Chemical Determination of an Unknown Liquid

Topics: Water, Temperature, Density Pages: 6 (1933 words) Published: May 23, 2013
I will identify an unknown liquid by obtaining an average density & boiling point for my liquid and comparing them with the known densities & boiling points of liquids in the CRC handbook of Chemistry & Physics. I will perform these experiments using three different measuring tools (graduated cylinder, burette & pipette) to evaluate the precision of each one.

Density 1, 2 & 3 calculated by using graduated cylinder to obtain volume. Density 4, 5 & 6 calculated by using volumetric pipette to obtain volume. Density 7, 8 & 9 calculated by using burette to obtain volume. All experimental values obtained have been included in this determination.

Average density calculated numerically:
Average density = sum of all densities divided by total number of densities measured Average density = (density 1 + density 2 + density 3 + density 4 + density 5 + density 6 + density 7 + density 8 + density 9) / 9 Average density = (0.771 g/mL + 0.778 g/mL + 0.786 g/mL + 0.788 g/mL + 0.7820 g/mL + 0.7820 g/mL + 0.7786 g/mL + 0.7737 g/mL + 0.7732 g/mL) / 9 Average density = 7.013 g/mL / 9 = 0.7792 g/mL

Average boiling point:
Average boiling point = sum of all boiling points divided by total number of boiling points measured Average boiling point = (boiling point 1 + boiling point 2 + boiling point 3) / 3 Average boiling point = (63.0 °C + 62.9 °C + 63.2 °C) / 3 Average boiling point = 189.1 °C / 3

Average boiling point = 63.03 °C

Average density calculated graphically:
Per graph created in excel the equation of the best fit line is y = 0.7827x - 0.0851, per mathematics the slope for a formula of style y = mx + b is equal to m, thus the slope of the best fit line is 0.7827, since this graph plots a line of mass over volume then the slope of that line is the density which is 0.7827 g/mL. See attached paper for graph.

Unknown Liquid Code: D Average Boiling Point: 63.03 Average Density (Numerically): 0.7792 g/mL Average Density (Graphically): 0.7827 g/mL

I am quite confident in stating that my unknown liquid is methanol. The density & boiling point of methanol are 0.791 g/mL & 64.7 °C respectively whereas I measured density & boiling point of 0.7792 g/mL (0.7827 g/mL graphically) & 63.03 °C in my unknown liquid. Also when looking at other liquids that have similar densities [2-propanone (0.791 g/mL), cyclohexane (0.779 g/mL), ethanol (0.789 g/mL) & 2-propanol (0.785 g/mL)] their boiling points [3-propanone (56.5 °C), cyclohexane (80.7 °C), ethanol (78.5 °C) & 2-propanol (82.4 °C)] are quite different from what I measured in my unknown liquid. Methanol is the only liquid that has both a comparable density & boiling point to my unknown liquid. Also per the United States Environmental Protection Agency's Technology Transfer Network Air Toxics Web Site Methanol is listed as a clear colorless liquid with an alcoholic odor, I made & noted these same observations for my unknown liquid.

The key data in identifying my unknown was the boiling point. I found that 5 of 8 non-water liquids listed in the lab manual had densities within 0.012 g/mL of each other (between 0.779 g/mL & 0.791 g/mL) considering the measured density of my unknown liquid is 0.7792 g/mL (0.7827 g/mL graphically) it could very well correspond with any of those 5 liquids, leaving me with a lot of uncertainty. Whereas the measured boiling point of my unknown liquid is 63.03 °C which corresponds with only 2 of the possible liquids, methanol (64.7 °C) & 2-methylpentane (62.0 °C). This data was key to quickly & accurately filtering through a long list of possible liquids. Since 2-methylpentane's density of 0.653 g/mL does not correlate well with the measured density of my unknown liquid and methanol's density (0.791 g/mL) does correlate with the measured density of my unknown liquid I feel safe in stating that my unknown liquid is methanol. All of the data I collected was useful in identifying my unknown liquid.

The graduated cylinder...
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