Materials Included In Kit

Additional Materials Required

Prelab Preparation

Prepare the following reference solutions by placing the ingredients in a suitable container, such as a large beaker, Erlenmeyer flask or bottle. Stir or shake well until all of the sugar has dissolved and the solutions are homogeneous.

• 0% sugar: 500 g of distilled or deionized water
• 5% sugar: 475 g of water, 25 g of table sugar and two drops of yellow food coloring
• 10% sugar: 450 g of water, 50 g of table sugar, one drop of blue and one drop of yellow food coloring
• 15% sugar: 425 g of water, 75 g of table sugar and two drops of blue food coloring
• 20% sugar: 400 g of water, 100 g of table sugar and one drop of blue plus one drop of red food coloring.

Safety Precautions

Although the materials in this experiment are considered nonhazardous, please follow all normal laboratory safety guidelines. Wear chemical splash goggles whenever working with chemicals, heat or glassware in the lab. Food-grade items that have been brought into the lab are considered laboratory chemicals and are for laboratory use only. Do not taste or ingest any materials in the chemistry laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All solutions may be rinsed down the drain with plenty of water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The experimental work for this lab can be completed within a typical 2-hour lab period. This experiment can also be downscaled to a 2-mL scale using plastic disposable syringes or disposable pipets.
• Juices and sports drinks such as Gatorade^® can be used as is from the bottle. Carbonated beverages must be “flattened” first to remove the gas bubbles. This can be done by pouring the beverage back and forth between two large containers. Carbonated gas or air bubbles will interfere with the accuracy of the volume measurement. Juices containing a large amount of pulp should also be filtered before use.
• Review with students the correct use of a pipet and how to read a meniscus. Students should be encouraged to work in pairs, but do not allow one student in a pair to become the professional data recorder. Each student should learn and practice the correct pipetting techniques.
• Volumetric pipets provide the most accurate and precise volume transfer. If these are not available, Mohr pipets or inexpensive, disposable serological pipets may also be used.
• Consider investing in pipet fillers to replace pipet bulbs. Pipet fillers are much easier for students to use.
• This experiment gives excellent results. Choose beverages whose sugar contents are relatively far apart (more than 1 g of sugar per serving size). See the Sample Data results table for examples.
• The “exception that proves the rule” in this study is Powerade (and other types of sports drinks). The working assumption in this experiment is that sugar is the main ingredient whose concentration contributes to the density of the solution—the other ingredients should not be present in large enough amounts to affect the density. This assumption is true for sodas and juices, but not for sports drinks, which contain large amounts of salts (particularly sodium and potassium chloride) to maintain electrolyte balance. Sports drinks thus provide a good example to stimulate classroom discussion of the experiment and its underlying assumptions.
• Fructose (“fruit sugar”) is the main sugar present in fruit juices, fruit drinks, such as Snapple^®, and carbonated sodas. In fact, “high fructose corn syrup” is usually the second ingredient (after water!) listed on the ingredients label. Table sugar is sucrose, a disaccharide composed of one molecule of fructose combined with one molecule of glucose.
• We tested fructose and sucrose reference solutions in this lab to see which one gave better agreement with the information provided on the nutrition labels. Both fructose and sucrose gave excellent graphs of density versus percent sugar concentration. Percent sugar concentrations were calculated for all of the beverages using both sets of data (fructose and sucrose). Both sets of results agreed equally well with the nutrition label information—there was no difference in the accuracy (percent error) of these two methods. For this reason, the experimental section for this lab was written using the more readily available “table sugar” (sucrose) rather than fructose.
• Students may be confused about how to read a nutrition label. Consider the sample label shown here. The relevant information on this label is the grams of sugars, not the Percent Daily Value. This soda contains 40 g of sugar per 355 mL of beverage.
  {14025_Hints_Figure_2}

• Volumetric pipets are designed and used to accurately deliver a specific volume of liquid from one container to another. A 10-mL volumetric pipet has a tolerance of 10.00 ±0.02 mL. Fill the pipet to the graduation mark as shown in Figure 1. After filling the pipet, wipe the sides of the pipet only with a clean paper towel. Do not blot the tip of the pipet. To transfer the liquid, place the pipet in a new container. Holding the pipet at a slight angle, allow the liquid to drain slowly into the container. Never pipet by mouth.
  {14025_Hints_Figure_1_How to use a pipet}

Answers to Prelab Questions

1. If the following mass and volume data are used to calculate the density of solution, how many significant figures are allowed in the calculated density? Mass of solution = 12.53 g; volume of solution = 8.27 mL.

   The number of significant figures allowed in the calculated density is three.

2. Calculate the density of the solution described in Question 1.
   {14025_PreLabAnswers_Equation_1}
3. According to its nutrition label, orange soda contains 49 g of sugar per 355-mL serving. If the density of the beverage is 1.043 g/mL, what is the percent sugar concentration in orange soda? Hint: This is a 2-step problem. First, use the density to convert the 355-mL serving size to grams. Then, calculate percent sugar in the beverage.
   {14025_PreLabAnswers_Equation_2}
4. How well does the sweet taste of a beverage correlate with the amount of sugar it contains? Based on your memory of taste, predict the relative sugar content of the following beverages that will be tested in this lab: cola, grape juice and sports drink.

   Student predictions will vary. Remind students not to taste any beverages in the lab.

5. The following graph is a calibration curve for the density of an aqueous salt solution versus percent salt concentration. If the density of seawater is 1.025 g/mL, what is the percent salt concentration?
   {14025_PreLabAnswers_Figure_3}
   According to the graph, seawater contains 3.8–3.9% salt (sodium chloride).

Sample Data

Laboratory Report

Answers to Questions

1. Plot density versus concentration for the five reference solutions on a graph. The concentration is the independent variable (x-axis), and the density is the dependent variable (y-axis). Use a spreadsheet program or ruler to draw a “best-fit” straight line through the data points.
   {14025_Answers_Figure_4}
2. Use the graph to estimate the unknown sugar concentration in each beverage. To do this, locate the point on the y-axis that corresponds to the density of the beverage. Follow that point on the y-axis across horizontally to where it meets the best-fit straight line. Read down vertically from this point on the best-fit line to the x-axis to estimate the percent concentration of sugar in the beverage. Construct a results table and record the density of each beverage and its estimated percent sugar concentration.

   See the Sample Data results table.

3. Calculate the actual or accepted value of the sugar concentration in weight percent for each beverage, using the nutrition label information and the measured density value. Hint: See Prelab Question 3 for how to do this calculation. Record both the nutrition label information and the actual percent sugar concentration in your results table.

   Sample calculation for cola:

   Nutrition label: 42 g of sugar per serving (355 mL)
   Density (determined in this experiment): 1.038 g/mL

   {14025_Answers_Equation_3}
   See the Sample Data results table.
4. Calculate the percent error in your experimental determination of the sugar content in each beverage. Enter the percent error in the results table.
   {14025_Answers_Equation_4}
   See the Sample Data results table.
5. What was your measured density for pure water (0% sugar solution)? The density of water is usually quoted as 1.00 g/mL, but this precise value is for 4 °C. Comment on why your measured density might be higher or lower than 1.00 g/mL.

   The measured density of pure water was 0.998 g/mL. This is less than the quoted density of 1.00 g/mL because the temperature was higher than 4 °C.

6. This lab looks at the relationship between the density of a beverage solution and its sugar content. What assumption is made concerning the other ingredients in the beverage and their effect on the density of the solution? Do you think this is a valid assumption? Explain.

   The assumption in this lab is that sugar is the major ingredient in each beverage. It is further assumed that the other ingredients are present in small enough amounts that they do not affect the beverage density. This assumption is not valid for certain types of beverages, such as sports drinks, that contain other salts in high concentrations.

7. When plotting data, such as that obtained in this experiment, why is it not appropriate to “connect the dots?” If you were to repeat the lab, do you think you would get exactly the same results? Comment on sources of error in this experiment and their likely effect on the results.

   You cannot “connect the dots” with the data points on the graph because this would “hide” the straight-line relationship through the data. Each point should actually be represented by error bars that define the precision or reproducibility of each measurement. Thus, if a density “point” at 1.032 g/mL were measured again, it might come out 1.029 or 1.036 g/mL. Sources of experimental error include variations in volume transfer using the pipet, contamination of the pipet from one solution to another, the presence of bubbles in carbonated beverages, etc.

Introduction

Seawater is more dense than freshwater due to the presence of dissolved salt in the ocean. As a result, our buoyancy—ability to float—is greater in salt water than in freshwater. What factors determine the density of a solution? Can the density of a solution be used to determine how much of a particular substance is dissolved in it?

Concepts

• Density
• Solution
• Concentration
• Calibration curve

Background

The density of a pure substance is a characteristic physical property that can be used to identify the substance. Density is defined as the ratio of mass per unit volume. It is an “intensive” property, that is, it does not depend on the amount of the substance. The density of any material is determined by measuring its mass and volume and then dividing the mass by the volume. The mass of a substance can be measured directly using a balance. The volume of a liquid can also be measured directly using laboratory glassware, such as a graduated cylinder, buret or pipet. In this experiment, liquid volumes will be measured using a pipet, which is designed to deliver an accurate and precise volume of liquid.

The density of a solution depends on its concentration (i.e., how much solute (solid) is dissolved in the solvent, or liquid). The higher the concentration of solute, the greater the density of the solution. A convenient way to express concentration is in units of weight percent, which corresponds to the number of grams of solute that are present in 100 g of solution. A 20% salt solution is prepared by dissolving 20 g of sodium chloride in 80 g of water. (Notice that the final mass of the solution is 100 grams.) If the density of a solution is plotted on a graph against the concentration of solute, a regular pattern is evident. Density is directly proportional to concentration. A 20% salt solution, for example, has a greater density than a 10% salt solution. If the densities of several solutions of known concentration of the same substance are determined experimentally, a calibration curve (graph) can be constructed that shows a straight-line relationship between the density of the solution and the concentration of solute. The calibration curve can then be used to determine the concentration of solute in an unknown solution.

Experiment Overview

The purpose of this experiment is to determine the percent sugar content in beverages. The density of five sugar reference solutions will be measured and plotted on a graph to obtain a calibration curve of density versus percent sugar concentration. The reference solutions contain known amounts of sugar (0–20%) and have been dyed with food coloring to make it easier to tell them apart. The densities of two beverages will also be determined and the calibration curve used to find how much sugar they contain. The results will be compared against the information provided on the nutrition labels for these beverages.

Materials

Prelab Questions

1. If the following mass and volume data are used to calculate the density of solution, how many significant figures are allowed in the calculated density? Mass of solution = 12.53 g; volume of solution = 8.27 mL.
2. Calculate the density of the solution described in Question 1.
3. According to its nutrition label, orange soda contains 49 g of sugar per 355-mL serving. If the density of the beverage is 1.043 g/mL, what is the percent sugar concentration in orange soda? Hint: This is a 2-step problem. First, use the density to convert the 355-mL serving size to grams. Then, calculate percent sugar in the beverage.
4. How well does the sweet taste of a beverage correlate with the amount of sugar it contains? Based on your memory of taste, predict the relative sugar content of the following beverages that will be tested in this lab: cola, grape juice and sports drink.
5. The following graph is a calibration curve for the density of an aqueous salt solution versus percent salt concentration. If the density of seawater is 1.025 g/mL, what is the percent salt concentration?
   {14025_PreLab_Figure_1}

Safety Precautions

Although the materials in this experiment are considered nonhazardous, follow all normal laboratory safety guidelines. Wear chemical splash goggles whenever working with chemicals, heat or glassware in the lab. Food-grade items that have been brought into the lab are considered laboratory chemicals and are for laboratory use only. Do not taste or ingest any materials in the chemistry laboratory.

Procedure

Density of Reference Solutions

1. Place an empty 100-mL beaker on the balance and press the tare or rezero button. The scale should read 0.00 g.
2. Draw up 10.00 mL of 0% sugar (distilled water) into a pipet and transfer the liquid to the empty beaker.
3. Measure and record the mass of the solution.
4. Rezero the balance using the tare button.
5. Blot the tip of the pipet gently with a paper towel to clean out any residual solution.
6. Repeat steps 2–5 four times to measure the masses of the four sugar reference solutions. Proceed in order from the least concentrated to the most concentrated reference solution.
   • Rinse the pipet once with each new solution before using the pipet to transfer the new solution to the beaker.
   • Drain the rinse solutions into the Erlenmeyer flask.
   • Remember to rezero the balance prior to each new mass measurement.
7. Calculate the density of each solution and record the results. Hint: Recall that the volume of each solution should be 10.00 mL.

Beverage Densities

1. Determine the density of two beverages of your choice.
2. Use clean glassware and record all mass and volume data.
3. Rinse the pipet with the second beverage between successive beverage measurements.

Student Worksheet PDF

14025_Student1.pdf