Practical - Lab report 1 - 2014/2015 PDF

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lab report 1 - 2014/2015...

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Purpose: The ultimate goal is to determine the identity of an unknown solution. In order to identify the solution successfully, data from physical properties such as density and boiling points are needed to analyze. To accurately determine density and boiling points of the solution, series of repeated mass measurements, volume measurements, and temperature measurements were completed using a graduated cylinder, a burette, a pipette, and a Bunsen burner. Unknown liquid code: X68 Results: Sample Calculations To calculate mass of liquid for all intruments, simply subtract total mass (Mass cylinder + liquid) to mass of empty device. This method is used to calculate mass using all intruments in the experiment. Trial 1 of table 1: Mass of liquid = 83.16g – 75.69g = 7.47 g, solution X68

To calculate density of liquid, divide total liquid mass over total liquid volume. This method is used to calculate density values using all intruments in the experminent. Trial 1 of table 1: Density of liquid = 11.46g/ 15.5ml = 0.739g/ml, solution X68 To calculate the average density, 2 points located on the line of best fit of the graph Mass vs. Volume were used. y2 − y1 23.41 −15.02 Slope = = = 0.777 g/ml , solution X68. 31.0 −20.2 x2 − x1 To calculate average temperature, add all 3 values then divide by 3. 85.5+ 80.5 + 78.5 Average temperature = = 81.5 °C 3 Results: Using different intruments. Graduated Cylinder: The average value for density of solution X68 measured by a graduated cylinder is 0.754 g/ml. This value can be calculated by combining the three trial values in Table 1 indicated on the first data sheet, then dividing by three to get the result. Volumetric pipette: The average value for density of solution X68 measured by a volumetric pipette is 0.738 g/ml. This value can be calculated by combining the three trial values in Table 2 indicated on the second data sheet, then dividing by three to get the result. Burette: The average value for density of solution X68 measured by a burette is 0.793 g/ml. This value can be calculated by combining the three trial values in Table 3 indicated on the third data sheet, then dividing by three to get the result. Graph 1: This graph represents the relationship between Mass vs. Volume of solution X68. The slope of this graph represents the density of solution X68.

Mass vs. Volume of solution X68 40 35

Mass (g)

30 25 20 15 10 5 0

5

10

15

20

25

30

35

40

45

50

Volume (ml)

The calculated density of solution X68 is

23.41 −15.02 31.0 −20.2

= 0.777 g/ml

Average boiling point: The average boiling point of solution X68 is 81.5°C. Results from all trials were used in the calculation. Identity of solution X68: The combination of average boiling point value, density value, and qualitative observations suggest that solution X68 is cyclohexane. Its average density value was 0.777 g/ml, slightly smaller than the theoretical density value of 0.779 g/ml. Discussion: Determination of identity for an unknown solution can be done in many different ways. For this particular experiment, measurements of density and boiling points were used to identify solution X68.The density results were also crucial in identifying the solution, it represents the compactness of the liquid. Density data was somewhat important in identifying the solution because every pure substance has its own unique characteristic compared to other substances. My solution’s density was calculated to be 0.777 g/ml, only 0.002 g/ml less than the theoretical density of cyclohexane. However, density goes along with temperature, the theoretical density was measured at 20°C and the experimental result was measured at room temperature which can range from 21°C - 26°C. Thus, this is a random error that will affect to the probability of a correct answer. The boiling point was also important, it can be used to compare between the literature value and experimental value to correctly identify the liquid. The range of boiling points was from 56.5 °C - 100°C among the available solutions so it was beneficial to have specific boiling points to narrow down the results. The solution’s physical properties such as color and transparency were important in describing the solution as well. The clear color and its liquid state are characteristics of cyclohexane according to literature information. The data that was somewhat valuable was the smell of the solution. Its irritating smell resembled the smell of nail polish remover. In contrast to the literature odor information, it was hard to say that it

smelled like chloroform. Thus, the smell of a solution X68 is not a concrete support in determining its identity. The density was obtained by collecting mass and volume data of the solution. It was simple to measure the mass using the digital scale. The volumes were determined using three devices including a graduated cylinder, a volumetric pipette, and a burette. The accuracy varies among these three devices. First of all, the graduated cylinder’s accuracy was low because the readings were on a larger scale compared to other devices. It was difficult to pour the liquid into the cylinder and inspect how much has been poured in at the same time. The literature relative precision for graduated cylinder was 5 parts in 100, which means there is 5% error. Next, the volumetric pipette provided exceptional precision of 1 part in 1000, which means there is 0.1% error. However, its method of measuring was not efficient because using the index finger to release excess liquid so that the bottom of meniscus lies above calibration mark was difficult. Lastly, the burette was the easiest device to accurately measure solution X’s volume. The device allowed easy adjustments along with precise readings. Its relative precision was 4 parts in 1000, meaning there is a 0.4% error. The pipette was the most accurate with 0.1% error but I would use the burette for future laboratories based on its ease of use. On the other hand, there were many sources of error in the experiment. Temperature was a source of error because it affected the probability of the correct answer. The theoretical density values were measured at 20°C, and the laboratory temperature could have ranged from 21°C -26 °C. Thus, it is inaccurate to compare the two values to get the correct solution because higher temperature tends to increase the density value. The nature of the devices is another source of error. Specifically, the reading on the graduated cylinder was not detailed enough, meaning that the calculated density value may be different from the literature value. Answer question 4 Accuracy of boiling points must be consistent so identify the solution X68. The result accuracy could be affected by the atmospheric pressure by a small degree. The laboratory’s pressure was given to be 101.4 kPa, higher than the STP value of 101.3 kPa (MSDS, 2007). Higher pressure results in higher boiling points. However, experimental and literature values can be compared because the difference of atmospheric pressure is small. The data recorded for each trial range from 78°C – 85.5°C. The temperature range was large because it was difficult to observe the exact temperature at which the liquid emerges into the capillary tube because of its clear color blending with its surrounding. When comparing the accuracy of determinations between density and boiling points, the density determination was more accurate. It was obvious that the wide range of results from Table 4 alone can prove determination of boiling point is less accurate. The temperature range was from 78°C – 85.5°C, whereas the density values remained consistent at around 0.777 g/ml. Detailed measurements can be obtained when determining the density using the devices. These devices provide reliable data that usually are not affected by random errors such as atmospheric pressure. On the other hand, determining boiling point relied on the observer; he/she could have missed the sudden rise of solution X68 inside the capillary tube and recorded incorrect temperature. In addition, boiling point determination can also be affected by atmospheric pressure. To conclude, determination of density was more accurate because it has a lower chance of error. To increase the reliability of this experiment, a modification should be made in the boiling point determination process. It was stated in the lab manual to “use hot water from the previous step” (pg. 48), the results could be more reliable if the water is renewed for every trial.

Renewing the water will cool the beaker itself and the test tube that contained the solution X68. It would be more logical to have the water and the solution at around the same temperature because some chemicals boil at lower temperatures (ie. 2-propanone boils at 56°C). If hot water was used from the previous step, and his/her chemical was 2-propanone, then the observer may never see liquid rising up the capillary tube. Conclusion: The unknown solution X68 was determined to be cyclohexane. The average density was 0.777 g/ml, 0.002g/ml less than the literature value of 0.779 g/ml. The average boiling point was 81.5 °C, 0.7°C larger than the literature value of 80.7°C.

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Table 1: Density of solution X68 determined when using a graduated cylinder.

Volume of liquid Mass of empty cylinder Mass of cylinder + liquid Mass liquid Density of liquid

Trial 1 15.5ml 72.66g 84.12g 11.46g 0.739g/ml

Trial 2 31.0ml N/A 96.07g 23.41g 0.755g/ml

Trial 3 44.5ml N/A 106.82g 34.16g 0.768g/ml

Table 2: Density of solution X68 determined when using a 10ml volumetric pipette. Trial 1

Trial 2

Trial 3

Volume of liquid Mass of empty cylinder Mass of cylinder + liquid Mass liquid Density of liquid

10.2ml 75.69g 83.16g 7.47g 0.732g/ml

20.2ml N/A 90.71g 15.05g 0.743g/ml

30.4ml N/A 98.12g 22.43g 0.738g/ml

Trial 2 13.0 ml 28.9 ml 15.9 ml 28.9 ml N/A 98.39g 22.7g 0.7855 g/ml

Trial 3 28.9 ml 42.0 ml 13.1 ml 42.0 ml N/A 108.22g 32.53g 0.7750 g/ml

Table 3: Density of solution X68 determined using a burette. Initial burette reading Final burette reading Volume of liquid added Total volume of liquid in beaker Mass of empty beaker Mass of beaker + liquid Mass of liquid Density of liquid

Trial 1 0 ml 13.0 ml 13.0 ml 13.0ml 75.69g 86.32g 10.63g 0.8177 g/ml

Table 4: Boiling point determinations for solution X68

Boiling point

Trial 1 85.5 °C

Trial 2 80.5 °C

Trial 3 78.5 °C

Discussion: Little amount of liquid was spilled during trial 1 of the transferring process of solution X into the beaker using the burette; it resulted in a relatively high density value of 0.8177 g/ml. The reason that it was high is because the correct volume was secreted out on the burette, but not all of the volume was collected in the beaker due to spilling. Therefore, it resulted in a smaller mass and the density value increased for that trial. However, there is uncertainty associated with the apparatus....