Gravimetric analysis is the term for a group of techniques that allow analytical chemists to identify unknowns by measuring mass.  The simplest type of gravimetric analysis involves the formation and collection of a precipitate.  In this lab, we are going to react silver nitrate with an unknown chloride compound.  By collecting and massing the silver chloride precipitate, we can calculate the amount of chloride ion present.  We can then use this information to determine the percentage of chlorine in the original unknown.  This will help us to identify the unknown.

Silver nitrate is expensive, so don't waste any!  We use it because it forms insoluble salts with most anions.  The general reaction will be

	XCln (aq) + AgNO3 (aq)  X(NO3)n (aq) + AgCl (s)

X is the unknown metal ion that you're looking for.  Since we don't know the ion, we don't know its charge and therefore don't know the subscript on the chloride and nitrate ions.  Luckily, our mole calculation involves only the silver chloride product, so we don't need to know the other subscripts or even have a balanced equation.

The silver chloride product is a solid, which we call a precipitate.  We need to collect it and find its mass.  We can separate the solid from the liquid by filtration, which is simply pouring the mixture through a porous filter paper.  The solid particles get trapped in the paper while the liquid passes through into a new beaker.

The problem is that some solid particles may be small enough to slip through the pores of the paper.  Before filtering, we increase the size of the solid particles by a process called digestion of the precipitate.  The mixture is heated to the boiling point and then allowed to cool slowly.  Some of the precipitate dissolves back into the hot solution, then recrystallizes when it cools.  The process works because the size of the crystals formed depends on the speed with which they form.  Since we cool the solution slowly, the crystals have time to grow large.

Methodology

1.	Obtain roughly 0.5 g of an unknown chloride compound.  Be sure to record which unknown you took.

2.	Dissolve the unknown in 20 mL of distilled water.  Make sure it is completely dissolved before proceeding to the next step.  You may add more water if needed to make it dissolve.  Acidify the solution with 1 mL of HNO3.

3.	Pour 10 mL of 1.0 M silver nitrate solution into the unknown sample.

4.	Digest the precipitate by bringing the solution to a boil, then allowing it to cool.

5.	While the solution is cooling, dry a piece of filter paper in the oven.  When it is dry, measure its mass.  Do not touch it with your hands once it is dry.  

6.	When the solution has cooled, pour it through the filter paper, making sure that all the precipitate is collected by the paper.  Wash the precipitate with small amounts of distilled water.

7.	Test the filtered liquid with a drop of silver nitrate.  If it turns cloudy, chloride ion is still present in the sample.  Add another 5 mL of silver nitrate and repeat the digestion and filtration steps.

8.	Gently warm your precipitate until it appears dry, then find its mass.

9.	Reheat the precipitate for 5 minutes and mass again.  Continue to reheat and mass the precipitate until the last two readings agree to within ±0.02 g.  This is called reheating to a constant mass.

Analysis

The precipitate is silver chloride.  You know the mass of the precipitate, so you can convert it to moles.  Since each silver chloride formula contains only one chloride ion, that is also the number of moles of chloride.  Now here’s the trick.  The only source of chloride in the beaker was the unknown compound, so the moles you calculated are also the number of moles of chloride in the original compound!  You can multiply the number of moles by the atomic mass of chlorine to find the number of grams of chloride in the original unknown.  You know the grams of chloride and the total grams of the sample (which was roughly 0.5 g), so divide them to get the percent chlorine.  Figure the percent compositions of the compounds that it could have been and see which is the best fit.  Identify the compound and do a relative error on the chloride percentage.

Questions

1.	Write a balanced equation the reaction of your unknown with silver nitrate.

2.	If you used tap water instead of distilled water, how would that affect your results?

3.	Silver nitrate is very expensive.  Could you suggest a less costly substitute that would work in this experiment?

4.	What would happen if you filtered the solution while it was still hot?

5.	A student is given 0.52 g of an unknown chloride compound and is told that the metal comes from group IIA on the Periodic Table.  He performs this experiment and collects 0.50 g of silver chloride precipitate.  After a few moments of calculation, the student runs from the lab in terror.  Go through his calculations and explain why he is terrified.