Cuyamaca College Potassium Chlorate Worksheet

Formal Report Check ListCover (5%)



Name
Date exp started
Exp name
/5
Abstract (10%)



Only 3 sentences, 1 paragraph
Answers why, how, your results
No pronouns
/10
Background (35%)





5 paragraph essay.
3 Concepts relating to the experiment/lecture, defined, with examples
No comments about the experiment or materials used in the experiment
Not restating the procedure
How much effort was put unto this section
/35
Lab Notebook (30%)





All parts in notebook
Observations do not have graphs, calculations, or opinions
Has a Data Analysis section for graphs, math, etc.
Prelab and Postlab questions as needed.
Did the student have the lab notebook ready for the lab?
/30
Conclusion (20%)



3 major errors inherent in the experiment.
Effects of these errors on the data.
How to prevent, minimize these errors.
/20
EXPERIMENT
6
Laurie LeBlanc
Analysis of Potassium Chlorate
The goal of this experiment is to perform a quantitative analysis of a chemical reaction. The reaction is the
decomposition of potassium chlorate. When potassium chlorate is heated over a Bunsen burner, it decomposes
to potassium chloride and elemental oxygen, according to the following equation:
potassium chlorate Ò potassium chloride + oxygen
Solid potassium chlorate contains potassium, chlorine and oxygen. Upon heating potassium chlorate,
oxygen escapes as a gas, and a solid residue, potassium chloride, is left behind. The experimental percentage
of oxygen in the original sample of potassium chlorate can be calculated using data collected when this
decomposition reaction is performed quantitatively. Its accuracy can be determined by comparing it to the
theoretical percent of oxygen calculated using the formula of potassium chlorate.
From the experimental data, you will be able to calculate the oxygen lost from your starting sample of
potassium chlorate upon heating. Using that information, you will determine your experimental percent
oxygen and ultimately calculate the error in that value using the following equation:
% Error
(|theoretical %O
experimental %O|/theoretical %O)
100 (**The
difference in the numerator is an absolute value so the % error will be positive.)
Experimental Procedure
A: Quantitative Analysis of KClO3
1 Cleaning the crucible. Place a clean, dry crucible (uncovered) on a triangle
and heat for 5 minutes at the maximum flame temperature. The tip of the
sharply-defined inner blue cone of the flame should touch and heat the
crucible bottom to redness. Allow the crucible to cool to room temperature. Handle only with crucible tongs or paper towel from now on to
avoid contamination or added weight from the oils on your hands.
Figure 6.1
29
2 Take the mass of the cooled crucible and its cover; record into your notebook. Add 1 to 1.5 g of potassium
chlorate to your crucible and replace the lid. Record the mass. The difference of these two masses will be the
weight of your potassium chlorate starting sample.
3 Decomposing potassium chlorate. Place the partially covered crucible on the triangle as shown in Figure 6.1
and heat gently until no more changes occur. This will help you to avoid the loss of molten material from the
crucible. The lid can now be placed completely over the crucible. You will then heat for an additional five
minutes at red hot to ensure that all oxygen is driven off. (Be sure that the sample is heated to a high enough
temperature by making sure that the crucible is a dull red color during this period. If not, adjust your Bunsen
burner.)
4 When finished, allow the crucible to cool for two minutes in the triangle, then grasp the crucible just below
the cover with the concave part of the tongs and very carefully transfer it to the benchtop. Allow to cool
completely to room temperature and weigh. Do NOT remove the lid from the crucible to avoid losing material
that may have adhered to the inside of the lid.
5 After weighing, reheat the first sample (without removing the crucible lid) for an additional 5 minutes at the
maximum flame temperature (bottom of the crucible heated to a dull red color); cool and reweigh. If the residue
is at constant weight, the last two masses should be in agreement. If the mass of the residue decreased by more
than 0.05 g between these two mass measurements, repeat the heating and weighing until two successive masses
agree within 0.05 g.
B: Qualitative Examination of Residue
6 After your sample residue has reached a constant mass, number and label three test tubes.
7 Preparation of three test tubes. Put a pea-sized quantity of potassium chloride into tube 1 and the same amount
of potassium chlorate into tube 2. Add about 10 mL of dI water to each of these two tubes; stopper and shake to
dissolve the salts. For test tube 3, add a small amount of your crucible residue (enough to cover the tip of your
spatula). You may need to scrape the residue out of your crucible to get enough. Add 3 mL dI water to the test
tube; stopper and shake. If the sample doesn’t dissolve within a minute, too much residue was added. Add
more water.
8 Testing with silver nitrate. Test the solution in each of the three tubes in the following way: Add 5 drops of
6 M nitric acid (which should serve to completely dissolve any undissolved solid) to each tube and then add
5 drops of 0.1 M silver nitrate solution to each. Mix thoroughly and vortex. Record your observations. This
procedure using nitric acid and silver nitrate is a general test for chloride ions. The formation of a white
precipitate is a positive test and indicates the presence of chloride ions. Be sure to record complete observations for each of the 3 test tubes.
SAFETY NOTE
7 Potassium chlorate is a strong oxidizing agent and
can cause fires if put in a trash can. Dispose of
the excess in a sink and flush with water.
7 This experiment is particularly hazardous because
of particles that may fly out of the crucible while
heating. Be sure to wear safety glasses at all
times!
7 Dispose of solutions and precipitates containing
silver in the heavy metal waste container provided.
30
Chemistry 120
While performing Experiment 6, after two successive heatings,
a student collected the following data:
Mass of crucible and lid
Mass of KClO3
(1st heating)
Mass of crucible/lid and residue
Mass of crucible/lid and residue
22.09 g
(2nd heating)
Using the information shown above, answer problems 1-6 showing all calculations and work for credit.
1 Will this student have to do a third heating? Include a calculation as proof of your answer.
2 Calculate the mass of the residue left behind in the crucible after the second heating.
3 Calculate the mass lost during heating.
4 Calculate the experimental % oxygen in the potassium chlorate sample.
5 Calculate the theoretical % oxygen in potassium chlorate.
6 Calculate the % error in the experimental % oxygen determination.
7 Write the balanced equation for the decomposition of potassium chlorate; include the states of all species in the
reaction.
8 (a) After each heating, why should you wait until your crucible is room temperature before weighing it?
(b) If you weigh a warm crucible, would this be considered a systematic or random error?
9 Potassium chlorate is a strong oxidizing agent like the sodium chlorate used in Experiment 2. How should the
student dispose of excess potassium chlorate?
10 Potassium chlorate and potassium chloride look very similar but react very differently with silver nitrate.
(a) Write the balanced equations for the reaction of silver nitrate with potassium chlorate and the reaction of silver
nitrate with potassium chloride. Indicate the states of all species using a solubility table.
(b) How might a student verify that the residue left in his crucible at the end of Experiment 6 has changed from
potassium chlorate to potassium chloride?
Cuyamaca College
Experiment 6 7 Analysis of Potassium Chlorate
31
**For questions below, answer the following questions showing
all calculations and work for credit.
Part A
EXPERIMENT 6
POST-LAB
QUESTIONS
1 What is the mass of your original sample (KClO3)?
2 Calculate the mass of oxygen lost during heating.
3 Calculate your experimental % oxygen in the potassium chlorate sample.
4 Calculate the theoretical % oxygen in the potassium chlorate sample.
5 Calculate the % error in your experimental % oxygen determination.
6 If you had forgotten to read the label on the jar carefully and put potassium chloride in the crucible instead of
potassium chlorate, how would your results have been affected?
7 (a) If your sample of potassium chlorate was contaminated with potassium chloride, how would your % oxygen
change?
(b) How, specifically, might this error occur in the lab?
8 (a) If some of your potassium chlorate popped out of the crucible while you were heating it, how would your %
oxygen have been affected?
(b) What systematic error might a student have done to cause this to happen?
Part B
9 In Part B, what observations did you make that led you to believe your residue was potassium chloride? Explain
fully for credit.
10 A student hasn’t properly washed her test tubes (1-3) used for Part B of Experiment 6 and they are contaminated
with sodium chloride (NaCl). How will this negatively affect her results for this portion of the lab?
32
Chemistry 120




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