Lab Report> CHEMISTRY 11101 Lab Report#2: this lab was to estimate Avogadro’s number using stearic a

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Briana Turner Lab Report #2 TA: Ryan Wood

Introduction: The purpose of this lab was to estimate Avogadro’s number using stearic acid. We dropped a precise amount of stearic acid onto the surface of water using a wellcalibrated pipette, enough to form a monolayer “cake” of molecules onto the surface of the water. The polar head of the stearic acid molecule and the non-polar end, which sticks away from the water, allows a monolayer to form on the surface of water. From the volume required to form a monolayer, the number of drops of stearic acid was counted. The thickness of the layer was then used to compute the volume of a singular carbon atom, and this value was then used to determine the volume of a mole of carbon atoms used. From this value, we were finally able to estimate Avogadro’s number.

Experimental Methods: A 15cm watch glass was cleaned with detergent solution and rinsed with both cold and distilled water. The watch glass was then dried and placed on plain piece of white paper. 10mL of hexane was poured into a 50mL beaker. A plastic pipette was used to place drops of stearic acid into a graduated cylinder. The number of drops required to deliver a volume of 1.00 mL was averaged from four trials. A 10mL graduated cylinder was filled with 1.0 mL of n-hexane. The number of drops of n-hexane needed to reach the next mL was counted, repeated twice, and averaged so the error was within The diameter of the watch glass was measured within 0.1cm accuracy and repeated twice.

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The watch glass was filled with distilled water. Stearic acid with a concentration of 0.1480 g/L was poured into a 10mL graduated cylinder. The plastic pipette was rinsed with n-hexane and then filled with stearic acid. The drops of stearic acid placed onto the surface of the water in the watch glass was counted, while the pipette remained vertical.

Once a drop’s form persisted for approximately 30 seconds, one more drop than necessary to form the monolayer was added to the surface of water. The pipette was used to remove some of the solution from the watch glass. The remaining solution was poured into a 600mL beaker, and the watch glass was cleaned with soap water, tap water, and distilled water. This procedure to drop the stearic acid on the water to form a monolayer was repeated two times.

Data Analysis: Calibration of plastic pipette Trial Drops needed to meet next mL Performed by 1

56 drops

Briana

2

54 drops

Ellie

3

53 drops

Briana

4

56 drops

Ellie

Average number of drops per mL:

Trial Diameter of watch glass (cm) 1

14.8

2

14.8

3

14.8

2

Average measure diameter of watch glass: Stearic acid concentration 0.1480g/L Trial Number of drops of stearic acid to form monolayer 1

9

2

9

3

9

Calculations 1. Volume of stearic acid solution to form the monolayer

2. Using the concentration of the stearic acid solution (g/mL), calculate the mass of stearic acid in grams.

stearic acid 3. Calculate volume of the monolayer of the stearic acid in mL. Density of stearic acid is 0.941 g/mL. mL 4. Calculate the area of the water surface

5. Calculate the height of the monolayer

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6. Calculate the volume of a carbon atom

7. Calculate the molar volume of a carbon atom using the density of diamond (3.51g/mL)

8. Calculate Avogadro’s number from the equation Avogadro’s number in the form of

and express

. Is your number within a power of

10 of the accepted value

Yes, this experimentally estimated value for Avogadro’s number is within a power of 10 of the accepted value for Avogadro’s number.

Calculations: 1. What is the worst assumption(s) in this experiment? Why? The worst assumption made in this experiment is to assume that the stearic acid molecules will stack together to form a specific arrangement to form a monolayer. The reason why this is a bad assumption to make is because the thickness of the monolayer is not accurate. Some of the stearic acid molecules could have slipped in between the spaces of the particular arrangement of molecules, thus altering the thickness of it.

2. If you were invited to improve this experiment, what would be your suggestion(s)?

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In order to improve upon the experiment, the pipette should be calibrated more times to get a more accurate reading of the amount of drops of nhexane per mL. Also, the pipette should be calibrated with stearic acid in addition to the n-hexane to maintain consistency when actually using the pipette to drop the stearic acid on to the water.

Conclusion: We were experimentally able to determine Avogadro’s number within a power of 10, and the value we calculated for Avogadro’s number falls within a reasonable range of the accepted value. The accepted value for Avogadro’s number is

, and the

number we calculated in the lab was

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