-corrected-the Molar Mass Of Volatile Liquid 406954

Lab report Determinating the mollar mass of a Volatile liquid

Student: Katarina lalović Professor: Jasminka Omerović

Date: 28.11.2012.

Aim: The aim of this experiment is to evaporate a sample of a liquid substance and measure certain physical properties of the substance as it condenses and to determine its molar mass.

Materials: 1. materials: hot bath 3 test tubes aluminium foil holder of a test tube needle analytic balance tissues googles gloves labcoat 2. chemicals: unknown volatile liquid ice cubes water

Hypothesis: As the unknown liquid evaporates the data obtained used in the ideal gas equation should dfetermine which substance it is.

Variables: Dependent variables: mass of the liquid Independent variables: molar mass of the unknown volatile liquid Control variables: temperature, pressure, time.

Procedure: A small test tube was covered with a piece of aluminium foil. A needle was used to make a hole in the aluminium foil. The sensitive balance was used to measure the mass of test tube covered with the foil. A hot water bath was prepared for this experiment. Now, a liqud sample of an unknown volatile compound was obtained. It was poured about 0.5 ml of the liquid into the test tube, which was then quickly covered with the aluminium foil. Then, test tube was placed in the hot-water bath so that the aluinium foil was above the water level. Test tube was heated to boiling untill the liquid vaporized. When it vapourized, test tube was held in the bath for three more minutes. For that time, the ice-water bath was prepared (ice cubes were put in the water). Then, test tube was taken out. The temperature and pressure were recorded and will be used in the ideal gas law calculations. The test tube holder was used to quickly transfer test tube to the ice water bath. Tets tube was cool for one minute, then removed and dried completely. Mass of the test tube was measured again after the cooling. Now, for the end, the test tube was rinsed and filled with tap water till the top and was covered with aluminium foil. Then, the mass of test tube, auminium foil and water were measured and recorded. At the end we repeated the procedure two more times to get more precise results.

Safety: For this experiment, googles, labcoat and gloves were necessary to prevent the possibility to get burns and injuries.

Results : Data collection Qualitative: The liquid was colorless and had a sharp smell. It vaporizes at the temperature about 88.8 ᵒC. Quantitative: Test tube+foil/g (m1) Test tube+foil after heating/g (m2) Test tube+foil+tap water/g Temperature/°C Pressure /kPa*

Trial 1 19.458

Measurements Trial 2 Trial 3 19.266 19.453

Average

Error

Unc/%

19.392

0.001 g

±0.005

19.497

19.312

19.499

19.436

0.001 g

±0.005

43.210

42.563

42.899

42.890

0.001 g

±0.002

89.5

87.4

88.8 100.78

88.5

0.5 °C

±0.5

*unc=(error/value)*100 * p = const.

Data processing: In order to calculate the molar mass of volatile liquid, ideal gas equation is used pV=nRT, where: p – average room pressure; p = 100. 78 kPa * 103 = 100 780 Pa V – average volume of test tube used in experiment, the space which gas occupied after vaporization; mass of the test tube will be multiplied by 10 -6, to get the volume in m3; V = 42.890 * 0.000001; therefor V= 0.000042890 m3 R – gasseous constant; R = 8.314 J/Kmol T – average temperature needed for vaporization of the liquid; T = 88.8 + 273 = 361 K First, number of moles will be calculated: n = pV / RT n = 100 780 Pa * 0.000042890 m3 / 8.314 J/Kmol * 361 K n = 4 Pa m3 / 3001 J/mol n = 0.001 mol Number of moles of the liquid that vaporized is 0.001 mol. Now, when we have this number, we can calculate molar mass: n=m/MM=m/n M = (m2 – m1) / 0.001 mol M = 0.044 g / 0.001 mol  M = 44 g/mol

At, the end, molar mass of my volatile liquid is 44 g/mol. The book value for this experiment was 46.07 g/mol, which is bigger than mine result, but according to mistakes I’ve made during the experiment, the difference is not that big. Considering all the possible mistakes my final result is 44 g/mol ± 0.5 %.

Conclusion: book value−experimental value ∗100 = book value 46.07−44 ∗100 = = 46.07 2 = *100 = 46.07 = 0.04 * 100 = 4 %

Total % error =

Total error is 4 % . Random error = ∑ of all unc. = ± ( 0.005 + 0.005 + 0.002 + 0.5) = ± 0.5 % Random error is ± 0.5 % . Systematic error = total % error – random error = 4 – 0.5 = 3 % Systematic error is 3 %.

Evaluation: According to all the data I’ve collected, this liquid fits into the description of ethanol . Ethanol, with formula C2H5OH, is an alcohol with molar mass 46.07 g/mol, it vaporizes on the temperature around 78.9 °C and it is colourless, as mine liquid. In this experiment, lots of different mistakes, could have happened without paying attention on some details. First mistake, was mine left-handedness. Maybe, I didn’t pay attention on how much liquid I put in the test tube, that is needed to vaporize. I put few drops less, which can lower the mass of the gas that occupied the test tube after vaporization, but also, a mistake could be made, if the liquid didn’t vaporize properly, which means that a few drops were left in the test tube. One mistake, that was mine fault, was that I didn’t wait the hot bath to reach the certain temperature, but I put the test tube after the water reached temperature of the 40 °C, so I needed to wait more time for my liquid o vaporize. But also, I had problems with hot bath, because it needed lot of time to reach the right temperature. My boiling temperature, is slightly bigger than the regular one on which liquid vaporizes. When collecting the data, I also could have made a mistake, because of, for example, temperature wasn’t constant, and if I didn’t record the temperature in the same time as I took my test tube out of the hot bath, it could change slightly, or even for one whole degree. And for the end, I should mention filthy test tubes. In mine, I noticed some white particles, that could affect the boiling temperature of the liquid.

In order to improve this experiment, I would recomend to wait untill the hot bath reaches the right temperature, which was about 80 °C, which is the average temperature needed for alcohols to start evaporation. If I had enough time, which I didn’t, I would wash each test tube once again and dry it completely. Each, smallest particle could affect the result. After I put the test tube in hot bath, I would wait until it has fully vaporized, and leave it for more than 3 minutes in it, to make sure that everything has vaporized. I would be accurate in recording the data. Each small change should be recorded in the notebook, because it could somehow affect my result. Especially, accuracy is needed when recording the mass and the temperature. When talking about the mass, I would make sure that I fully dried my test tube before the measurement, because, few drops of water could increase mass abnormally for these conditions. While measuring the volume of my test tube, I noticed that in one of the test tubes I spilled few drops, which could also affect my volume recordings, so next time I could be more precise while filling the test tube with water. And last but not the least, I would do more trials, because they can always help me to understand where did I make the previous mistake. So in order to improve the experiment, I have to improve my manipulative skills, be precise, accurate and take care of the instruments in the lab.