Week of 10/17


Today we worked on gravimetric analysis problems. We started by reviewing double displacement reactions in the packet and then I went over a typical ga problem.  The rest of class you worked on textbook problems.


Today we started with the nomenclature quiz.  After that you worked on designing a lab to determine the alkali metal in an alkali metal carbonate.


No class b/c of PSATs


Worked on designing the lab to determine alkali metal. Here are example procedures.

HW: Chemical formulas lab due Friday morning!


Alkali Metal Carbonate Lab

Example Avogadro Lab Conclusion:

The Determination of Avogadro’s Number Lab was used to estimate Avogadro’s number experimentally. It’s also used to find the dimensions of an atom and determine the validity of the Oil Slick method of estimating Avogadro’s number. The experimental method used to determine Avogadro’s number was the oil slick method, which uses Oil (containing oleic acid and ethanol), lycopodium powder and water. The results of this experiment found that the Oil Slick method is not a valid method of determining Avogadro’s number as there is too much error involved in the process and false assumptions were made about the properties of the oil slick and molecule.

The mass and volume of oleic acid in a drop of oil was calculated by weighing several then dropped into a mixture of lycopodium powder and water. The ethanol in the oil then evaporated or dissolved in the water leaving the oleic acid(C17,H33COOH). The oleic acid displaced the water/lycopodium mixture as the oleic acid is nonpolar and the water is polar so the two don’t form a solution (also the water doesn’t dissolve the lycopodium, the powder rests on top of the water so when the water is displaced so is the powder, allowing to see the area of the oil slick). The nonpolar oleic acid lycopodium powder and formed a roughly circular monolayer(one molecule thick) slick on the surface of the water. The diameter of the circle formed was measured and then used, along with the volume of the oleic acid to estimate Avogadro’s number. We performed this procedure for three trials. To achieve more consistent results during this procedure, we had the same individual perform a specific task (ex. measuring the diameter of the slick, dropping the oil) for each trial.

Our group did the calculation of Avogadro’s number two different ways. For one set of calculations it was assumed that the height of one molecule of oleic acid was 20 atoms tall and for the other calculation it was assumed that one molecule was 19 atoms tall. Both these heights were supported as possible by a three dimensional model we formed of oleic acid. The calculated values for Avogadro’s number when using a molecule height of 20 atoms were: 1.12 x 1022 molecules (98.14%, 10.7 cm), 1.07 x 1022 molecules (98.22%, 10.9 cm), and 1.36 x 1022 molecules (97.74%, 11.1 cm). The calculated values for Avogadro’s number when using a molecule height of 19 atoms were 1.00 x 1022 molecules (98.34% error, 10.7 cm), 1.01 x 1022 molecules (98.32%, 10.9 cm) and 1.12 x 1022 molecules (98.04%, 11.1 cm). The difference between the use of 19 or 20 atoms made no appreciable difference on the result.

Our data is not valid because of the high percent error in our estimate of Avogadro’s number and the inaccuracy of our results. The percent error was over 98% for every trial that was conducted. The extreme inaccuracy in our calculations of Avogadro’s number can be contributed to several factors. This experiment made several assumptions, that if false, would’ve led to miscalculations and an incorrect estimate of Avogadro’s number. The assumption that the oil slick is one monolayer tall could be incorrect. If it was incorrect, it would invalidate the results of all the calculations made based on the idea that the layer was one molecule deep. This in turn would affect our results for Avogadro’s number. For instance, if the oil slick was actually five molecules tall, the average calculation for Avogadro’s number would be 5.35X1023, which is distinctly different from the result we found operating under the assumption that it was a monolayer. The assumption that the molecule was 20 atoms tall or 19 atoms tall could also be incorrect. It’s more likely that the Oleic Acid molecule is 19 atoms tall as when we did our calculations and accounted for 19 atoms in the height of the molecule, the percent error of Avogadro’s number was smaller. The average percent error using 20 atoms was 98.03%, whereas the average percent error for 19 atoms was 98.23%. Though the difference in percent error is small, the 20 atoms calculation still are closer to the accepted value it can be inferred that the molecule is more likely to be 20 atoms as opposed to 19 atoms.

Our experiment was also not accurate due to a systematic error made in measuring the diameter of the oil slick, which would lead to miscalculations in estimating Avogadro’s number. The average diameter of the oil slick was 10.9, but if you calculate the diameter of the oil slick by working backwards from Avogadro’s number, it’s closer to 17. The largest diameter, 11.1 cm, had a smaller percent error than the two smaller diameters of 10.7 cm and 10.9. The 11.1 cm had a percent error of 97.74% (assuming that the molecule is 19 atoms tall) and the 10.7 cm and 10.9 cm diameters have percent errors of 98.14% and 98.22%. If we were to increase the diameter of the oil slick to 17 cm and readjust the calculations, the estimate of Avogadro’s number is now 1.616 x 1023. The percent error of the new calculation is 73.16%, which is still high, but substantially lower than the original calculations, which had an average diameter of 10.9 cm and a percent error of 98.04%. As we increase the diameter of the oil slick, the estimate of Avogadro’s number gets closer to the accepted value.

As our estimates of Avogadro’s number are inaccurate, but relatively precise, it can be assumed that a systematic error was made in measuring the oil slick. When we measured the oil slick, we measured the largest diameter and the smallest diameter and averaged the two values, using the average as the diameter. This could’ve led to the diameter being smaller than it actually was. In addition, the circle formed by displacing the water and lycopodium powder was not even close to a perfect circle. It had many irregularities in its curve and diameter, which could’ve led to incorrect measurements of the diameter. The mass of the added lycopodium powder to the water could’ve added more resistance to the oil slick, making it harder to displace the water. Since it was harder to displace, the water wouldn’t have been pushed back as far, and thus the diameter was smaller than it actually should’ve been.

Even accounting for systematic error in measuring the diameter of the oil slick, there was still a percent error of 73.16%. This could’ve been due to the assumption that the height of the oil slick was only one monolayer, or we could have miscalculated the amount of oleic acid in one drop. If the amount of oleic acid in one drop had been too much, it would’ve decreased the estimate of Avogadro’s number, putting it further from the accepted value. It’s likely that we overvalued the weight of oleic acid in one drop of oil, leading to us underestimating Avogadro’s number.

This method was proven to not be a valid way to estimate Avogadro’s number. This was due to the tremendous amount of error that can not be accounted for when calculating on the molecular level. One major flaw was the fact that the desired thin film of lycopodium powder could not be achieved, this would lead to the oil not taking the shape of a perfect circle. Without having a perfect circle measuring an accurate diameter is practically impossible, and when calculating on the subatomic level having this measurement off can drastically jar the results. To improve this experiment a better substitute for the lycopodium powder should be used. The substitute should be able to settle in one uniformly thin layer across the surface of water, allowing for the oil slick to take the shape of a perfect circle. However, it should be noted that this experiment has a large amount of variables to control, requires measurements of the utmost accuracy and makes many assumptions. Therefore it is highly recommended that a different method should be used for those looking to execute an experiment with a low percent error.