The Molar Mass by Vapor Density lab PDF

Preview

Summary

Download The Molar Mass by Vapor Density lab PDF

Description

The Molar Mass by Vapor Density

Lizet Castillo Date: November 28, 2019 Class: Chem1100-08

Purpose: The purpose of this experiment was and is to use the Dumas Method so that we can determine the vapor density and find the molecular weight of the unknown liquid we receive vaporizing it. Theory/Principles of the lab: When you start to do the Dumas Method the vapor is to stay in the flask and after the experiment was completed then we weigh the liquid to get the molecular weight from the unknown we got. But in order to get the molecular weight, we needed to use the ideal gas law with the formula (PV=nRT). This method finds the substituting n=grams/molecular

weight into the ideal gas laws and determines the molecular weight of gas giving using PV=nRT=g/MW RT. Within the vaporizing of the unknown solution #20 in a flask heated in boiling water we got the measurements of the weight of the empty 125mL of flask + rubber septum, and the mass of the flask + rubber septum + condensed unknown liquid. After we wrote down the barometric pressure and temperature of the flask that was being heated and mass of flask + rubber septum + water, we obtained the temperature of the water we had. After we did that we convert the proper units to obtain the unknowns molecular weight of g/mol and find the volume of water by using V=m/d. We converted the barometric pressure to atm using 760 mmHg(P=mmHg/760mmHg). To get the volume of water, we first subtract the flask + rubber + water by the mass of the empty flask + rubber septum and then divided it by the mass over our density of the water. last, of all, we got the density of water at that specific temperature in which we convert mL to L. Afterward plug them into the Dumas equation to get an answer.

Experimental procedure: Step 1: weigh a 125mL Erlenmeyer flask with a rubber septum (obtained from the stockroom) on an analytical balance. Both the septum and flask must be clean and dry. Record this mass. Obtain about 3 to 4mL of the unknown which is assigned to you by your instructor. Pour this into the flask and place a small piece of capillary tubing through the puncture in the septum to allow the air and excess vapor to escape when the flask is heated. Step 2: Clamp the flask so that it is up to its neck in a large and excess vapor to escape when the flask is heated. Clamp the flask so that it is up to its neck in a large beaker (400 or 600mL) about half to three-quarters full of water (see illustration). Add 3 or 4 boiling stones to the water in the beaker. Heat the water to a boil for two minutes after the liquid seems to have disappeared from the flask. Step 3: Do not remove the flask from the heat before this time, as air will reenter the flask and cause erroneous results. (How would this affect your final weight?) when the liquid phase is unknown has been gone for two mins, stop heating and carefully remove the flask from the beaker of hot water. Cool the flask under a stream of cool tap water to condense the vapor in the flask. Then remove the capillary. Dry the outside of the flask well with a paper towel, making sure that you also dry around the edge of the septum.

Step 4: weigh the flask, septum, and condensed liquid on an analytical balance immediately. The condensed unknown will evaporate at a perceptible rate. Record this mass (the mass of the flask, septum, air, and condensed liquid). The condensed liquid occupies a much smaller volume than that of the flask, so the weight of the air essentially the same as that in the first weighing.

Data Tables/Summary: Table 1: data was used for our measurements of barometric pressure(mmHg), mass of flask + rubber septum(g), mass of flask + rubber septum + condensed liquid of unknown g #20, the temperature of boiling water (C), mass of flask + rubber septum + water(g) and the temperature of water(C). 1.Unknown #20 first round: Patm(mmHg) =

752mmHg

Empty 125ml Flask + Rubber Septum =

89.6027g

Condensed Mass 125ml Flask + Rubber +

89.6027g

Liquid = Temperature of Boiling Water =

98.3*C

125ml Flask + Rubber + Water =

232.310g

Temperature of H2O (*C) =

22.5*C

● Results: Formula Used: MW=mRT/VP ● (T= 98.3 + 273= 371.3K) ● (m= 89.6027g - 89.0701g=0.5326g) ● (R= 0.08206L ∙ atm/mol ∙ K) ● VH2O= 232.310g - 89.0701g/0.9976g/mL=143.58mL→0.14358L ● P= 752mmHg/760mmHg=0.989 atm MW= (0.5326g) (0.08206 L ∙ atm/mol ∙ K) (371.3K)/ (0.14358L) (0.989 atm) = 111.78 g/mol 2. Unknown #20 Second round:

Patm(mmHg) =

752mmHg

Empty 125ml Flask + Rubber Septum =

89.0701g

Condensed Mass 125ml Flask + Rubber +

89.7127g

Liquid = Temperature of Boiling Water =

99.6*C

125ml Flask + Rubber + Water =

232.108g

Temperature of H2O (*C) =

22.0*C

● Results: ● (T= 99.6 + 273= 372.6K) ● (m= 89.7127g - 89.0701g= 0.6426g) ● (R= 0.08206L*atm/mol*K) ● VH2O= 232.180g - 89.0701g/0.9978g/mL=143.43mL… so, 0.14343L ● P= 752mmHg/760mmHg= 0.989 atm MW= (0.6426g) (0.08206 L*atm/mol*K) (372.6K)/ (0.14343L) (0.989 atm) = 138.39 g/mol Average of MW: 111.78 g/mol + 138.39 g/mol= 250.17 g/mol ● So, the unknown liquid molecular weight for the first one should be111.78 g/mol. And second unknown liquid molecular weight should be 138.39 g/mol. overall average weight for the unknown liquid would be 250.17 g/mol.

Conclusion: At the beginning of this experiment I was a little lost but we soon figured out that we needed to use the Dumas method but first we need to understand how to use it properly and after trying to figure it out me and me lab partners realized that we need to get the vapor density and we also need to use the equation the Demus method gave us. And after doing all the math like After we we convert the proper units to obtain the unknowns molecular weight of g/mol and find the volume of water by using V=m/d and more we managed to to get the molecular weight of the unknown liquid we were given.

Discussion Questions: 1. So after we did the experiment we realized that the liquid hadn’t completely vaporized like how we thought it would and also the apparent molecular weight was bigger when you compared it. Done properly would cause the pressure inside the flask to below which would give us a much lower number of moles. 2. This so for the uncertainty of the pressure: ● The Average= 752mmHg + 752mmHg/2= 752mmHg ● Percentage of the uncertainty= 3mmHg/751mmHg times 100% to get 0.4% 3. Has to be the Avogadro’s law using this formula PV=nRT ∝n. 4. 1.783 g/L ● STP=1atm and T= 0°C ● Molar mass=(22.4L/1mol)(1.783g/L)=39.94g/mol Argon 5. So my partners and I decided that our uncertainty would have been the same as for question two. Works Cited: Goldwhite, Harold et al. Experiments In General Chemistry. Macmillan Learning Curriculum Solutions, 2018....