Title (V Title)



Experiment #2

Determining the Concentration of an Unknown

Beer’s Law

The primary objective of this experiment is to determine the concentration of an unknown copper (II) sulfate solution. You will use a Colorimeter (a side view is shown in Figure 1) to measure the concentration of each solution. In this experiment, red light from the LED light source will pass through the solution and strike a photocell. A higher concentration of the colored solution absorbs more light (and transmits less) than a solution of lower concentration. The Colorimeter monitors the light received by the photocell as percent transmittance.

[pic]

Figure 1

You will prepare five copper (II) sulfate solutions of known concentration (standard solutions). Each solution is transferred to a small, rectangular cuvette that is placed into the Colorimeter. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. When you graph absorbance vs. concentration for the standard solutions, a direct relationship should result. The direct relationship between absorbance and concentration for a solution is known as Beer’s law.

You will determine the concentration of an unknown CuSO4 solution by measuring its absorbance with the Colorimeter. By locating the absorbance of the unknown on the vertical axis of the graph, the corresponding concentration can be found on the horizontal axis. The concentration of the unknown can also be found using the slope of the Beer’s law curve.

OBJECTIVES

In this experiment, you will

• Prepare and test the absorbance of five standard copper (II) sulfate pentahydrate solutions.

• Calculate a standard curve from the test results of the standard solutions.

• Test the absorbance of a copper (II) sulfate solution of unknown molar concentration.

• Calculate the molar concentration of the unknown CuSO4.5H2O solution.

Prepartion of Solutions (direct method):

In order to calculate the amount (grams) of a compound to use in a solution you need to know the concentration (mole/L), the volume (V), and the molar mass (g/mole) of the compound. Remember that Molarity x Volume = moles:

Let’s say you want to make a 200 ml of a 0.5 M solution of CuSO4.5H2O. The molar mass(MM) is 249.68 g/mol. How many grams of copper sulfate pentahydrate do you need to measure out?

M x V x MM = grams

You would then measure out 24.968 g of copper sulfate pentahydrate and bring the volume of water up to 200 mL for a 0.5000 M solution. Please note that you do not simply add 200 mL of water. Why? Displacement. 24.968 g is a handful and will displace a certain volume of water. Therefore, it will not take exactly 200 mL to make the solution.

Preparation of Solutions (dilutions):

Suppose that you are given 0.5031 M solution of CuSO4.5H2O and asked to make 50 mL that is 0.2 M! You can’t use the above to figure out how much copper sulfate to weigh out. So you use something called dilutions. This is a process used to make a concentrated solution of something less concentrated. For example, you buy Miracle Grow at Wal-Mart as a concentrated liquid. You put that on your plants and you kill them. However, if you read the instructions you will see that you should add 10 drops of Miracle Grow to 1 quart of water- that is a dilution!!!!

When doing dilutions remember, moles before must equal moles after. You are not changing the number of moles, only the volume that they are in. So, the following must be true:

Molesbefore = Molaritybefore x Vbefore and Molesafter = Molarityafter x Vafter

This gives us the equation M1V1 = M2V2

Back to the above problem:

Mbefore (or M1) = 0.5031 M

Vbefore( or V1) = ? (what you are trying to solve)

Mafter (or M2) = 0.2 M

Vafter (or V2) = 50 mL

(0.5031 M) x V1 = 0.2M x 50 mL

V1 = 19.88 mL

You would take 19.88 mL of the original solution and dilute with water in a 50 mL volumetric flask. That gives you a new solution with a molarity of 0.2 M.

Absorption of light by Solutions:

If light is passed through a solution, a certain amount will be absorbed and a certain amount will be transmitted. Transmittance is simply a ratio of the light shone through the solution (Io) and the light exiting the sample (I). Values range from 0 to 1 with 1 meaning 100 % transmittance. The spectrophotometer will give you a % T value.

A more common way of measuring light in solutions is absorbance which is defined as

-log T. The absorbance of a solution is related to the concentration of a solution using Beer’s Law:

A = ε c l

Where

A = absorbance

c = concentration (M)

ε = molar absorptivity (M-1/cm)

l = path length (cm) of sample container (usually taken as 1.0 cm)

Determining concentration from a standard curve:

You have collected the following data in the lab. Based on that data you need to construct a standard curve and determine the concentration of that unknown from the standard curve.

| Molarity | %T | T | A |

|0.5031 | 19.3 | 0.193 | 0.714 |

|0.3522 | 32.0 | 0.320 | 0.495 |

|0.2516 | 45.3 | 0.453 | 0.344 |

|0.1006 | 72.5 | 0.725 | 0.140 |

|unknown | 33.1 | 0.331 | 0.480 |

Take this data and make a graph in Excel of absorbance (Y-axis) vs molarity (x-axis).

To determine the concentration of the unknown, draw two lines; one from the y-axis to the line and from the line down to the x-axis. That will be the concentration of your solution.

Unknown concentration = 0.32 M (from graph above)

PROCEDURE

1. Obtain and wear safety goggles.

2. Into a plastic weigh boat, weigh approximately accurately 5 grams of powdered copper(II) sulfate pentahydrate, CuSO4.5H2O. .

Place a small funnel into the neck of a clean 50 mL volumetric flask. Transfer the CuSO4.5H2O quantitatively to the volumetric flask by carefully raking the solid from the weigh boat into the funnel and thus into the flask. Do very small amounts at a time to that the funnel will not get clogged up. After all is transferred use a wash bottle to add a few milliliter of water to the weight boat and dissolve any residual solid. Pour this water into the funnel and then use the wash bottle to rinse any solid on the inside of the funnel into the volumetric flask. Keep the flask on the desk and do not shake it during this process.

Carefully fill the volumetric flask to the mark, put a stopper in it and shake until all of the solid is dissolved. Note that the level of liquid is below the mark on the neck. The sum of the volumes of water and CuSO4.5H2O separately is larger than when they are mixed. The volumes are not additive.

Carefully fill the flask to the mark again and shake to mix. Calculate the molarity of the solution (should be about 0.4M). Mark this solution 4.

3. Obtain a 2.0 mL and a 5.0 mL transfer (they have a bulge in the middle and no graduations) from the soaking apparatus. Rinse them (see procedure below) with water and then rinse with the above 0.4 M solution using a rubber bulb for suction.

4. Obtain three clean 10 mL volumetric flasks. Into one of them use the 2 mL pipette to place 2 mL of the above 0.4 M solution. Mark this solution 1. Use the 5 mL pipette to place 5 mL of the 0.4 M solution into a second 10 mL volumetric flask. Mark this solution 2. Use both the 2 mL and the 5 mL pipettes to place 7 mL of the 0.4 M solution into the third 10 mL flask. Mark this solution 3.

Calculate the molarity of these three solutions. They should be about 0.08M, 0.2M and 0.28 M respectively, the exact values depending on the exact molarity of the solution being diluted.

The transmittance of the three diluted solutions and the 0.4 M solution will be measured with the colorimiter.

RINSE THE PIPETS AS FOLLOWS: HOLD THE PIPETTE UPSIDE DOWN AND PUT THE TIP ON THE SPIGOT OF THE FAUCET. TURN THE WATER ON CAREFULLY AND ALLOW WATER TO RUN THROUGH THE PIPETTE SLOWLY TO RINSE IT. THEN RUN A SMALL AMOUNT OF DISTILLED WATER THROUGH THE PIPETTE TO RINSE OUT THE TAP WATER. RETURN THE PIPETTE (TIP UP) TO THE DISTILLED WATER SOAK CONTAINER

5. Connect a Colorimeter to Channel 1 of the Vernier computer interface. Connect the interface to the computer using the proper cable.

6. Start the Logger Pro program on your computer. Open the file “Experiment #2 Beer’s Law” from Logger Pro. (This program is now on the desktop under the File “CLAB 124 Experiments”)

7. Calibrate the Colorimeter.

a. Prepare a blank by filling an empty cuvette ¾ full with distilled water. Place the blank in the cuvette slot of the Colorimeter and close the lid.

b. Set the wavelength on the Colorimeter to 635 nm, press the CAL button. When the LED stops flashing the calibration is complete.

8. You are now ready to collect absorbance-concentration data for the five standard solutions.

a. Click [pic].

b. Remove the cuvette from your Colorimeter and pour out the water. Using solution 1, rinse the cuvette twice with ~1 mL amounts, and then fill it ¾ full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid.

c. When the absorbance readings stabilize, click [pic], type the concentration of Test Tube 1 in the edit box, and press the ENTER key. The data pair should now be plotted on the graph.

d. Discard the cuvette contents as directed. Using solution 2, rinse and fill the cuvette ¾ full. Wipe the outside, place it in the Colorimeter, and close the lid. When the absorbance readings stabilize, click [pic], type the concentration of Test Tube 2 in the edit box, and press the ENTER key.

e. Repeat the procedure for solutions 3 and 4. Do not test the unknown solution until Step 9.

f. When you have finished testing the standard solutions, click [pic].

g. Examine the graph of absorbance vs. concentration. Click the Linear Regression button, [pic]. A best-fit linear regression line will be shown for your five data points.

9. Record the absorbance values, for each of the four trials, in your data table.

10. Determine the absorbance value of the unknown CuSO4 solution.

a. Obtain about 5 mL of the unknown CuSO4 in a clean, dry, test tube. Record the number of the unknown in your Data Table.

b. Rinse the cuvette twice with the unknown solution and fill it about ¾ full. Wipe the outside of the cuvette, place it into the Colorimeter, and close the lid.

c. Read the absorbance value displayed in the Meter window. (Important: The reading in the Meter window is live, so it is not necessary to click [pic] to read the absorbance value.) When the displayed absorbance value stabilizes, record its value as Trial 6 in your data table.

d. Dispose of any of the remaining solutions as directed.

DATA TABLE

|Trial |Concentration (mol/L) |Absorbance |

|1 | | |

|2 | | |

|3 | | |

|4 | | |

| |Unknown number ____ | |

DATA ANALYSIS

1. Calculate the linear regression (best-fit line) equation of absorbance vs. concentration for the five standard CuSO4 solutions. Print a graph showing the data and linear-regression equation for the standard solutions.

2. Determine the concentration of the unknown CuSO4 solution using the slope-intercept equation.

-----------------------

[pic]

[pic]

[pic]

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download