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Goals
- Experimental determination of the mass fraction of oxygen in the compound potassium chlorate (\(\ce{KClO3}\)) by thermal decomposition of a potassium chlorate sample.
- To demonstrate qualitatively that the residue resulting from the decomposition of potassium chlorate is potassium chloride.
All compounds are made up of elements chemically combined in fixed proportions—they obey theLaw of constant composition. One way to express the proportion of each element in a compound is percent by mass or percent by mass.
In part A of this laboratory, a sample of potassium chlorate is experimentally analyzed to determine the mass percentage of elemental oxygen it contains. To do this, the potassium chlorate must be heated to temperatures above 400 °C, which causes it to thermally decompose into potassium chloride and free oxygen:
\[\ce{2KClO3 (s) ->[heat] 2KCl(s) + 3O2 (g)}\]
\[\text{Potassium Chlorate} \ce{->} \text{Potassium Chloride} + \text{Oxygen}\]
Students perform a quantitative analysis of the reactants and products of this reaction, measuring the initial mass of solid potassium chlorate used (before heating) and the mass of solid potassium chloride product or residue remaining after heating. Using the law of conservation of mass, the difference between these measured masses is the mass of oxygen released (from the original potassium chlorate sample). This data becomes theExperimental-Mass percentage of oxygen in potassium chlorate are determined:
\[\text{mass percent oxygen (experimental)} = \frac{ \text{mass released oxygen}}{ \text{mass consumed potassium chlorate}} \times 100\]
Mass percentages of elements in compounds can also be calculated theoretically using molar masses along with the compound's known chemical formula. And that's how it happenedtheoreticallyMass percent oxygen in potassium chlorate would be calculated using the expression:
\[ \text{Massenprozent Sauerstoff (theoretisch)} = \frac{3 \times \text{(Molar Mass of O)}}{ \text{Molar Mass of } \ce{KClO3}} \times 100\]
Students can therefore assess their accuracy in this experiment by comparing their experimental results with the true theoretical value and calculating their percent error.
In Part B of this laboratory, the residue remaining after heating is qualitatively analyzed to show that it is chemically different from the initial potassium chlorate sample. In particular, the residue is tested for the presence ofchloride ionsby the addition of nitric acid and aqueous silver nitrate. A positive test is indicated by the formation of awhite precipitate. The actual identity of the residue is then definitively verified by comparing this result with those obtained from identical tests on known samples of potassium chlorate and potassium chloride.
Proceedings
materials and equipment
Solid potassium chlorate (\(\ce{KClO3}\)), solid potassium chloride (\(\ce{KCl}\)), 6 M nitric acid (\(\ce{HNO3}\)), 0.1 M silver nitrate (\ (\ce{AgNO3}\)), two crucibles with lids, stand and ring clamp, clay triangle, crucible tongs, Bunsen burner, three medium-sized test tubes, test tube rack, stirring rod and an electronic balance.
Security
Be especially careful when using the Bunsen burner and handling hot equipment. Remember that most items look exactly the same whether they are hot or cold. Heat the potassium chlorate sample slowly to avoid spattering. Be aware that silver nitrate can stain and discolor the skinNitric acid can burn the skin. If any of the chemicals are spilled, rinse them under running water and report the accident to your instructor. Spilled nitric acid can also be neutralized with the sodium bicarbonate solution at the sinks.
Part A: Mass percent oxygen in potassium chlorate
The following steps should be performed for two separate samples of potassium chlorate.
- Clean both crucibles and their lids (received from storage) by rinsing them thoroughly with distilled water and then drying them as completely as possible with a paper towel.
- Weigh the first crucible and lid on an electronic scale and record this mass on your report form.
- Add about 1 gram of potassium chlorate to the crucible.Don't do this over balance!Then weigh and record the mass of the crucible, lid, and potassium chlorate sample.
- Get a stand and ring clamp from the back of the lab. As shown in the illustration and photo on the following page, place your clay triangle on the ring and then place the crucible containing the sample on the triangle. Cover the crucible with the lid.
- Heat the crucible and sample with a Bunsen burner for a total of 12 minutes. Make sure the pan is covered and that the top of the flame touches the bottom of the pan.
- For the first 6 minutes the rehearsal should besoftheated by adjusting the Bunsen burner flame to a low to medium temperature. Note that overheating the sample at this point could result in sample loss through spatter and cracking of the crucible.
- For the last 6 minutes the rehearsal should bestarkheated by setting the Bunsen burner flame to a high temperature.
- Allow the crucible to cool to room temperature. Then weigh and record the mass of the crucible, lid, and remaining residue. Note that the weight of your sample is expected to decrease by at least 30% of its original mass (~0.3g).
- Now heat the sample a second time for another 6 minutes with a high-temperature flame. Then let it cool down to room temperature again. After this second heat, weigh the cooled crucible, lid, and sample and record the mass. If this mass is within 0.050 grams of your mass measurement after initial heating (see step 6), no further heating is required and you can begin Part B.Do not discard the leftovers as you will need them for Part B.
- If the sample from step 7 is not within 0.050 grams of the mass from step 6, reheat a third time, cool and record the mass.
- Repeat all steps for your second crucible and potassium chlorate sample.
- Analyse:For each analyzed sample, determine the mass of potassium chlorate before heating, the mass of \(\ce{KCl}\) residue after heating, and the mass of evolved oxygen. For these calculations, use the residue mass after the last heat. Then, using these values, determine the experimental mass percentage of oxygen in potassium chlorate (see Theory section for the required equation).
Part B: Qualitative residue analysis
- Place three medium test tubes in the test tube rack. The test tubes should be thoroughly cleaned and rinsed with distilled water. Label them Tube #1, Tube #2, and Tube #3. Continue to use only distilled water for the remainder of Part B.
- Add a pea-sized amount to Tube #1potassium chlorate. Then fill tube #1 half full with distilled water and mix with a stir bar until the solid dissolves. If the solid does not completely dissolve, decant the clear portion of the solution into a clean test tube and label #1 instead.
- Add a pea-sized amount to Tube #2potassium chloride. Then fill tube #2 half full with distilled water and mix until the solid dissolves. Be sure to rinse your stir bar with distilled water first or you will contaminate your sample.
- Put some distilled water in your crucible andResidue. The residue should dissolve. Then pour the resulting solution into tube #3. Repeat the process until tube #3 is half full (same as tubes 1 and 2).
- Add to each of tubes #1-36 drops of nitric acidimmediately followed by6 drops of silver nitrateSolution. Then enter your observations in the report form. Discard all test tube waste in the container provided.
- Analyse:Based on your observations for these tests (and any other observations made), what evidence do you have that the residue in your crucible is really potassium chloride?
Pre-laboratory task: The composition of potassium chlorate
- In Part A of this lab, you will analyze a sample of potassium chlorate to determine the mass percentage of oxygen it contains. To perform the analysis, you will decompose the potassium chlorate by heating it. Write down the word equation and the balanced formula equation for this decomposition reaction.
- word equation:
- formula equation:
- The potassium chlorate sample is heated in a special "container".
- What is the name of this container?
- Will this container be covered or uncovered during heating?
- You must heat your potassium chlorate sample at least twice.
- How long does the sample have to be heated the first time (total)?
- How long does the sample have to be heated the second time?
- After the heating is completed, a residue of potassium chloride remains in the "container". Do you expect it to weigh more, less, or the same as the original potassium chlorate sample? Why?
- In Part A, you perform several mass measurements. What aretwoPrecautions you need to take when using the electronic scale?
- In Part B of this exercise, you will analyze the residue in the left "container" to experimentally confirm its identity. You will need three test tubes for this. Potassium chlorate is added to tube #1, potassium chloride to tube #2, and the residue to tube #3. These solids are all dissolved in distilled water.
- Wastwoare chemicals then added to each of these substances to test them?
- What will you observe if you get a positive test for chloride ions?
Laboratory report: The composition of potassium chlorate
Part A: Mass percent oxygen in potassium chlorate
experimental data
| Probe 1 | Probe 2 | |
|---|---|---|
| Mass crucible + lid | ||
| Mass crucible, lid + \(\ce{KClO3}\) | ||
| Crucible mass, lid + residue after 1st heating | ||
| Crucible mass, lid + residue after 2nd heating | ||
| Crucible mass, lid + residue after 3rd heating |
data analysis
Use your data to determine the experimental mass percentage of oxygen in \(\ce{KClO3}\). Clearly indicate your work for each step in the table below.
| Probe 1 | Probe 2 | |
|---|---|---|
| Mass of the original \(\ce{KClO3}\) sample | ||
| Mass of \(\ce{KCl}\)-Remainders | ||
| Mass of oxygen released | ||
| Mass percent oxygen in \(\ce{KClO3}\) | ||
| Average mass percent oxygen | ||
Calculate the theoretical mass percent of oxygen in \(\ce{KClO3}\) using the molar masses together with the well-known formula of potassium chlorate. Show your work clearly.
Calculate the percentage error between your average experimental value and the theoretical value for the mass percent oxygen in \(\ce{KClO3}\). Show your work clearly.
Part B: Qualitative analysis of residues
Observations and Analysis
| Rohr | Observations (after addition of nitric acid and silver nitrate) |
|---|---|
| #1: Potassium Chlorate | |
| #2: Potassium Chloride | |
| #3: Residues from Crucible |
Explain how your observations in the table above confirm that the residue in your crucible after heating is potassium chloride.
Are there anyother observationsyou made during this experiment (notthose in the table above), which would indicate that the potassium chlorate was converted to a new substance upon heating?
Questions
- Was your average experimental mass percentage of oxygen in potassium chlorate higher or lower than the theoretical value (circle one)? higher lower
Which of the following error sourcescouldused to explain this discrepancy (circle one)?
- The potassium chlorate sample was not heated strongly or long enough.
- Some of the potassium chloride product spurted out of the crucible during the heating process.
Explain your choice. Your answer should include an analysis of the calculations you performed on your raw data to get your experimental percentage of oxygen.
- Suppose the camp made a mistake and gave you a mixture of potassium chlorate and potassium chlorite. If you analyzed this mixture, would you get more or less mass percent oxygen than an equal mass sample of pure potassium chlorate (circle one)? Bigger smaller
Explain your choice. Your answer should include an analysis of the formulas of the compounds involved.
- Show your calculations clearly. Suppose you are given a sample of 36.55 g of potassium chlorate.
- What mass of oxygen should theoretically be released upon heating?
- What mass of potassium chloride residue should theoretically remain after heating?