Density Determination, Melting Point and Refractive Index

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Introduction

This laboratory was designed to examine the physical properties of organic compounds. The three properties determined were density, melting point, and refractive index. Every compound has unique physical properties. In this laboratory the experimental results will be compared to the compounds actual cataloged properties, accuracy will then be calculated through percent error.

Theoretical Basis

One physical property is density. Density is the measure of mass per area volume (Density = Mass/Area). Another important physical property is a compound's melting point. The melting point or range is very useful when determining a compounds purity. If the organic solid is pure, there are no contaminants present, then the substance will melt over a very short range (1- degrees). If the organic substance is a mixture then the melting point will be lowered and the melting range extended. By plotting the different melting point on a graph one can see an obvious trend when working with a bimolecular mixture. The eutectic point is a point on the graph where the two curves meet, at this point the mixture of the two substances is combined is such a way that the mixture will actually behave as if it is a pure substance.Cheap Custom Essays on Density Determination, Melting Point and Refractive Index

Refractive index is yet another physical property. The refractive index is a precise measurement used to track the purification of liquid samples. When light passes through an organic compound it is bent, or refracted. The refractive index can be measured precisely to four decimal places. The refractive index is measured by the equation light velocity in a vacuum ÷ light velocity in the sample. This number will always be greater then one. The refractometer, the device used to measure the refractive index, utilizes a sodium line to visually line up the distance the light is refracted. This is what the n stands for. The sodium "D" line at 0 degrees C. If the substance is impure than the refractive index will be different than refractive index of the pure substance.

Procedure

Density

A 1 mL syringe was weighted and the result recorded. The syringe was filled with precisely l mL of Toluene. The syringe filled with 1 mL of Toluene was then weighted again and the result recorded. Density of the substance was then determined. This was repeated for compounds Ethylene Glycol as well as Cyclopentenone.

Melting Point Determination

A small amount of solid t-Cinnaminic acid was placed into a closed capillary. The capillary was placed into a system used to determine melting and boiling points. The sample was then heated to its melting point. The melting point as well as the range of melting were recorded.

A small amount of solid 6% Cinniminic acid- 7% Urea mixture was then placed into a capillary and inserted into the same heating device used to determine the melting point of pure Cinnaminic acid. The melting point and range were recorded. A total of seven different mixtures were melted and the temperature range recorded and plotted onto a graph to determine the eutectic point.

Refractive Index

A drop of Toluene was placed onto a refractometer cell, which was previously cleaned with Methanol. The refractive index was determined by lining up the bottom of the refracted light with the sodium line through the eye lens. Again this was repeated with Ethylene Glycol and Cyclopentenone.

Results

Density

Density = Mass / Volume

Mass of 1mL syringe = .5 grams (+/- .001 g)

Mass of 1mL syringe + 1mL Toluene = .8 grams (+ .001 g)

Mass of 1mL Toluene = 0.85 grams (+ .001 g)

Volume of Toluene = 1mL

Density of Toluene = 0.85 g / 1 mL = 0.85g/mL

Actual Density of Toluene = 0.866g/mL

Percent error = 0.8667g/mL-0.861g/mL

X 100 = .88%

0.861g/mL

Density of Ethylene Glycol = 1.108 g / 1 mL = 1.108g/mL

Actual Density of Toluene = 1.11g/mL

Percent error = .45%

Density of Cyclopentanone = 0.81 g / 1 mL = 0.81g/mL

Actual Density of Toluene = 0.51g/mL

Percent error = %

Melting Point Determination

t-cinnamic acid urea

100%0% 85%15% 71%% 6%7% 50%50% 5%75% 0%100%

1-18 4-118 8-100 88-100 85-8 -10 100-105

10-14 -108 110-15 100-10 87-100 110-10 18-10

100-10 105-11 80-5 6-100 14-16

11-1

A graph of the previous melting points can be found attached to the back page of this report. The Eutectic point was found to be between 50%-50% to 6%-7%. The actual eutectic point is 6%7% t-cinnamic acid urea.

Refractive Index

Experimental Refractive Index of Toluene n 1.47

Actual Refractive Index of Toluene = n 1.461

Percent error = .07%

Experimental Refractive Index of Cyclopentanone n 1.44

Actual Refractive Index of Toluene = n 1.47

Percent error = .1%

Experimental Refractive Index of Ethylene Glycol n 1.4

Actual Refractive Index of Toluene = n 1.41

Percent error = .07%

Discussion

All of the results found were very accurate. The small discrepancies between the experimental and actual values was due to human or mechanical error, or possibly even the contamination of the substance. It is most likely human error.

The melting point experiment proved to be just as successful. The pure solid, Cinnaminic acid, melted at exactly the theoretical temperature and range. When the impure cinnaminic acid was heated to its melting point, the melting range was longer by several degrees Celsius and the initial melting point lowered considerably. The melting point accurately displayed impurities in the organic solid. The eutectic point using the average of the experimental values was 50-50%. The actual point being at 6-7%. When drawing the graph lines, if the high numbers were used and the low values ignored then a 6%-7% eutectic point was achieved. There were groups that had consistently low melting points, this could be that the group lacked proper technique or they used a fault melting point machine. Certain mixtures may have also been cross contaminated by other mixtures in the area.

The experimental refractive indexes were all within a percentage point of the actual value. Although the number obtained through experimentation was close to the actual number cataloged, there was still human error. The refractive index is supposed to be an exact measurement to the fourth decimal place. The experimental value was only correct the first.

The physical properties of an organic compound can be easily determined and clearly show whether the sample is free of impurities and contaminants.

Conclusion

All of the physical properties measured in the organic compounds showed to be correct within one percentage point of their theoretical values. This laboratory was accurate, precise, and successful.

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