General Chemistry w/Lab I
Experiment: Density of a Liquid Mixture
In this laboratory you will determine the density of liquid mixtures and use graphical techniques to determine the composition of an unknown mixture. You will learn to use MS Excel to graph data and determine the slope, and understand the difference between accuracy and precision as it applies to experimental data.
When two liquids are mixed, how does the density of the solution compare to the density of the pure liquids? How does this density change as the percentage of each liquid changes? In this laboratory you will investigate these questions by determining how the density of an isopropanol/water mixture varies as the fraction of isopropanol increases.
You will begin by using the known density of pure water to determine the volume of a pycnometer, and then determine the densities of pure isopropanol and several isopropanol/water solutions. The results will be graphed, and the relationship between the density of the solution and percent isopropanol will be determined. This graph can then be used to find the percent composition of an unknown mixture.
For this experiment you are to work in pairs. The first task will be to determine the volume of the pycnometer. This is accomplished by weighing the pycnometer empty, filling it with deionized water and reweighing. From this data, the mass of water can be calculated. Then, using the known density of water (available in published tables) the volume of the water, and thus the volume of the flask, can be determined.
The density of all remaining solutions are to be determined in a similar way. Once the volume of the pycnometer is accurately known, it can be filled with the desired solutions and weighed. Using the mass and volume of the unknown solutions, each density can be calculated.
Each pair of students will be assigned a different isopropanol/water mixture. You will need to prepare this solution by mixing the appropriate amounts of isopropanol and water. You will also be given a solution with an unknown percentage of isopropanol, and are to determine its density, and from that, its composition. Lastly, every pair of students will determine the density of pure isopropanol.
A graph of density vs percent isopropanol will be prepared from the class data and linear regression will be used to determine the slope and intercept. This can then be used to determine the composition of the unknown solution.
Finally, we will judge the accuracy and precision of our methods by pooling the class results for the density of pure isopropanol.
- Obtain a pycnometer. If it is clean and dry, proceed to step 2. If not, wash it carefully with soap and water and rinse it with deionized water. To dry, squirt a small amount of acetone inside, swirl to coat the sides, and discard the liquid in the waste container in the hood. Blowing air into the flask with an empty, dry plastic squeeze bottle will speed the drying process.
- Weigh the clean, dry pycnometer with the cork inserted, and record the result.
- Obtain a beaker of deionized water and record the temperature. Look up the density of water at this temperature and record the density in your data table in your notebook.
- Fill the pycnometer with the deionized water. This requires a little care. First fill to just above the cap stem, tap gently to dislodge bubbles, and insert the cap. Water should squeeze out the top and no air bubbles should remain.
- Dry the outside of the flask, weigh, and record the result. Calculate the volume of the pycnometer. Repeat the procedure a second time to ensure that you are achieving consistent results.
- Record the solution you have been assigned to prepare. Figure out the amount of water and isopropanol to mix in order to prepare 60.0 mL of this solution. Mix the liquids in a graduated cylinder by stirring, and cap with a rubber stopper when you are done to minimize evaporation.
- Fill the dry pycnometer completely with this solution and weigh. Use the known volume of the flask to calculate the density of your solution. Repeat the procedure a second time and record both results on the whiteboard.
- Rinse the pycnometer twice with small amounts of your assigned unknown, then fill it with the mixture. Weigh the flask and contents, record the result, and calculate the density of the mixture.
- Rinse the pycnometer twice with small amounts of pure isopropanol, then fill with pure isopropanol. Weigh the flask and contents, and record the result. Calculate the density of pure isopropanol, and record your result on the whiteboard.
- Construct a graph (by hand) of density vs percent isopropanol for all class data obtained. Do NOT use the graph paper in your lab notebook; it is not accurate enough (although you may want to use it for a rough sketch). Instead, use the graph paper provided by your instructor. Staple the completed graph into your lab notebook. You must complete a graph by hand and have it approved by your instructor prior to using a computer to generate a graph.
- Determine the slope and intercept of the handdrawn graph using standard (rise over run) methods.
- Each student is to use MS Excel to build a spreadsheet containing the same data as the handdrawn graph, being sure to arrange the data in columns, with clear labels and proper units included at the top of the columns. Use the Format Cells command to format the data so that the correct number of significant figures are displayed. While constructing your spreadsheet pay attention to appearance as well as substance. Be sure to include you name in the title at the top of the spreadsheet, and print a copy for inclusion in each of your reports.
- Use the Chart feature to prepare a graph of the data, and use the regression feature to determine the slope of the line. Print a copy of the graph to include in your lab report. Since the graphs will appear very similar it is essential that you place your name on the graph before printing.
- Use your calculated slope and intercept (obtained in MS Excel using linear regression) to determine the composition of your unknown solution.
- Enter the data for the pure isopropanol into another spreadsheet, and format as you did for the previous data set. Then use the formulas in MS Excel to compute the mean (average) value and standard deviation for the density of pure isopropanol. Include all class results in your calculation. Print a copy of this data table for inclusion in your report.
- Use the CRC to look up the accepted value for the density of isopropanol. Calculate the percent error for both the class average and your result, and comment on the accuracy and the precision of the class result as well as your own.
Writeup and Analysis
Your report in your lab notebook should include:
- Tables of all data collected, with correct units and significant figures.
- Two graphs of density vs percent isopropanol, the first prepared by hand and the other using MS Excel. Be sure to label the axes and title the graphs.
- A section showing all calculations, including the calculation for the slope of your handdrawn graph. All values should contain the correct units and significant figures.
- Your calculation of the density of your unknown solution, and determination of its composition.
- The class average for the density of pure isopropanol, the accepted value, the percent error and standard deviation of the class results.
- Your analysis/discussion of the accuracy and precision of the class, and your own, results, and discussion of possible sources of error.
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