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Goals
- Experimentally determine the mass fraction of oxygen in the compound potassium chlorate (\(\ce{KClO3}\)) by thermal decomposition of a sample of potassium chlorate.
- Qualitatively demonstrate that the residue resulting from the decomposition of potassium chlorate is potassium chloride.
All compounds consist of elements chemically combined in fixed proportions: they obeyLaw 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, potassium chlorate must be heated to temperatures above 400 °C, causing it to thermally decompose into potassium chloride and free oxygen:
\[\ce{2KClO3 (s) ->[kalor] 2KCl(s) + 3O2 (g)}\]
\[\text{Potassium Chloride} \ce{->} \text{Potassium Chloride} + \text{Oxygen}\]
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 in these measured masses is the mass of oxygen released (from the original potassium chlorate sample). This data becomes theExperimental-The mass fraction of oxygen in potassium chlorate is determined:
\[\text{mass percent oxygen (experimental)} = \frac{ \text{mass oxygen supplied}}{ \text{mass potassium chlorate consumed}} \times 100\]
The mass percentages of elements in compounds can also be calculated theoretically using molar masses along with the known chemical formula of the compound. therefore, thetheoreticallyThe mass percentage of 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\ ]
Therefore, students can judge their accuracy in this experiment by comparing their experimental results with the actual 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 analyzed for the presence ofchloride ionsAddition 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
Potassium chlorate solid (\(\ce{KClO3}\)), potassium chloride solid (\(\ce{KCl}\)), nitric acid 6 M (\(\ce{HNO3}\)), silver nitrate 0.1 M (\ (\ce{AgNO3}\)), two crucibles with lids, holder and ring clamp, clay triangle, crucible clamp, Bunsen burner, three medium-sized test tubes, test tube holder, stirrer and 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. Please note that silver nitrate can stain and discolor the skinNitric acid can burn the skin. If any of the chemicals are spilled, rinse them off with running water and report the accident to your instructor. Spilled nitric acid can also be neutralized with baking soda solution in sinks.
Part A: Mass Percent of Oxygen in Potassium Chlorate
The following steps must be performed for two separate potassium chlorate samples.
- Clean both crucibles and their lids (store-bought) by rinsing them well with distilled water and drying them with paper towels as completely as possible.
- 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 make it about balance!Then weigh and record the mass of the crucible, lid, and potassium chlorate sample.
- Look for a stand and ring clamp at the back of the lab. As shown in the diagram and photo on the next page, place your clay triangle in the ring and then place the crucible with the sample in 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 the top of the flame touches the bottom of the pan.
- The sample must stand for the first 6 minutessoftHeat by adjusting the Bunsen burner flame to a low to medium temperature. Note that overheating the sample at this point may cause the sample to splatter and crack the crucible.
- The rehearsal must be during the last 6 minutesstarkHeat 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 must decrease by at least 30% of its original mass (~0.3 g).
- Now heat the sample a second time for another 6 min with a high-temperature flame. Then cool back down to room temperature. 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 Part B can begin.Do not discard the waste as it is required for Part B.
- If the sample from step 7 is not within 0.050 grams of the mass from step 6, heat a third time, cool and record the mass.
- Repeat all steps for the second crucible and the second potassium chlorate sample.
- Analyse:For each sample tested, determine the mass of potassium chlorate before heating, the mass of the residue \(\ce{KCl}\) after heating, and the mass of released oxygen. For these calculations, use the residual mass after final heating. Then, using these values, determine the experimental mass percentage of oxygen in potassium chlorate (see the Theory section for the required equation).
Part B: Qualitative analysis of residues
- Place three medium test tubes in the test tube rack. Test tubes must be carefully 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 chloride. 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.
- 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 (as well 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 waste test tubes in the container provided.
- Analyse:Based on your observations for these tests (and any other observations you have made), what evidence do you have that the residue in your crucible is in fact potassium chloride?
Homework before the lab: 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, decompose potassium chlorate by heating it. Write the verbal equation and the balanced formula equation for this decomposition reaction.
- word equation:
- formula equation:
- The potassium chlorate sample is heated in a special "vessel".
- 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 should the sample be heated the first time (in total)?
- How long should the sample be heated a second time?
- After heating, a residue of potassium chloride remains in the "pot". 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. Which are theyof theWhat precautions should you take when using the electronic scale?
- In Part B of this exercise, you will analyze the debris left in the "Dumpster" to experimentally confirm its identity. You will need three test tubes for this. Potassium chlorate is placed in tube #1, potassium chloride in tube #2, and the residue in tube #3. All of these solids will dissolve in distilled water.
- Wasof theare chemicals then added to each of these substances to test them?
- What will you see if you test positive for chloride ions?
Laboratory report: The composition of potassium chlorate
Part A: Mass Percent of Oxygen in Potassium Chlorate
experimental data
| Probe 1 | Probe 2 | |
|---|---|---|
| Crucible mass + lid | ||
| Crucible mass, lid + \(\ce{KClO3}\) | ||
| Crucible mass, lid + residue after 1st heating | ||
| Crucible mass, lid + residue after the second 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 sample \(\ce{KClO3}\) | ||
| Mass-Residue \(\ce{KCl}\) | ||
| released oxygen mass | ||
| Mass fraction of oxygen in \(\ce{KClO3}\) | ||
| Mass Average Oxygen Fraction | ||
Calculate the theoretical mass fraction of oxygen in \(\ce{KClO3}\) using the molar masses together with the well-known formula for potassium chlorate. Show your work clearly.
Calculate the percentage error between your mean experimental value and the theoretical value for the mass percentage of oxygen in \(\ce{KClO3}\). Show your work clearly.
Part B: Qualitative analysis of waste
Observations and Analysis
| sugar cane | Observations (after addition of nitric acid and silver nitrate) |
|---|---|
| No. 1: Potassium chlorate | |
| #2: Potassium Chloride | |
| No. 3: Crucible residue |
Explain how your observations in the table above confirm that the residue in your crucible after heating is potassium chloride.
are there anyother observationsWhat did you do during this experiment?NOthose in the table above) suggesting that potassium chlorate transformed into a new substance when heated?
Questions
- Was your experimental mass average percentage of oxygen in potassium chlorate greater or less than the theoretical value (circle one)? bigger smaller
Which of the following error sourcesit couldused to explain this discrepancy (circle one)?
- The potassium chlorate sample was not heated strongly or long enough.
- Some of the potassium chloride product flowed out of the crucible during the heating process.
Explain your choice. Your answer should include a discussion of the calculations you performed on your raw data to get your experimental oxygen percentage.
- Suppose the store made a mistake and gave you a mixture of potassium chlorate and potassium chlorite. After analyzing this mixture, would you get a greater or lesser mass percentage of oxygen than a sample of the same mass of pure potassium chlorate (circle one)? Bigger smaller
Explain your choice. Your answer should include a discussion 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?