Analysis Of Alum Postlab 475g1m

Analysis of Alum, AlK(SO4)2•12H2O Post-Lab David Danjul, Chem AP, Period 1-2, Mobley

Procedure: Part I. Melting Point Determination of Alum 1. pulverize a small amount (about 0.5 g) of dry alum using a mortar and pestle 2. Pack the alum in a capillary tube to a depth of about 0.5 cm. 3. Turn the tube so the open end is up and bounce the bottom of the tube on the desk. 4. Fasten the capillary tube to the thermometer with a rubber band. Alum should be level with the thermometer bulb 5. Using a thermometer clamp or a universal clamp and notched cork stopper (or split rubber stopper), fasten the thermometer to a ring stand. 6. Immerse bottom of capillary and thermometer in a beaker of water and heat. Slow the heat of the water bath as the temperature approaches the melting point of alum in order to get an accurate temperature reading of the melting point. Materials for Part 1 7. Record the temperature range. -beaker, 150 mL / Thiele 8. Repeat again, using a new sample of alum and capillary tube. -Capillary tubes, 22 Part II. Determination of the Water of Hydration in Alum Crystals. -mortar and pestle 1. Set up Bunsen burner on ring stand beneath ring clamp. -notched stopper to hold 2. Adjust height of ring clamp to 1 cm above the burner. thermometer 3. Place crucible with cover in clay triangle and heat over burner -rubber bands until red hot. -thermometer -clamp 4. Turn off gas and remove burner Materials for Part 2 5. Remove crucible and place on wire gauze with crucible tongs. -balance 6. Allow crucible and cover to cool for 10 mins minimum -Crucible and cover 15-30 mL 7. Find mass using balance scale. Handle with tongs -crucible tongs 8. Record mass -ring 9. Add 2g alum crystals to crucible. Weigh. -triangle, pipe stem 10. Set crucible at an angle and heat gently to remove the water -wire gauze of hydration. Materials for Both Part 1 and Part 2 11. After bubbling stops, heat strongly for 5 mins -gloves 12. Turn off gas and remove burner -apron 13. Remove crucible with tongs and place on gauze. -goggles -Aluminum potassium sulfate, 14. Let cool for 10 mins AlK(SO4)2*12H2O, 2.5g 15. Measure and record mass of crucible, cover, and anhydrous -bunsen burner alum -ring stand 16. Repeat drying it until a constant mass is obtained 17. Record final mass 18. Dispose anhydrous alum and clean crucible.

Analysis and Calculations: Data: Class Melting Point Data Group 1 Trial 1 91.2 C Trial 2 91.6 C 91.2 + 91.6 Average 2 = 91.4𝐶 Result: Melting Point Statistics Mean Melting Point

2 91.7 C 91.4 C 91.55 C

91.4 + 91.55 + 93.35 + 93.1 + 91.75 + 97.2 6

=93.0583

≈ 93.1 C

3 94.4 C 92.3 C 93.35 C

4 93.3 C 92.9 C 93.1 C

5 91.5 C 92.0 C 91.75 C

Deviation from Mean

6 97.6 C 96.8 C 97.2 C

Standard Deviation

(93.1 − 91.4) + (93.1 − 91.55) + (93.35 − 93.1) + (93.1 − 93.1) + (93.1 − 91.75) + (97.2 − 93.1) 6

=1.492C

√

(1.72 + 1.552 + 0 + 1.352 + 4.12 ) 6−1

=√4.785C= 2.18746C≈2C

Q-Test applied to Group 6 Data:

97.2−93.35

𝑄 = 97.2−91.4 =

3.85 5.8

= 0.66

0.66 is much larger than the table value of 0.56 (taken from the Q-Table for 6 measurements) Therefore the deviant (Group 6) may be rejected with 90% confidence that the result is in error. Data: Water of Hydration Data Group 1 2 Mols 0.00389 mol 0.00426 mol Alk(SO4)2 Mols H2O 0.0581 mol 0.0561 mol

3 0.00393 mol

4 0.00378 mol

5 0.00055 mol

6 0.0044 mol

0.0571 mol

0.0572 mol

0.008 mol

0.0521 mol

Ratio of 1:14.936 1:13.17 1:14.615 1:15.13 1:14.54 1:11.8 AlK(SO4)2 to water (not rounded) Result: Water of Hydration Statistics Mean Deviation from Mean Relative Precision . 904 + .862 + .583 + 1.098 + .508 + 2.232 . 9042 +. 8622 +. 5832 + 1.0982 +. 5082 + 2.2322 14.936 + 13.17 + 14.615 + 15.13 + 14.54 + 11.8 AlK( √( ) 6 6−1 6 SO4) =14.031833≈14.032 =1.0312 =1.292 2* H2O Q-Test applied to Group 6 Data:

13.17−11.8

1.37

𝑄 = 14.936−11.8 = 3.136=0.4369 0.4369 is larger than 0.56 (taken from the Q-Table for 6 measurements). Therefore the deviant (Group 6) may be rejected with 90% confidence that the result is in error.

Post-Lab Questions 1. Why must objects be cooled before their mass is determined on a sensitive balance? Rising hot air from the heated object will cause instability in the recorded mass of the object. It’s best to allow objects to reach room temperature before attempting to measure its mass. 2. Comment on the results of the different tests used to that the sample tested was alum. The different tests used to that the substance tested was alum was the melting point at which we knew that the crystal alum would melt at 92.5 C. The results were relatively accurate to the literature except one outlier (Group 6). For the Water of hydration test, the results were also pretty accurate except Group 6. 3. What other tests could be used to the composition of alum? There are other tests which can be made to the compositions of alum. For example, we can use the percentage of the sulfate in the alum and use it to find its mole ration and figure out how much water is in the synthesized alum. Another way is to find the freezing point or boiling point of the solution.

Result/Conclusion The experiment, which consisted of finding the samples melting point and water of hydration, was conducted in an effort to determine and analyze whether the sample given was indeed alum. Results of the experiment indicated that the melting point was 91.4 C. Compared to the class average of 93.1 C our results were relatively accurate (the class average includes the outlier from Group 6). Otherwise the average would be 92.23 C without the deviant. Compared to the literature value of 92.5 C, our results would be considered accurate, although the class average would be accurate and precise. Our percent error (100 - [(Measured melting point) / (Literature melting point) * 100]) is 1.18% and the class average percent error (with group 6) is 0.649% and without group 6 would be 0.292%. The results of the experiment also indicated that the ratio of AlK(SO4)2 to water was 14.936. The literature value reveals that it is 12. Therefore our percent error (100 * (((Mole ratio) - (actual mole ratio)) / (actual mole ratio))) is 24.46% and the class average of 14.032 has a percent error of 16.93%. The outlier, 11.8 is surprisingly the closest one to the literature value with a percent error of 1.67%. Although there was some margin of error, the experiment has generally proven that the sample was indeed alum.

Error Analysis There were several possible sources of errors while performing the experiment. While figuring out the melting point for the alum crystals, it was difficult to determine whether the alum crystals started to melt exactly due to the boiling bubbles that obstructed the view of the capillary tube. Observing the melting point of alum in the experiment was precise but not accurate since the exact point where the alum melts wasn’t really observable through the human eye. During the process of melting the alum, we did not stir while it was heating, which may have caused some unevenness in heat distribution. Also, the first trial we heated the sample too quickly which may have caused inaccuracies in determining the melting point of the alum. Because it was hard to tell whether it melted or not, and because we did not know what to expect when it should melt, our first trial was heated too quickly, and a lot of it evaporated. The second trial revealed that we added 1 cm alum instead of 0.5 cm. Although I do not believe the quanitity of alum would affect its melting point, it may have be altered the data slightly due to the different masses between trial one and trial two. The experiment may have had an error when dehydrating the alum. The alum may have not fully dehydrated, therefore it would cause the mass of anhydrous alum to increase which lead to the increase of the mole ration. In order to prevent this from happening again, the heating time should extend to at least 15 minutes. The mass of the crucible, cover, and anhydrous alum may also have errors since the crucible wasn’t completely cool at room temperature due to the time constraints of the class period. A way to resolve this issue is to use a fan to speed up the process of lowering the temperature of the crucible to room temperature in order to reduce the error of the mass of the crucible, cover, and anhydrous alum.