Selective precipitation of ions
        In this experiment, you will be given a solid mixture of three solid components, Na2CO3, NaCl and KNO3. You must determine the mass% of each in the original solid sample. We will use selective precipitation to separate the various substances and gravimetric analysis to determine the amount of each substance in the mixture.
     Gravimetric analysis, by definition, includes all methods of analysis in which the final stage of the analysis involves weighing. In the most basic case, this could involve simply heating a sample to dryness and weighing to determine the amount of volatile components. In this account, however, I shall limit myself to gravimetric methods which rely on the use of precipitation reactions.
     The quantitative determination of a substance by precipitation followed by isolation and weighing of the precipitate is called gravimetric analysis. The basic method of gravimetric analysis is fairly straightforward. A weighed sample is dissolved after which an excess of a precipitating agent is added. The precipitate which forms is filtered, dried or ignited and weighed. From the mass and known composition of the precipitate, the amount of the original ion can be determined.

For successful determinations the following criteria must be met;
1.    The desired substance must be completely precipitated. In most determinations the precipitate is of such low solubility that losses from dissolution are negligible. An additional factor is the "common ion" effect, this further reduces the solubility of the precipitate. When Ag+ is precipitated out by addition of Cl-
Ag+ + Cl- <-> AgCl(s)
the (low) solubility of AgCl is reduced still further by the excess of Cl- which is added, pushing the equilibrium to the right.
2.    The weighed form of the product should be of known composition.
3.    The product should be "pure" and easily filtered. It is usually difficult to obtain a product which is "pure", i.e. one which is free from impurities but careful precipitation and sufficient washing helps reduce the level of impurity.
In this experiment, you will be given access to a master solution that contains a known TOTAL mass of solids and volume of water used to dissolve said solids. The three compounds dissolved,  Na2CO3, NaCl, and KNO3 will be in some unknown mass ratio (which is really what youi are trying to find in this experiment) No mass ratio of any one compoenent is greater than 50%. You will take a small (<1mL) sample of this master solution, add 1.0 M CuSO4 to selectively precipitate one of the dissolved ions (which one?) then to the filter solution, you will add a solution of 1.0 M AgNO3 to precipitate one of the other ions (which one?) The third component will be determined by process of elimination.

Lab report: Short form memo

Pre-lab questions: Read them very carefully!!
1.    When CuSO4(aq) is added to our dissolved unknown solution, what will be the identity of the precipitate?
2.    Assuming there is 0.342 g of NaCl in the original sample, what is the minimum V of 1.0 M AgNO3(aq) you need to add to completely precipitate all the dissolved Cl ion as AgCl?
3.    In a particular experiment, 1.034 g of sample was dissolved.  After addition of excess amount of AgNO3, there was 0.322 g of AgCl recovered. What is the mass % NaCl in the original sample?
4. Write the puprose statement for your short form memo of this experiment.

~MEO 30Oct08