Sample Report: Chemistry 111 Laboratory Section B,
Spring 1998
Paul T. Jackson
Kristin St. Olaf, Partner
Chem 111 Lab B
Experiment #1
February 13, 1997
Introduction.
This laboratory experiment illustrates how to use the properties
of inorganic ions to identify the chemical composition of unknown
solutions. Generally, inorganic compounds exist as ions when dissolved
in water; an ion consists of an atom or group of atoms with a
+ or - electrical charge. Often the resulting solution is clear
and may possess some color indicative of the compound dissolved.
However, when certain combinations of inorganic ions encounter
each other in solution, they interact and form a solid (called
a precipitate). The formation of precipitates does not happen
sporadically, but follows well established rules. One can use
the solubility rules and the observed interaction of solutions,
to ascertain the identity of the inorganic materials present in
the initial "unknown" solutions.
Experimental.
One alteration to the published procedure was implemented. Since
reagent solutions were shared among lab members, the solutions
were not added in order from A to H, but were added in a random
sequence due to local availability. The identity of solutions
in particular rows and columns were maintained as published in
the lab manual. Additionally, a white piece of paper was used
under the 96 well assay tray to aid in determining precipitate
or solution color.
Results.
After adding reagents to the appropriate wells in the assay tray,
interactions between some combinations of inorganic ions were
observed. Not all combinations produced a precipitate or resulted
in a color change - both indicators of chemical reactions. Table
1 shows the results obtained from the 8 x 8 grid in our assay
tray. Colors of precipitates are noted in the table below (lt
= light, brk = brick, yel = yellow, NR = no reaction, -- = ions
remain in solution).
Surprisingly, no precipitate was observed when solution F was
combined with any of the other unknown solutions. This suggests
that all combinations of these ions are completely soluble in
water. The diagonal of the assay tray also serves as a control
in the experiment and a base with which to compare other solution
combinations.
Discussion.
Using the solubility rules, the list of solutions, and the description
of all possible precipitates, listed in the manual as a guide,
one can deduce the identity of the inorganic substance in each
of the eight solutions (A-H). The approach used starts with the
most identifiable precipitate and branches out from there. The
brick-red precipitate was identified as silver chromate; thus,
chromate and silver ions must be present in either solution D
or H. The second most characteristic precipitate was the brown
of silver hydroxide. Since this precipitate is the wells containing
solution D and C, solution D must contain the silver ion source,
solution H the chromate ion and solution C the hydroxide. We now
have 3 of the eight species identified.
We can continue identifying chemical species with the precipitates
associated with hydroxide ion. Only lead(II)hydroxide is a possible
precipitate and it occurs when C is combined with A. Thus, A contains
lead ions. Lead iodide is a characteristic yellow color and appears
when solution A combines with solution G. Therefore, we deduced
that iodide is in solution G. We now know all precipitates with
iodide; we need a different solid to sort out the last three solutions.
After looking through the solubility rules and precipitate colors,
we noted that barium makes a light yellow solid when combined
with chromate and a white solid when combined with sulfate. This
pattern is noted when B is mixed with H and when B is mixed with
E. Hence, solution B contains barium and solution E contains sulfate.
As mentioned earlier, solution F does not yield a precipitate
so all species are soluble. This must be the potassium nitrate
solution! We have now identified all species by deductive reasoning
through identification of the precipitates by employing solubility
rules and the list of possible reagents. A summary of the results
is shown in Table 2.
| Identity | |
| Lead(II)nitrate | |
| Barium chloride | |
| Sodium hydroxide | |
| Silver(I)nitrate | |
| Aluminum sulfate | |
| Potassium nitrate | |
| Sodium iodide | |
| Potassium chromate | |
Created 24 February 1998 by Paul T. Jackson