Dumas Method Lab PDF

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Experiment 1: Dumas Method Purpose Due to the Ideal Gas Law you will identify the unknown organic liquid by experimentally determining its molar mass using the Dumas Method.

Introduction The Ideal Gas Law is the theory that all gasses, when experiencing the same specific environment, are found to exhibit an ideal behavior1. The environment at which gasses behave ideally is at 100kPa and within 50 degrees of room temperature. Their behavior is represented in the following formula where pressure ( P), volume (V in liters), moles (n), the gas constant (R=8.31451 J/molK), and temperature is (T in Kelvins): PV = nRT In order to obtain the Molar mass of the inorganic liquid, we must obtain its mass and volume (V) in its gaseous state, and the temperature (T), pressure (P). This method is known as the “Dumas Method ”2. While there are several assumptions of ideal gasses, there are two key assumptions because they are the two most important ways in which ideal gases differ from normal gases 3. They are that: 

The volume occupied by molecules themselves is negligible relative to the volume of the container. Therefore, the volume of the container = V (liters) of the gas



There are no (or negligible) intermolecular forces between the gas molecules, and its structure remains the same from as its liquid form.

Moreover, after the gas has been heated into vapour form, it must be cooled to take the final mass of the liquid remaining. Using the final mass, and the computed number of moles, the molar mass can be determined and the unknown organic liquid can be identified.

Procedure Before starting the lab, a snorkel was placed over the hot plates. Two warm water baths were then prepared. The initial weight of the Erlenmeyer flasks, their foils, and rubber bands were taken as the initial mass for each trial. The organic solvent was dispensed in the flasks then carefully covered with the foils. An additional foil seal was placed on each flask. The first two trial flasks were placed in the warm water baths which were slowly brought to a gentle boil until 5 minutes after the liquid solvent was completely dissolved. The temperatures of the water baths were recorded when the flasks were removed. As flasks cooled, liquid condensed in the flasks, and the third trial was placed in the water bath. The outer foil and rubber band were removed from the first and second flask and the left-over moisture that was trapped was removed with a paper towel. The flasks and their contents were weighed for the final mass. The difference of the initial and final masses was then computed for each trial, for the value of the mass of the organic liquid. The organic solvent was then removed from the flask, and the actual volume of the flask was measured by filling the flask with water then pouring it into a graduated cylinder (this was done in two portions). After the experiment was completed the station was cleaned up according to the clean-up guidelines.

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Observations Table of Qualitative and Quantitative Observations

Water bath as it begins to boil Organic solvent

Graduated Cylinder

Qualitative Observations  Bubbling  Steam is produced  Clear and colourless  No liquid present when removed from water bath  Some liquid present after cool down Quantitative Observations  Uncertainty: +/- 0.02 mL

Questions 1. Calculate the mean (molar mass) and standard error of the sample for your results. M= 58.2g/mol Sx= 1.70 2. Based on the molar mass you determined, identify the particular organic compound you used from the list of options presented on the datasheet. Acetone (C2H6O) 3. Give three sources of experimental error that may have caused your answer to be incorrect and explain how your error would have changed your final conclusion. A. Fingerprints on materials may have added additional weight to the final mass B. Leftover vapour trapped under the foil added to the final mass of the organic compound C. Vapour of the organic compound escaped the Erlenmeyer flask despite the foil seals, thus affecting the accuracy of the PV=nRT calculations.

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Experiment 1 data sheet. This sheet is available for download from the onQ web site.

e x a n dr aBr o mb e r g Na me :Al

Pa r t ne r :Da n i e l l eCut e r

St u de n tNo :2 0 15 04 10

St ud e ntNo :2 01 50 45 5

La bSe c t i on :1

Be n c h#( o nc o mp ut e rs c r e e n ) :3 4

DATA SHEET

Sa mp l enumb e rf r o mt h es t o c k b o t t l ey ouus e d .

Ma s s fl as k +2. 5 ”x 2 . 5”Alf o i l = +r u bb e rban d Ma s s o fabo v e+c o mp ou nd = a f t e rhe at i n g Ma s s m= c on de n s e dv ap ou r Va po urDa t a Vo l ume Te mp e r a t u r e Pr e s s u r e Mo l a rMa s s Me a nofMo l a rma s s e s

V=

2 Tr i a l1*

Tr i a l2

Tr i a l3

9 4. 4 25 5

9 5. 1 76 9

9 6. 2 13 2

g

9 4. 5 52 4

9 5. 4 89 8

9 6. 5 10 0

g

0 . 32 69

0 . 31 29

0 . 29 68

1 61 . 0

1 61 . 0

1 61 . 0

99 . 7

8 8. 1

1 00 . 0

102 . 3

102 . 3

1 02 . 3

6 1. 5

5 7. 1

55 . 9

T= P= M=

´ M σx

u ni t s

g

mL K kPa 1 gmol

5 8. 2

St a n da r dEr r o ro ft h e me a nmol a rma s s 1 . 39 [* Because you are doing statistical determination of your uncertainty, you do not need to propagate uncertainties in each calculation. You really need at least three data sets to do a proper uncertainty calculation statistically. If one of your sets of data is no good, for example, a mass of compound is way too large or way too small or the calculated molar mass is very different from the other two, then borrow a dataset from one of the other groups who used the same sample number as you did. Indicate below this line who you borrowed the data from. ]

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References 1. What is the ideal gas law? (n.d.). Retrieved from https://www.khanacademy.org/science/physics/thermodynamics/temp-kinetic-theory-ideal-gaslaw/a/what-is-the-ideal-gas-law. 2. Libretexts. (2019, June 5). 11B: The Dumas Method (Experiment). Retrieved from https://chem.libretexts.org/Ancillary_Materials/Laboratory_Experiments/Wet_Lab_Experiments /General_Chemistry_Labs/Online_Chemistry_Lab_Manual/Chem_11_Experiments/11B:_The_D umas_Method_(Experiment). 3. Clark, J. (2017, June). IDEAL GASES AND THE IDEAL GAS LAW. Retrieved from https://www.chemguide.co.uk/physical/kt/idealgases.html....