STUDIES OF A PRECIPITATION REACTION
Name
Lab Section
Lab Partner
Date of Lab
Instructor Signature_____
Problem Statement: What is the nature of the reaction between calcium nitrate and potassium iodate?
I. Data Collection: Interaction of Calcium Nitrate and Potassium Iodate
A. Obtain, in clean labeled beakers, about 40 mL of Ca(NO3)2 and 60 mL of KIO3 solution. Record
the precise molar concentrations of these stock solutions in the table. Using volumetric pipets
with bulbs, prepare the following solutions in two clean labeled beakers.
Solution 1: 25.0 mL Ca(NO3)2 + 25.0 mL KIO3
Solution 2: 15.0 mL H2O + 10.0 mL Ca(NO3)2 + 25.0 mL KIO3
Mix each solution with a separate stirring rod, rubbing the tip of the rod against the bottom or
sides of the beakers (a process called scratching the solution). Record your observations below.
Let these solutions stand for 15-20 minutes, stirring them occasionally. (You will want to
recover all the precipitate later, so it is best to leave the stirring rods in the solutions – do
not take them out or move them between containers.) In the meantime proceed with part I.B.
B.
Assuming the reaction involves the coming together of dissolved ions, what are the
possible identities of the precipitate formed? Think up and carry out (using other chemicals
available in the hood) experiments which will distinguish between the possibilities. Describe
what you did and the results of these experiments.
C.
Write a balanced chemical equation that represents the reaction and is consistent with the
data obtained so far.
II. Data Collection: Weight of the Precipitates
A. Label (write your initials in PENCIL) and then weigh two pieces of filter paper. Record these masses
in the table below. Following the directions from pre-lab discussion, filter each solution, catching the
supernatants (the clear liquids which pass through the filter paper) in labeled, clean beakers.
Using the appropriate supernatants, wash out the original beaker onto the filter paper to transfer any
precipitate that remains in the beaker or on the rod. When all the precipitates have been trapped in the
Experiment derived from lab I-1 in Inquiries in Chemistry Waveland Press
Page 1 of 3
paper, set the supernatant solutions aside for later use.
Rinse the precipitates with four separate 5 mL portions of methanol. (CAUTION!!! The methanol
is flammable—use no open flames and no smoking in the lab.) Gently remove the filter papers from the
funnels and place them on watch glasses. Let these air dry for about 5 minutes; then place them in a 90110 °C drying oven – did you remember to label them? Leave them in the oven to dry for at least 30
minutes; then cool, weigh, and record the masses in the table. In the meantime proceed with the
remaining parts of the experiment.
Concentration of Stock Ca(NO3)2
__________________________________
Concentration of Stock KI03
__________________________________
SOLUTION #1
SOLUTION #2
a.
mL Ca(NO3)2 solution
_____________________
_____________________
b.
mL KI03 solution
_____________________
_____________________
c.
mass of filter paper
_____________________
_____________________
d.
mass of paper and ppt.
_____________________
_____________________
e.
mass of ppt.
_____________________
_____________________
f.
moles of Ca(NO3)2
_____________________
_____________________
g.
moles of KI03
_____________________
_____________________
h.
expected mass of ppt.
_____________________
_____________________
B. Calculate the number of moles of Ca(NO 3)2 and KI03 added to each solution. Using these amounts
and the chemical equation you deduced in part I.C, calculate the expected mass of the precipitates (Is
there a limiting reagent in each case?). Show your calculations and reasoning below and record the
numbers in parts f, g, h of the table.
III. Data Collection: Tests of the Supernatants
A. Obtain about 5 mL of 0.3 M Na3PO4, 5 mL of 1 M HCl and about 2 cm3 of solid KI. Place about 1
mL of the stock Ca(NO3)2 and KIO3 solutions in separate test tubes.
Ca+2 test: add about 10 drops ( ½ mL) of Na3PO4 solution to the 1 mL of Ca(NO) 2 solution.
Describe the results.
The reaction is: 3Ca2+(aq) + 2PO43-(aq)  Ca3(PO4)2(s).
I03 - test: Dissolve about ½ cm3 of KI into the 1 mL of KIO3 solution; then add about 1 mL of 1
M HCl. Describe the results.
The reaction is IO3 (aq) + 5I-(aq) + 6H+(aq)  3I2(aq) + 3H2O.
B.
Using these tests, check the two supernatants from part II.A for the presence of Ca t and 103.
Describe the results.
IV Dropwise Mixing
Using a dropper add the stock Ca(NO 3)2 solution 5 to 10 drops at a time to your remaining sample
of stock K103 solution. After each addition stir with scratching (rubbing the stirring rod against the
beaker bottom or sides). Record your observations after each addition.
V Data Analysis
A. How do the actual precipitate masses compare to the masses you predicted by calculation?
Calculate the percentage error assuming your theoretical yield is the correct value. If they do not
compare well, make a list of at least 4 possible explanations.
B. Are the test results for the presence of Ca+2 and IO3-1 in the supernatants consistent with the limiting
reagent, excess reagent calculations in part II.B? Are there any inconsistencies between the
calculated and test results? Explain your reasoning.
C. Describe the results of the Dropwise Mixing experiment. Were the results unexpected in any way?
(i.e., given the chemical equation deduced in part I.C, what should have happened with the first
addition of Ca(NO 3 ) 2 to KIO3 ?) Explain your reasoning.