1 Fv 6-26-13 Experiment 5C: Physical Measurements: The

FV 6-26-13 EXPERIMENT 5C: PHYSICAL MEASUREMENTS: THE DENSITY OF A SOLUTION

MATERIALS: 25 mL graduated cylinder; 25 mL pipet; 50 mL buret; 100 mL beaker; plastic bottle with cap; plastic dropper; degassed Coke® and Diet Coke®.

PURPOSE: The purposes of this experiment are: (1) to compare the precision of common volumetric glassware; (2) to determine the densities of different aqueous solutions.

LEARNING OBJECTIVES: 1. Identify and use the following volumetric glassware: graduated cylinder, buret, pipet 2. Calculate the density of a solution. 3. Describe the differences in the precision of the volumetric glassware used. 4. Describe the relationship between experimental measurements, precision, and significant figures. 5. Apply the rules for significant figures. 6. Explain the difference between intensive and extensive quantities.

DISCUSSION: Density is a physical property, defined as the ratio of a substance’s mass divided by its volume. Density is expressed 3 in units of g/cm for solids, or g/mL for liquids or solutions. We can determine a sample’s density by measuring its mass and volume, and taking the quotient. Mass can be measured precisely and accurately with an analytical balance. The volume of a liquid is measured with volumetric glassware. Some common types of volumetric glassware are the graduated cylinder, buret, and pipet. Your instructor will demonstrate how to use each of these volume measurement devices.

How accurate a density calculation is depends on the accuracy and precision of the instruments measuring mass and volume. In this experiment, the analytical balance is the most precise instrument. Because the volume measurement devices are less precise than the balance, they limit the precision of the density calculations. The accuracy and precision of the volumetric glassware depends on the type of glassware and the skill of the operator.

PROCEDURE: The instructor will assign one of the two sample solutions (Coke or Diet Coke) to each group. Each group will perform all of the measurements on their assigned sample solution. Ensure that all glassware is clean before you start. If it is not, clean it with soap and water, and then rinse with tap water, and finally with distilled water.

Part A. Graduated Cylinder 1. Empty any residual liquid from the plastic bottle. Cap the bottle, and make sure that the outside of the plastic bottle and cap is dry. Using the analytical balance, weigh the empty plastic bottle with cap. Record the mass, including units, for Trial I in the Data Section. 2. Using the 100 mL beaker, obtain approximately 60 mL of your assigned solution from the laboratory stock bottle. Record the identity of the solution in the Data Section.

3. Rinse the graduated cylinder with the sample solution you are about to use. To rinse the glassware, place a small amount of the solution in the cylinder, manipulate the glassware until all interior surfaces have been contacted by the solution, and then discard the rinse. 4. Fill the graduated cylinder to the 25 mL mark with the sample solution. Record the volume, with units and the correct number of significant figures, in the Data Section. 5. Pour the "25 mL" sample into the previously weighed plastic bottle. Recap the bottle, and use the analytical balance to determine the mass of the capped bottle plus sample. Record the mass, including units, in the Data Section. 6. Repeat Steps A.4 and A.5. Record the data from Trial II in the Data Section.

Part B. Buret 1. Repeat steps A.1-A.2 2. Rinse the buret with the assigned sample solution the same way as in step A.3. 3. Mount the buret on a stand using a buret clamp. Fill the buret to the 0.00 mL mark with the sample solution, ensuring no air bubbles are in the glass tip below the stopcock. 4. Deliver a 25 mL sample into the previously weighed plastic bottle, and record the total volume delivered in the Data Section. Record the exact volume (to the hundredths place) delivered, even if slightly off the target value. 5. Recap the bottle. Using the analytical balance, record the mass of the capped bottle plus sample in the Data Section. 6. Repeat Steps B.4 and B.5. Record the data from Trial II in the Data Section.

Part C. Pipet 1. Repeat steps A.1-A.2 2. Rinse the pipet with the assigned sample solution the same way as in step A.3 3. Draw 25 mL of sample into the pipet and deliver this sample into the previously weighed plastic bottle. Allow it to drain completely. Any drops clinging to the outside of the pipet tip should be touched off into the plastic bottle. Do not use your bulb to blow out the remaining liquid. Record the volume in the Data Section. (The pipet should deliver 25.00 mL at calibration temperature. Temperature effects are minimal and will be ignored). 4. Recap the bottle. Using the analytical balance, record the mass of the capped bottle plus sample in the Data Section. 5. Repeat Steps C.3 and C.4. Record the data from Trial II in the Data Section.

Clean Up: Rinse all glassware with distilled water. Lay the graduated cylinder on its side, and allow it to drain. Invert the beaker and plastic bottle and place them on a paper towel to allow them to drain. Invert the buret and mount it in the buret clamp on the ring stand, with stopcock open; allow it to drain. At your station, clean your work area and leave on the bench top only the material that was originally there.

DATA AND ANALYSIS SECTION Group #______Sample:______(NOTE: all data and calculations should adhere to significant figures standards)

Part A. Graduated Cylinder Trial I Trial II Mass of capped bottle without sample (g) Mass of capped bottle with sample (g) Mass of Sample (g) Volume of Sample (mL) Density of Sample (g/mL) Average Density of Trials I and II (g/mL)

Part B. Buret Trial I Trial II Mass of capped bottle without sample (g) Mass of capped bottle with sample (g) Mass of Sample (g) Volume of Sample (mL) Density of Sample (g/mL) Average Density of Trials I and II (g/mL)

Part C. Pipet Trial I Trial II Mass of capped bottle without sample (g) Mass of capped bottle with sample (g) Mass of Sample (g) Volume of Sample (mL) Density of Sample (g/mL) Average Density of Trials I and II (g/mL)

Class Data: Average Density (g/mL) Graduated Cylinder Buret Pipet Graduated Cylinder Buret Pipet Group Coke Coke Coke Diet Coke Diet Coke Diet Coke 1 ------2 ------3 ------4 ------5 ------6 ------7 ------8 ------9 ------10 ------

QUESTIONS

1. In this experiment, what term (mass or volume) determined the number of significant figures in your density calculations? Explain your answer.

2. Based on your class data, which piece of glassware provides the least precise and most precise density values? Explain your answer.

3. What, if anything, can be said about the accuracy of the measurements made with the three different pieces of volumetric glassware? Explain your answer.

PRE-LAB QUESTIONS EXPERIMENT 5C

1. For each of the following pairs, circle the property that best describes a solution of Coke.

a) homogeneous or heterogeneous

b) pure substance or mixture

2. What is the correct volume reading (in mL) for the liquid in the graduated cylinder shown below? Record your answer with the correct number of significant figures.

3. How many “uncertain digits” should be included in any measurement or calculation?

4. List two intensive properties and two extensive properties:

INTENSIVE: ______

EXTENSIVE: ______