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.
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
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;
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-
+ 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.
The weighed form of the product should be of known
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
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
Lab report: Short form memo
Pre-lab questions: Read them very
1. When CuSO4(aq) is
added to our dissolved unknown solution, what will be the identity of
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?
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.