Coke Floats (Or Does It?)

Transcription

1 Date Lab Time Name Objective Coke Floats (Or Does It?) Scientific measurements will be introduced using common soft drinks as examples. Studentcollected data will be pooled leading to a discussion of accuracy and precision. Background The measurements physical scientists make are often used in calculations to obtain other related quantities. A number of common devices enable us to make simple measurements of a substance s properties: the meter stick measures length, the graduated cylinder measures volume, the balance measures mass and the thermometer measures temperature. These physical measurements allow for characterization and possible identification of a substance. A Physical property that is a characteristic of substance include melting point, boiling point, and density. All chemically pure water samples, for example, melt at 0 o C, boil at 100 o C under one atmosphere pressure, and have a density of approximately 1.0 g/ml. If a particular measurement is repeated, precisely the same result is usually not obtained because each measurement is subject to some type of experimental error. Suppose you perform a series of identical measurements of a quantity. The term precision (P) refers to the closeness of a set of values to each other which were obtained from identical measurements of the same quantity. Accuracy (A) is a related term that refers to the closeness of a single measurement to its true or excepted value. Look at the dart boards below as examples. High A Low P Low A High P High A High P Low A Low P Coke Floats (Or Does It?) 1

2 Date Lab Time Name Materials needed ml graduated cylinders 12-oz can of room temp. balance 12-oz can of Diet room temp. pitcher of sweetened room temp. Ruler Procedure Part I Regular Soda Measurements Density: 1. Using an electronic balance, measure the mass of an empty 100 ml graduated cylinder and record the mass on your data sheet. 2. Add approximately 75 ml of one of the regular soft drink to the empty graduated cylinder. Read the actual volume of the soft drink as accurately as possible. Record the volume of the soft drink on your data sheet. {When reading volumes using graduated devices, accurate and precise values are obtained by reading at eye level and using the bottom of the meniscus}. 3. Using the same electronic balance you used in Step 1, measure the mass of the soft drink plus the graduated cylinder. Record the mass on your report sheet. 4. Calculate the mass of just the volume of soft drink and record. 5. Using the expression below, calculate the mass density of the soft drink and record. {Mass Density is computed by dividing the mass of a substance by the volume it occupies. This relationship is expressed mathematically as} mass m mass density = or ρ = volume V Verifying the Manufacture s Claims: 6. Using your graduated cylinder, determine and record the total volume of soft drink in the can by repeating Step 2 until all the soda in the can has been measured. 7. Record the total volume of the soda as listed on the side of the can. 8. Calculate (do not measure) the total mass of the soft drink using the soda s mass density from Step 5 and your result from Step Determine the % Sugar in the can of soda by dividing the amount of sugar per serving (as listed on the side of the can) by your total calculated mass from Step Using the graph on page 4, determine the % sugar in your sample of soft drink and record. {Draw a horizontal line at the same density reading as your calculated density. At the point where it crosses the Regression Line, draw a vertical line downward. The point where it touches the bottom axis will represent the % sugar in your soft drink.} 11. One member of your group go to the board and write your values for total volume and density under the appropriate columns for class analysis. Part II Diet Soda Measurements Repeat the entire procedure in Part I using a diet soda. Part III Unknown Measurements Repeat Steps 1 6 of the procedure in Part I using an unknown sample of lemonade. Coke Floats (Or Does It?) 2

3 Date Lab Time Name Report Sheet Data Analysis Part I - Regular Soda Measurements Mass of 100 ml graduated cylinder empty Combined Mass of ~75 ml of soft drink & 100 ml graduated cylinder Mass of ~75 ml of soft drink only Actual Observed Volume of soft drink in cylinder Density of soft drink /ml Total measured volume of soft drink in can * Total volume of soft drink as listed on side of can Total calculated mass of soft drink in can * Percent sugar in soft drink (Manufacturer s Claim) % Percent sugar in soft drink (from graph) % Calculations: Part II - Diet Soda Measurements Mass of 100 ml graduated cylinder empty Combined Mass of ~75 ml of soft drink & 100 ml graduated cylinder Mass of ~75 ml of diet soft drink only Actual Observed Volume of diet soft drink in cylinder Density of diet soft drink /ml Total measured volume of diet soft drink in can * Total volume of soft drink as listed on side of can Total calculated mass of soft drink in can * Percent sugar in diet soft drink (Manufacturer s Claim) % Percent sugar in diet soft drink (from graph) % Calculations: Coke Floats (Or Does It?) 3

4 Date Lab Time Name Part III - Unknown Measurements Mass of 100 ml graduated cylinder empty Combined Mass of ~75 ml of unknown & 100 ml graduated cylinder Mass of ~75 ml of unknown sample only Actual Observed Volume of unknown sample in cylinder Density of unknown sample /ml Percent sugar in unknown sample (from graph) % Calculations: Coke Floats (Or Does It?) 4

5 Date Lab Time Name Post-lab questions: 1. Using both the Soda and Diet Soda, compare the % sugar you calculated and your measurements for the total volume for each can against the manufacturer s claim (see label on can). What is your percent error in the total volume for each case? 2. Suppose a certain individual can have 1 glass of sweetened lemonade a day without getting ill so long as the sugar content is less than 5 %. Would a glass of lemonade from your unknown sample be safe for them to drink? Why? 3. Why was it necessary for the soft drinks to be open at room temperature? 4. Copy the Results of the Classroom from the board below and then calculate the average values of total measured volume and calculated mass density for both the regular and diet soft drinks. Comment on the accuracy and precision in the data. Regular Soft Drink Diet Soft Drink 5. Why can t you make any statements about the precision of your individual data? 6. What specific sources of error (do not include human error) in this experiment could have produced inaccurate results? Coke Floats (Or Does It?) 5

6 Date Lab Time Name Summary/Conclusions: Coke Floats (Or Does It?) 6