Experiment 1

BPD: Balances, Pipets, and Data Analysis

Introduction:

            The purpose of this experiment is to familiarize the student with:

·        the use and care of a digital top-loading milligram balance

·        pipeting (obtaining a volume to a high degree of accuracy)

·        uncertainty in measurements, including the concepts of accuracy, precision, and significant figures.

·        units of measurement including density

·        the proper way to report scientific results

·        data analysis including generating a graph and basic statistics (mean and standard deviation, relative standard deviation, percent difference).

Determining the density of distilled water will develop these skills.

 Background reading:

Cooperative Chemistry Laboratory Manual:

Pages

Topic

13-15  

Safety Rules, Laboratory Etiquette

17

Waste Disposal

36-41

Reporting Numerical Results, Significant Figures, Graphs

45-47

Containers, Measuring Liquids by Volume

68

Reading a Meniscus

66

Using a Volumetric Pipet

49

Laboratory Balance

 Chemistry & Chemical Reactivity

Section 1.4                   Physical Properties (pp20-21)

Section 1.6-1.7            Measurements (30-34)

Section 1.8                   Mathematics of Chemistry (pp35-44)

 

Chemical Principles, The Quest for Insight

Section A.1                  Physical Properties (pp F6-F10)

Instrumentation and Equipment:

The milligram top-loading balances are expensive and should be handled with great care. Never pour a chemical into a container while the container is on the balance.  Remove the container from the balance, add the chemical to the container, and then place the container back on the balance.  Be sure to clean up any spills immediately.  Your instructor will provide instructions for cleaning up any chemical spill.

There are many ways to obtain measured volumes. These differ in the degree of accuracy. Graduated cylinders are glass equipment calibrated to contain (TC) a known volume. The graduated cylinder’s level of accuracy is limited to one decimal place beyond the smallest graduation. In addition, when transferred to another container, the volume becomes uncertain because some of the water sticks to the surface of the glass. The graduated cylinders are in your assigned equipment locker. Volumetric or transfer pipets are glass equipment calibrated to deliver (TD) a known volume (one volume only) to another container, such as a beaker or flask. They are generally the most accurate way to get a known volume (typically to 2 decimal places). However, volumetric pipets only come in a few common sizes (1, 2, 5,10,25,50 mL). Graduated or serological pipets (TD) are also common, but not as accurate (also limited to one decimal place beyond the smallest graduation). However, a graduated pipet will deliver different volumes. Pipets and rubber bulbs will be obtained from the common equipment area at the front of the lab. Clean and return all common equipment after use. Report any breakage to your instructor immediately.

Use the computers in the computer room to generate your graphs.  The use of these computers is limited to laboratory work. If additional computers are required, you can use the computers in the ACC lab on the second floor.  Follow all rules for the appropriate use of computers as given in the University Appropriate Use Policy.

 Chemicals

Distilled water (located in the plastic carboy on lab benches).

Safety Precautions

Wear safety glasses (goggles) at all times. Even though water is being used, other hazards still exist. If a piece of glassware is broken, dispose of glass in special containers at the front of the lab. It is your responsibility to clean up your accidents, not your instructors. A whiskbroom and dustpan are available at the front of the lab.

 

Experimental Procedure:

Part A: Density of Water by Graduated Pipet

*Your instructor will demonstrate the use of a graduated pipet.

The purpose of this part is to determine the density of water and evaluate the accuracy and precision of the result using basic statistics.

  1. Obtain a graduated pipet and rubber bulb from the common equipment area.
  2. Fill a 100-mL beaker approximately half full with distilled water. This should be enough water for the entire experiment.
  3. Weigh a clean, dry 100 or 150 mL beaker or flask and record the mass to three decimal places on the data sheet.
  4. Add about 5-mL of distilled water to the pre-weighed beaker or flask. Be sure to record the initial and final volumes of the pipet. (You will want to practice this skill a few times first.)
  5. Reweigh the beaker, and calculate the density of water.
  6. Repeat the procedure twice, and then follow the instructions on the data sheet for data analysis.

Part B: Density of Water by Volumetric Pipet

*Your instructor will demonstrate the use of a volumetric pipet.

The purpose of this part is to determine the density of water using a graphical approach.

  1. Obtain a 5, 10, and 25-mL volumetric pipet and rubber bulb from the common equipment area.
  2. Fill a 100-mL beaker approximately half full with distilled water. This should be enough water for the entire experiment.
  3. Weigh a clean, dry beaker or flask and record the mass on the data sheet.
  4. Using a 5-mL volumetric pipet, pipet distilled water into the pre-weighed beaker and reweigh. Record the mass. (You will want to practice this skill a few times first.)
  5. Repeat the procedure with each of the other two pipets (10 and 25-mL pipets), then follow the instructions on the data sheet for data analysis.

 

Part A Data Sheet

Part B Data Sheet