Problem n n 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 n Again, important to set up correct equation: n Rearranging and solving:
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