Scientific Measurement and Lab Skills

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

1. Measuring Diameter and CircumferenceExpand

   Error can be introduced in any experiment, even when the technique is performed carefully and correctly. It is essential to critically examine your experimental methods when performing any lab exercise in order to understand and evaluate uncertainty. In this part of the lab, you'll determine a procedure for measuring the circumference of different glassware and then assess sources of error in your measurements.

   • To begin, put on the appropriate personal protective equipment, including gloves, chemical splash goggles, and a lab coat.
   • Set a pad on the benchtop to prevent the breakage of glassware. Flip each beaker upside down on the pad so that the base of the beaker is facing the ceiling.
   • Place one end of the string as close to the edge of the base as possible and extend the string to the other side of the beaker to measure the diameter. Use a marker to mark the string, labeling the edges.
   • Remove the string and use the ruler to measure the distance between the endpoints marked on the string. Record the diameter (and the volume) in your notebook. Repeat the measurement two more times.

     Table 1: Measuring Diameter and Circumference

     		Trial 1	Trial 2	Trial 3
     50-mL beaker	Measured diameter
     	Measured circumference
     	Calculated circumference
     100-mL beaker	Measured diameter
     	Measured circumference
     	Calculated circumference
     250-mL beaker	Measured diameter
     	Measured circumference
     	Calculated circumference
     400-mL beaker	Measured diameter
     	Measured circumference
     	Calculated circumference
     600-mL beaker	Measured diameter
     	Measured circumference
     	Calculated circumference
     Measured Circumference vs. Diameter		Calculated Circumference vs. Diameter
     Slope		Slope
     y-intercept		y-intercept
     R2		R2

     Click Here to download Table 1
   • Place one end of the string at the edge of the beaker and extend the string around the beaker to measure the circumference. Note: Try to measure the circumference at the same height that you measured the diameter while keeping the string as level as possible.
   • Mark the string, measure the length using the ruler, and write down the circumference measurement in your notebook. Repeat the measurement two more times.
   • Collect diameter and circumference measurements for each of the different sized beakers and record them in your notebook.
2. PipettingExpand

   Pipettes are volumetric devices used in the lab to measure and deliver specific volumes of liquids. There are different types of pipettes, but the ones we will use today are called volumetric pipettes. These glass pipettes are labeled with the letters 'TD' and the volume, which denotes that it is designed to deliver the volume of liquid at the specified temperature. The error is also listed. The measurement is correct when the bottom of the meniscus is at the appropriate volume line. A pipetter or bulb is used to draw the liquid into the pipette. In this part of the lab, you will practice pipetting using different volumetric pipettes and examine your consistency and possible sources of errors. Note: You must never pipette by mouth.

   • To begin, fill a 50-mL beaker with tap water.
   • Weigh the empty 10-mL graduated cylinder on the balance and record the weight.

     Table 2: Pipetting 10 mL of Water

     	5-mL pipette			10-mL pipette
     	1st measurement	2nd measurement	3rd measurement	1st measurement	2nd measurement	3rd measurement
     Massempty 10-mL graduated cylinder (g)
     Massfull 10-mL graduated cylinder (g)
     Volumewater (mL)
     Masswater (g)
     Massempty 10-mL graduated cylinder (g)
     Massfull 10-mL graduated cylinder (g)
     Volumewater (mL)
     Masswater (g)

     Click Here to download Table 2
   • Attach a 5-mL pipette to the pipetter and draw up 5 mL of water. Adjust the volume so that the bottom of the meniscus is at the 5-mL measurement line. Then, dispense the water into the 10-mL graduated cylinder.
   • Repeat to add another 5 mL of water to the graduated cylinder.
   • Read the volume filled on the graduated cylinder and record it in your notebook.
   • Return the graduated cylinder to the balance and record the new mass. Subtract the initial mass from this value to obtain the mass of water added to the glassware.
   • Weigh the mass of the empty 10-mL volumetric flask and record the value. Use a 5-mL pipette to measure 10 mL of water into the flask. Return the flask to the balance and record the new mass. Subtract the initial mass to obtain the mass of water added.
   • Dump the water out of the graduated cylinder and volumetric flask and attach the 10-mL pipette to the pipetter.
   • Reweigh the empty graduated cylinder. Then, take up 10 mL of water using the pipette and dispense it into the graduated cylinder. Read the volume of liquid, record it in your notebook, and determine the mass of the water added.
   • Reweigh the empty volumetric flask and then measure 10 mL of water into the volumetric flask. Determine the mass of the water added.
   • Continue to practice pipetting by delivering volumes of liquid to both the graduated cylinder and the volumetric flask. Repeat at least three times for each pipette and glassware. Record all volume and mass measurements so that you can determine the consistency and accuracy of your technique.
3. Vacuum FiltrationExpand

   Vacuum filtration using a Büchner funnel is a technique often used to separate solids from liquids. A Büchner funnel is connected to a filter flask using a rubber adapter and is then attached to a vacuum with silicone tubing. After filter paper is placed inside the funnel, the solution is poured into it. The vacuum pulls the liquid into the flask with the solids remaining on the filter paper in the funnel. In this part of the lab, you'll practice this technique by filtering sand from water using the Büchner funnel setup. You will then check the efficacy of your technique by comparing the weight of the sand before and after filtration.

   • To begin, zero the balance. Place an empty weigh boat on the balance and record the weight in your notebook. Then, tare the balance.

     Table 3: Vacuum Filtration

     Initial mass of sand (g)
     Final mass of sand (g)
     Percent yield

     Click Here to download Table 3
   • Remove the weigh boat from the balance and use the scoopula to transfer sand to the weigh boat. Return the weigh boat back to the balance and check the mass.
   • Add or remove sand until you have about 3 g. Record the weight as the initial mass in your notebook.
   • Obtain approximately 20 mL of acetone in a labeled beaker, making sure to cover it with a watch glass as you bring it back to your workstation.
   • Measure 20 mL of deionized water using the 10-mL graduated cylinder and pour the water into the 100-mL beaker.
   • Obtain red dye solution and measure 5 mL of the dye solution using the 5-mL pipette.
   • Dispense the red dye into your beaker of water and then add in the sand. Thoroughly mix the sand, dye, and water using a glass stirring rod.
   • Set up the vacuum filtration system. Place the rubber adapter in the neck of the filter flask and connect the silicone tubing from the vacuum to the barbed arm of the filter flask. Place the stem of the Büchner funnel into the rubber adapter and make sure it is a snug fit. Center the filter paper inside the funnel, then, turn the vacuum on to begin suction.
   • Gently stir your sand mixture while slowly pouring it into the center of the filter paper. Use your glass stirring rod to direct the mixture to the center of the funnel. Scrape any leftover sand mixture into the funnel using the rubber policeman.
   • Once all of the water has gone through the funnel, turn off the vacuum. Check to make sure that no sand entered the filtrate in the flask. Note: If sand did enter, the filter paper was not centered in the funnel. Re-center the filter paper, then empty the filtrate into the beaker and filter the solution again.
   • Once the solution is properly filtered, turn the vacuum off and rinse the sand on the filter paper with about 3 - 5 mL of deionized water. Turn the vacuum on to remove the water and dye. Repeat the rinsing step until all the red dye is removed from the sand.
   • Turn off the vacuum and disconnect the filter flask from the silicone tubing.
   • Label the 600-mL beaker as 'aqueous waste' and pour the filtrate from the filter flask into the beaker.
   • Reattach the filter flask and tubing to the Büchner funnel. Dry the sand further by rinsing it with 3-5 mL of acetone. Turn on the vacuum to pull the liquid into the filter flask. Repeat this step 1-2 more times to dry the sand completely.
   • For the last drying step, place the large rubber stopper on top of the funnel for about 10 min.
   • Label a separate beaker for organic waste. Turn off the vacuum, disconnect the tubing from the flask, remove the funnel, and pour the acetone into the organic waste flask.
   • Take the funnel and filter paper to the balance and tare a clean weigh boat.
   • Carefully remove the filter paper from the funnel and scrape the sand into the weigh boat using the rubber policeman. Weigh the sand and record the mass as 'final mass'.
   • After 5 min, measure the mass again and note any changes.
   • To clean up from the lab, rinse the aqueous waste down the sink. Pour the organic waste into the designated organic waste container. Discard the sand, filter paper, and string in the trash. Rinse all used glassware with tap water and set them to dry.
4. ResultsExpand

   Now that you have practiced measuring, pipetting, and filtering, let's analyze your techniques.

   • First, let's look at the consistency of your beaker diameter and circumference measurements. Use spreadsheet software to prepare a scatter plot of measured circumference versus measured diameter.
   • Plot diameter as the independent variable on the x-axis, and plot circumference as the dependent variable on the y-axis.
   • Calculate the expected circumference for each of the measured diameters using the following equation: C=πD
   • Plot the expected circumference values as a function of diameter on the same graph.
   • Fit both sets of data using a linear best fit and determine the slope, intercept, and r-squared value of the fit line. The slope and intercept have more significant figures than the original measurement. Our measurements were only accurate to the tenths place. So, the equation is adjusted to y = 2.9x + 2.1.
   • If your measurements of circumference were accurate, the two trend lines should closely match, and the trend line equation should closely match the formula for circumference. You can see here our measurement of circumference, and that our line of best fit has some error.
   • Now, let's look at your pipetting skills. Remember that we used both a 5-mL pipette and a 10-mL pipette to measure 10 mL of water into a 10-mL graduated cylinder and then measured the weight.
   • First, subtract the mass of the empty graduated cylinder from the graduated cylinder containing water to obtain the mass of the 10 mL of water.
   • Here, we show the results of two trials for the 5-mL and 10-mL pipettes. Since the density of water is 1 g/mL, 10 mL of water has a mass of 10 g. The measurement of 10 mL of water was fairly accurate using either volume of pipette, with the 5-mL pipette being slightly more exact.
   • To analyze how well the sand was filtered using the Büchner funnel setup, calculate the percent yield. The percent yield is equal to the final mass of sand, or the actual yield, divided by the initial mass of sand, or the theoretical yield, times 100.

     • 

     Note: It is impossible to obtain a percent yield greater than 100%. As the final mass of sand, or the actual yield, cannot be higher than the initial mass, or the theoretical yield.
   • Here, we show the results from our filtration, where the percent yield is equal to about 75%. The closer the percent yield is to 100%, the better the filtration. Note: If your percent yield is higher than 100%, there could be an error in your calculation, the sand could have been weighed incorrectly, or the sand may not have been completely dry when weighed.