Lab Techniques

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Determining the Density of an EggExpand

   Density is a measure of how compact a substance or object is, and it is calculated as mass divided by volume. When a dense object is placed in a liquid with a lower density, the object sinks; when the object is less dense than the liquid, it floats. When the densities are the same, the object will be suspended in the liquid. In this experiment, we'll determine the density of an egg by creating a solution with the same density. We'll measure the density of the salt solution using different volume measurement glassware and determine which is the most accurate.

   • To begin, wear the appropriate personal protective equipment, including close-toed shoes, long pants, long-sleeved shirt, lab coat, chemical splash goggles, and gloves.
   • Measure the mass of the egg. Set a weigh boat on the balance and tare it. Carefully set the egg in the weigh boat while you record the mass in your notebook.
   • Prepare the salt solution that will be used to suspend your egg. Use the 600-mL beaker and fill it 3/4 of the way full with deionized water.
   • Gently place your egg in the beaker, and then add about one teaspoon of NaCl into the water. Stir the solution gently until the NaCl dissolves. Continue to add NaCl until the egg floats just below the surface.
   • Use different glassware to measure the volume of the salt solution. Weigh the 50-mL graduated cylinder on the top loading balance and record the mass. Then, measure 20 mL of your salt solution and record the volume in your notebook. Note: Place a blank sheet of paper behind the graduated cylinder to read the volume more clearly. Always view the measurement at eye level and measure the volume from the bottom of the meniscus. The lines on the graduated cylinder are 0.2 mL apart, meaning that you can read the volume to the hundredths place.

     Table 1: Measuring Mass

     Container	Mass of empty container (g)	Mass of full containter (g)	Mass of solution (g)	Percent error
     50-mL graduated cylinder
     10-mL volumetric pipette
     50-mL beaker
     20-mL volumetric flask

     Click Here to download Table 1
   • Weigh the filled graduated cylinder and record the mass in your notebook. Then, pour the salt solution back into the 600-mL beaker.
   • Use the volumetric pipette to measure volume. First, weigh the empty 50-mL beaker and record the mass in your notebook. Then, carefully measure 10 mL of the salt solution using the volumetric pipette and dispense the salt solution into the beaker.
   • Measure another 10 mL and add it to the beaker. Weigh the filled beaker and record the mass in your notebook. Then, empty, rinse, and dry the beaker.
   • Use the volumetric flask to measure volume. Weigh the empty 20-mL volumetric flask and record the mass. Then, carefully fill the flask with the salt solution to just below the 20-mL fill mark. Do not fill it all the way. Use an eyedropper to finish filling the flask so that the bottom of the meniscus just touches the fill line on the neck of the flask. Then, weigh the full flask and record the weight.
   • Lastly, use the 50-mL beaker to measure volume. Weigh the empty beaker, then fill the beaker with salt solution to the 20-mL line and weigh the full beaker. Record the masses in your notebook.
   • Measure the displacement of the egg when it is submerged in the salt solution. First, remove the egg and look at the level of the salt solution in the 600-mL beaker. Note: If it is not at a marked line, add deionized water to the solution so that the meniscus is at a readable volume and record the volume.

     Measurements	Salt solution using 20-mL volumetric flask	Egg
     Mass (g)
     Displacement of water (cm)	-
     Inner diameter of 600-mL beaker (cm)	-
     Volume (cm³)	-
     Volume (mL)	20
     Density (g/mL)

     Click Here to download Table 2
   • Gently place the egg in the beaker.
   • Use a ruler to measure the new height of the solution level and record the height. Then, place the ruler across the top of the beaker to measure the inner diameter.
   • Calculate the new volume from these measurements. Subtract the initial volume to obtain the displacement of the egg (volume).
   • To clean up from the experiment, return the egg to your instructor and pour the salt solution down the sink. Rinse all used glassware with tap water and set them out to dry. Finally, return the sodium chloride to your instructor.
2. ResultsExpand
   • Calculate the mass of the solution in each of the different volumetric glassware by subtracting the empty glassware mass from the full glassware mass.
   • Keep track of uncertainty in your measurement by using significant figures. For example, when we look at the graduated cylinder measurement, the weight of the empty container is 90.92, and the weight of the full container is 111.59. The weight measurements have significant figures to the hundredths place, so it is important that significant figures are maintained throughout our calculations. Thus, the mass of the 20-mL salt solution is 20.67 g.
   • The 50-mL beaker weighed 28.65 g when it was empty and 47.96 g when it contained 20 mL of salt solution. Therefore, the mass of 20 mL of the salt solution is 19.31 g.
   • Repeat the calculation to obtain the mass of 20 mL of salt solution that was measured using the 10-mL pipette and the 200-mL volumetric flask. Note: Generally speaking, volumetric measurements are more accurate when using a volumetric flask.
   • Look at the accuracy of the other methods compared to the volumetric flask in terms of percent error. Percent error is calculated by subtracting the theoretical value from the experimental value, then dividing by the theoretical value times 100.

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   • Use the volumetric flask measurement as the theoretical value to determine the percent error for the other volume measurement methods. As expected, the volume measurement that uses the markings on the beaker is the least accurate. Note: We also know this because the known density of pure water is 1 g/mL. Thus, 20 mL of pure water weighs 20 g. The density of the salt solution must be higher than 20 g since salt was added to the water.
   • Calculate the density of the salt solution using the volumetric flask measurement. Divide the mass of the solution by the volume of the solution. Thus, the density of the salt solution is 1.053 g/mL.
   • Compare the calculated density of the salt solution to the density of the egg. The volume calculated using the displacement of water in a beaker was found to be 61.14 cm³ or 61.14 mL. The weight of the egg was measured to be 60.15 g, so its density is 0.9838 g/mL. The disparity between the density of the egg and the density of the salt solution arises from error introduced in both calculations, primarily due to volume measurement readings.