Determining the Concentration of Sugarin a Commercial Beverage

PRE-LAB ASSIGNMENTS:

To be assigned by your lab instructor.

STUDENT LEARNING OUTCOMES:

Calculate concentrations in units of molarity Calculate dilutions Use density as an analytical tool Graph data and perform a linear regression analysis Distinguish and identify precision and accuracy Recognize factors in experimental error

EXPERIMENTAL GOALS:

You will use the concepts learned in previous labs to generate a standard curve of the density of sugar-water solutions. Using this curve, you will determine the concentration of sugar in a commercially-available drink or juice and compare to the advertised concentration.

INTRODUCTION:

Chemists often deal with solutions, which are homogenous mixtures of substances. Generally, a solution (often abbreviates as soln.) is composed of the solvent, which is the major component in the mixture, and solute, which is the minor component in the mixture. It is important to be aware of the relative amounts of solvent and solute in a solution, or the ratio

to understand its properties. Chemists refer to this ratio as a solution’s concentration.

In a previous laboratory, it was shown that chemists use the mole to designate the amount of substance. Recall that the mole, like any other designation of a quantity (e.g., dozen, gross, ream), can apply to counting anything, from atoms or molecules to marbles or softballs. The mole is a convenient means of expressing a quantity of substance present, because it is independent of the type of substance measured. This makes the mole a valuable quantity for comparing properties or reactivities of different substances. While the mole is a powerful conceptual tool, it is difficult to directly measure a mole in the laboratory. Instead, we can use the molar mass to determine the number of moles present from a given mass of substance,

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because mass is a much easier quantity to determine in the laboratory. As you have learned, the quantities of mass and the mole are related through molar mass.

Another unit often used by chemists is the liter, a measurement of volume. Volume is an extensive property, but when a given substance’s mass is divided by its volume, we can report the density of the substance, which is an intensive property. In a similar fashion, chemists use molarity to discuss a solution’s concentration. Molarity, often designated by capital M, is the ratio of moles of solute to liters of solution,

Molarity = [1]

Thus a 1.0 M solution of table salt in water (a so-called aqueous solution) has 1.0 moles (~58 g) of NaCl in 1.0 L of solution. Similarly, a 0.5 M solution of salt water has 0.5 moles (~29 g) of NaCl in 1 L of solution. It is important to note that the molarity of a solution is an intensive property, much like density, and therefore does not depend on the amount of solution present. Thus, we can use the molarity of a solution to determine the number of moles of solute present in a given volume in the solution.

It is often convenient to prepare a solution of lower concentration from a more concentrated solution in the chemistry laboratory using the technique of dilution. In everyday life, dilutions are important as well. An example of a dilution is when a pitcher of orange juice is made from frozen juice concentrate and water. The frozen orange concentrate is generally considered too intense to consume directly, so a predetermined volume of water is mixed with the concentrate to make a more palatable dilution. The dilution can be varied to taste, with some people preferring weaker orange juice and others preferring stronger. Food preparation has many other examples of dilutions.

Chemists use dilutions frequently in the laboratory. By starting with a given concentration of solution, solute or solvent may be added to adjust the concentration to a desired value. In practice, solvent is added, usually as a volume, to achieve the desired diluted concentration. Thus, if an experimenter had 1.0 L of a 1.0 M solution of salt water, a dilution to 0.50 M concentration could be made by adding enough water to the original solution to make a total of 2.0 L. The relationship between the initial concentration and the final concentration can be given by

Mi Vi = Mf Vf [2]

where Mi and Mf are the initial and final solution concentrations (in mol/L), respectively, and Vi and Vf are the initial and final solution volumes (in L), respectively.

In this laboratory you will make a stock concentration of sugar (sucrose, C12H22O11) in water and two dilutions. You will then measure the density of these solutions, and members of the class will share data to generate a standard curve of concentration vs. density for sugar dissolved in water. You will then use this data to determine the concentration of sugar in a commercially available beverage or juice that you will provide (please do not use a diet soda!).

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SAMPLE CALCULATIONS:

1. A student prepares a 0.97 M stock solution of sodium hydroxide (NaOH) in distilled water. What volume of the stock solution must be added to distilled water to give 100 mL of a 0.75 M solution?

Solution: Mi = 0.97 M, Mf = 0.75 M, and Vf = 100 mL = 0.100 L.

Rearranging Equation 2 we have

= 0.077 L = 77 mL

Hence it will require 7.7 mL of the original solution, diluted to 100 mL to yield a 0.75 M final solution.

2. A student diluted 20.0 mL of a 4.31 M solution of aqueous sodium chloride (NaCl) to 250. mL. What is the final concentration of the solution?

Solution: Mi = 4.31 M, Vi = 20.0 mL = 0.0200 L, and Vf = 250 mL = 0.250 L.

Rearranging Equation 2 we have

0.345 M

3. How many moles are in 250. mL of the final solutions prepared in Examples 1 and 2 above?

Solution: In Example 1, the final concentration is 0.75 M or 0.75 mol/L. Hence 250. mL of this solution will yield (0.75 M)(0.250 L) = 0.19 moles of NaOH.

In Example 2, the final concentration is 0.345 M or 0.345 mol/L. Hence 250. mL of this solution will yield (0.345 M)(0.250 L) = 0.0863 moles of NaCl.

PROCEDURE:

Safety Concerns: Once a beverage is opened in the labs, do NOT drink it. Use pipet bulbs with pipets. Never pipet by mouth!

In this experiment, you will work in groups of two (or three if there is an odd number of people in the class). Once all of the dilution data has been measured, the entire class will pool their data (in the Class Data Table in the report sheet) to generate a graph of sugar concentration (in Molarity) vs. density. Each pair will then use this data to determine the concentration of sugar in a commercially-available beverage.Do not use a diet soda as your commercial beverage!

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Begin degassing your soda at least fifteen minutes before beginning part C of the procedure.

A. Preparation of Stock Solutions

Three stock solutions of 1.500 M, 1.250 M and 1.000 M sugar (sucrose, C12H22O11) in water will be prepared. Each group will be assigned a different stock solution to make.

1. Determine the mass of a clean, dry 50-mL volumetric flask (with stopper) to the nearest 0.001 g.

2. Depending on which solution your group is preparing (1.500 M, 1.250 M, or 1.000 M), determine the number of grams of sugar necessary to make a 50.00 mL solution. (Record this in the Lab Report in the space marked “Calculated Mass of Sugar.”) Obtain this amount of sugar, and record the measured mass to the nearest 0.001 g. (Record this in the Lab Report in the space marked “Measured Mass of Sugar.”)

3. Add the sugar to the volumetric flask. Because of the small neck on the 50-mL flask, a funnel may be necessary to avoid spills. Make sure the funnel is clean and dry. Also, make sure that all of the sugar is added to the flask. While the funnel is still in the neck of the flask, wash the funnel down with distilled water making sure that all of the washings go into the flask.

4. Add enough DI water to the flask to fill the bulb about halfway. Stopper the flask, and invert several times to get the sugar to dissolve. Once most of the sugar has dissolved, add enough DI water to the flask to fill the bulb about ¾ of the way; swirl and invert the flask several times to ensure good mixing. (If your group is preparing the 1.500 M solution, you may need to use a warm water bath to help the sugar to dissolve.)

5. Once all the sugar is completely dissolved, use a plastic transfer pipet to fill the flask to just below the line on the neck. Again, invert the flask several times to make sure that the solution is homogenous.

6. Finally, using the transfer pipet, add enough water to the flask to reach the line, and invert the flask a few times. Make sure that all of the sugar has dissolved, and that the solution is homogeneous.

7. Once the solution is prepared, measure the mass of the volumetric flask with the solution to the nearest 0.001 g. Determine the mass of the solution, and calculate the density of the stock solution. From the actual amount of sugar weighed, and the volume of the flask used, calculate the actual molarity of your stock solution. Enter it on the appropriate line in the lab report.

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8. Transfer the contents of the volumetric flask to a beaker, and label the beaker “Stock solution.” Rinse the flask a minimum of three times with DI water, and dispose of the washings in the sink (do not add the washings to the stock solution!).

B. Preparation of Two Dilutions

First Dilution.

1. For the first dilution, determine the volume of prepared stock solution necessary to give 50.00 mL of the target concentration shown in Table 1.

TABLE 1: First Dilution.Stock

Solution Prepared,

M

Desired Dilution Conc.,

M1.500 0.45001.250 0.37501.000 0.3000

2. Carefully add the calculated volume to the rinsed 50-mL volumetric flask using a volumetric pipette.

3. Add enough DI water so that you are a few mL short of the fill line, stopper the flask and invert several times to ensure good mixing. Use a transfer pipet to add water to the line, stopper and invert a few more times.

4. Determine the mass of your diluted sugar solution by difference using the mass of the volumetric flask determined from Part A.

5. Calculate the density of this dilution. From the actual molarity of your stock solution and the actual volume of your stock solution that you diluted, calculate the actual molarity of your first dilution. Record the actual molarity on the appropriate line in the lab report. You may discard the diluted solution down the sink. Rinse the volumetric flask several times with DI water.

Second Dilution.

6. For the second dilution, add 10.0 mL of stock solution from Part A to the rinsed volumetric flask. Repeat steps 3-5 from this section of the procedure. You now know the density of your stock solution and the two dilutions made.

7. Report this data to the Class Data Table and obtain a best-fit line of Molarity vs. density either using graph paper or an Excel spreadsheet.

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C. Determining the Concentration of Sugar in a commercially-available beverage

1. Obtain ~ 120 mL of a beverage in a clean beaker, and record its identity, the grams of sugar per serving (g/serving), and the serving size (mL). Calculate the molarity of sugar in the beverage from this information.

2. If your beverage is carbonated, you must de-gas it to purge the solution of all of the carbonation (dissolved CO2). To de-gas your sample, add a magnetic stirbar to the beaker, and put the beaker on a magnetic stirring plate. You should agitate the soda for 5-10 minutes, or until no more bubbles appear to be forming. (You want “flat” soda.)

3. Now add the degassed beverage to the fill line of your rinsed 50-mL volumetric flask.

4. Determine the mass of the beverage by difference using the mass of the flask determined in Part A.

5. Calculate the density of the beverage.

6. Discard the beverage in the flask down the drain.

7. Repeat steps 3-6 of this section to measure the sugar concentration of the beverage a second time.

8. Calculate the average density of the two determinations.

9. Before returning the volumetric flasks and pipets to the cart, rinse them thoroughly with deionized water. A final rinse of the flasks with acetone will be done by the stockroom assistants to ensure that there is no water remaining in the flasks by the next laboratory period.

10. Collect the molarity vs. density data from every group in the lab (this data should be written on the blackboard by your instructor) and record it in the Class Data Table. Use this data to construct a graph of molarity (y-axis) vs. density (x-axis). Using this graph, and the average density for your beverage, you should be able to determine the molarity of sugar in your beverage.

11. Convert the molarity of the sugar calculated from the graph to units of grams of sugar per serving. Calculate a % difference between the grams per serving from the nutrition information and the grams per serving determined from the graph. (Take the difference between these two numbers, divide by the nutrition data molarity, and multiply by 100.)

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LAB REPORT

The Concentration of Sugar in a Commercial Beverage or Juice

Name ________________________________ Date _________ Report Grade ______

Partner ________________________________ Section _________

A. Preparation of Stock Solution

Stock Solution Target Concentration ______________

Calculated Mass of Sugar __________ Mass of Flask ____________

Measured Mass of Sugar __________ Mass of Flask + Soln. ____________

Mass of Solution ____________

Molarity of Solution __________ Density of Stock Soln ____________

B. Preparation of Dilutions

First Dilution

First Dilution Target Concentration (Mf) ____________

Calculated Volume of Stock Solution for Dilution (Vi) ____________

Actual Dilution Target Concentration ____________

Mass of Flask + Soln. ____________

Mass of Solution ____________

First Dilution Density ____________

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Second Dilution

Vol. Stock Soln. (Vi)___________ Mass Flask + Soln. ____________

Mass of Solution ____________

Conc. of Dilution (Mf)____________ Second Dilution Density ____________

C. Beverage Density Determination

Beverage or Juice: ___________________

Amount of Sugar per Serving from nutrition label: ____________________ g/serving

Serving Size from nutrition label: _____________________ mL

Trial 1 Trial 2

Mass Beverage + Flask ____________ ____________

Mass of Beverage ____________ ____________

Density of Beverage ____________ ____________

Average Density __________________

Molarity of Sugar in Beverage __________________ (calculated from graph)

Grams of Sugar Per Serving __________________ (converted from previous line)

% Difference __________________

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Class Data Table

Target Conc. Actual Conc. Density Target Conc. Actual Conc. Density

1.500 M 0.300 M

1.250 M 0.300 M

1.000 M 0.250 M

0.450 M 0.200 M

0.375 M

Your instructor will go over the data with the class and identify any outlier points that can be ignored.

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Name ________________________________

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