Introduction:
The purpose of this experiment was to get an experimental estimation of Avogadro’s number. We did this by determining the number of molecules of stearic acid necessary to form a monolayer on the surface of water. First we had to determine the volume of one drop from our pipettes by counting how many drops it takes to accumulate one milliliter.
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Next we counted the number of drops of stearic acid needed to form a monolayer on a 15cm watch glass. By know the number of drops needed to from a monolayer and the volume of one drop we can calculate the total volume of stearic acid needed. Once a monolayer of steric acid was formed on the surface of water we could divide the volume of steric acid used by the area to get the height/thickness of a molecule of steric acid. We know that a steric acid molecule consists of 18 carbon atoms. Thus by dividing the height of a steric acid molecule by 18 and then cubing this number we have the volume of a carbon atom. We know the volume of one mole of carbon by dividing the molar mass of carbon by its density. By dividing the volume of one mole of carbon by the volume of one atom we have an approximation of Avogadro’s number. Experimental Procedure: The procedure was the same as that given in the lab manual. Data Analysis: Data: Pipette Calibration Pipette #1 Pipette #2 Number of drops Number of drops Trial 1 60 Trial 1 63 Trial 2 56 Trial 2 61 Trial 3 57 Trial 3 64 Averages: Pipette#1: 58 drops 58 drops/mL 0.017 mL/drop Pipette #2: 63 drops 63 drops/mL 0.016 mL/drop Drops of Stearic Acid to form monolayer on water Pipette #1 Pipette #2 number of drops number of drops Trial 1 6 Trial 1 9 Trial 2 9 Trial 2 9 subtract 1 drop from each number of drops -1 number of drops -1 Trial 1 5 Trial 1 8 Trial 2 8 Trial 2 8 Concentration of Stearic Acid: 0.1480g/L Diameter of watch glass: 14.60cm Calculations: See attached sheet for calculation and questions
Discussion:
In this lab our major source of error arose from the assumption that every drop from the pipette was uniform and of equal volume. This is nearly impossible because the volume of each drop depends on many variables like the pressure exerted on the pipette, the amount of time between each drop and whether or not there were air bubbles in the pipette. In both of our pipettes there were definitely air bubbles. The rest of the lab calculations are based on the volume of one drop from the pipette. We made a major assumption very early on in the lab which has an impact on the rest of our results ensuring they had a degree in error. Although the volume of each of the pipette is most likely relatively close, they are not equal. In a lab such as this even a difference of 0.001mL makes a huge difference because the volume of stearic acid we are adding to the water is much less than a milliliter. Being of by half a drop or if one drop as small air bubble, this has a huge, huge impact on the results and explains why for both of our pipette we got a number × 1025 as oppose to 1023. Another assumption we made which would be a source of error is that we assumed the area of water we were trying to cover with a monolayer of stearic acid was the same for each trial. Because we did not measure the amount of water we filled the watch glass with it varied by some amount in each trial. While the difference in area was small between trials, even one drop more or less of water could mean the difference between 9 and 10 or 8 and 9 drops of stearic acid which is huge when considering the scale of the experiment.
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