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Gravimetric Analysis of A Chloride Salt ( Theory )


Now we will discuss about Gravimetric Analysis Of Chloride Salt, in the next article we will discuss about the practicum.

Quantitative analysis is that aspect of analytical chemistry which is concerned with determining how much of one or more constituents is present in a particular sample of material. Two common methods used in analytical chemistry are gravimetric and volumetric analysis. Gravimetric analysis derives its name from the fact that the constituent being determined can be isolated in some weighable form. Volumetric analysis , onthe other hand , derives its name from the fact that the method used to determine the amount of a constituent involves measuring the volume of a reagent . Usually , gravimetric analyses involve the following steps :

1. Drying and then accurately weighing representative sample of the material to be analyzed.
2. Dissolving the samples.
3. Precipitating the constituent in the form of a substance of known composition by adding a suitable reagent.
4. Isolating the precipitate by filtration.
5. Washing the precipitate to free it of contaminants.
6. Drying the precipitate to a constant weight ( to obtain an analytically weighable form of known composition )
7. Calculating the percentage of the desired constituent from the weights of the sample and precipitate.

Although the techniques of gravimetric analysis are applicable to a large variety of substances , we have chosen to ilustrate them with an analysis that incorporates a number of other techniques as well. Chloride ion may be quantitatively precipitated from solution by the addition of silver ion according to the following ionic equation :

Ag+  +   Cl  ⇄ AgCl

Silver chloride is quite insoluble , the addition of silver nitrate solution to an aqueous solution containing chloride ion precipitates AgCl quantitatively. The precipitate can be collected on a filter paper , dried, and weighed. From the weight of the AgCl obtained , the amount of chloride in the original sample can then be calculated.

This experiment also ilustrates the concept of stoichiometry . Stoichiometry is the determination of the proportions in which chemical elements combine and the weight relations in any chemical reaction. In this experiment stoichiometry means specifically the mol ratio of the substances entering into and resulting from the combination of Ag+  and Cl- . In the reaction of Ag+ and Cl- in equation , it can be seen that 1 mol of chloride ions react with 1 mol of silver ions to produce 1 mol of silver chloride.

Moles Cl- = Moles AgCl = grams AgCl / gram molecular weight AgCl

Grams Cl in sample = ( moles Cl- ) ( gram atomic weight Cl )

= ( gram atomic weight Cl ) ( gram AgCl ) / gram molecular weight AgCl

= ( 35,45 ). ( Grams AgCl ) / 143,3 = ( 0,2474 ) . ( Grams AgCl )

The number 0,2474 is called a gravimetric factor. It converts grams of AgCl into grams of Cl. Gravimetric factors are used repeatedly in analytical chemistry and are tabulated in handbooks. The percentage of Cl in the sample can be calculated according to the following formula :

% Cl in sample = ( grams Cl in sample ) . ( 100 ) / gram weight sample

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