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 pptwhere: a is # of moles of analyte A r is # of moles of reagent R AaRr is a pure, insoluble precipitatewhich 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…