Gravimetric Analysis

Gravimetric Analysis can be used to determine the percent composition of a species in a compound. This is known as quantitative analysis. An agent (precipitating agent) is chosen which will cause the species of interest to precipitate out of solution. The precipitate is then isolated and weighed. Through a series of calculations, the amount of the species in the original sample or compound can then be determined.

1. Weigh accurately about 0.5 g of the unknown, and dissolve it in water. The volume of water is not important, but you do want to make sure the solid is completely dissolved.

2. Add the precipitating agent in the form of solution. For example, to precipitate a chloride use a soluble salt of silver (AgNO3).

3. Add the precipitating agent until no more solid seems to precipitate. Allow the mixture to sit for about 5 minutes, and then add a few drops more, observing to see if any more precipitation occurs. If you see more solid forming, keep repeating the steps until no more precipitate results. Now separate the solid from the liquid. The mixture can be filtered by vacuum filtration through a weighed filter paper. Other methods involve gravity filtration or use of a centrifuge. Allow the solid to dry until you get a constant weight for readings at subsequent times (at least overnight or through use of an oven).

4. From the mass of precipitated compound (ppt) and the formula (in this case AgCl), you can calculate the percent composition of a species (in this case Cl-) in the original sample.

5. Sample calculation:

The goal is to determine the %Cl in a sample.

Using the molar masses, determine the percent Cl- in AgCl which is 24.7%. 24.7 = (35.453/35.453+107.868)*100

The grams (g) of Cl- in the ppt = 0.247 * the weight of the ppt

%Cl in the original sample=calculated g Cl-/weight of original sample