Molarity Worksheet
“Beer’s Law” Lab – A lab in which you will practice making and using dilutions
in order to better understand “molarity”, what it means to make a “dilution”, as well as the M1V1 = M2V2 formula
Materials: LabPro interface with spectrophotometer, two 50-mL volumetric flasks, one 100-mL volumetric flask, one 250-mL volumetric flask, four 13 x 100 mm test tubes, 1 plastic cuvette, 1 50-mL graduated pipet, aspirator for the pipet (to be shared with another group), one 150-mL (or 250-mL) beaker, peg rack.
Background: Matter can absorb, deflect, and/or transmit specific colors of light. When different substances are dissolved in a solution, they will absorb and transmit light differently. Scientists know that a solution of a given color is transmitting that color, and absorbing colors that are “opposite” (on the color wheel) to the color of the solution. Scientists use their knowledge of light absorption, a law called “Beer’s Law”, and an instrument called a spectrophotometer in order to measure the amount of light that is being absorbed by a chemical that they are interested in. If no light is absorbed by their sample, no chemical is present. If lots of light is absorbed, lots of chemical is present. Depending on the substance being studied, scientists will choose one particular color of light to use to determine the presence or absence of a given chemical of interest.
Beer’s Law is most easily stated as this - the amount of light absorbed by a substance (when it is dissolved in a non-interfering solvent) will depend on the amount of the substance present in the solvent. The absorption of light will increase as the concentration of the substance increases. The more concentrated your Kool-Aid solution, the higher your absorbance measurements will be. The law is accurate only for dilute solutions; as deviations from the law occur in concentrated solutions. Today you are using light with a wavelength of 470 nm to detect the dye in a series of dilutions of grape Kool-Aid. Since your mixture of Kool-Aid contains sugar, the molarity of your solutions will be based on the sugar (C12H22O11) content.
Today you will measure out a mass of purple Kool-Aid as your solute, and then dissolve your sample using tap water as the solvent. Since you will know both the moles of sugar and the volume of your solution, you will be able to calculate the molarity of the concentrated sugar sample. This is the “M1” value in the M1V1 = M2V2 equation.
Then, you will perform a dilution by measuring a specific volume of your original solution (“V1”). You will transfer this to a flask, and add tap water to reach a given final volume (“V2”). You will then need to calculate the new molarity (M2). You will make 3 more dilutions and similarly calculate the molarity. After mixing thoroughly, you’ll use the spectrophotometer to record the absorbance measurements.
The computer program will generate a graph of absorbance versus molarity. After printing your graph, your teacher will provide you with an “unknown” Kool-Aid sample, and explain how to use your graph to determine the molarity of the sugar present in the unknown sample.
Procedure: Preliminary computer logon and calibration of instrument. Do asap. RIGHT NOW.
1. Logon on to computer. Check that the LabPro interface is 1) plugged in, 2) connected to computer via USB and 3) is connected via channel 1 into the spectrophotometer.
2. Either click on the LoggerPro3.8 desktop shortcut, or access it (through the start menu): Start ( Science Department ( Chemistry Lab ( LoggerPro 3.8
3. Click on the open folder icon (top toolbar). Select “Chemistry With Vernier”. Select Experiment 11 (Beer’s Law)
4. Pick up the plastic cuvette. Look at the plastic sides: 2 sides are rough, 2 are smooth. Wipe fingerprints off of the 2 smooth sides (by using a towel or tissue). Touch only the 2 rough sides from here on.
5. Calibrate the spectrophotometer (“spec”) by doing this:
a. Squirt distilled water from the squirt bottle into the cuvette until it is 3/4ths full. Slide the lid of the“spec”, so that you are looking into the sample holder. There is a line drawn in the sample holder to indicate where the light beam emerges. The light needs to shine through the smooth sides of the cuvette. The 2 smooth sides have to be positioned from left – to – right, and the 2 rough sides front – to – back so that the light beam will be shined correctly through the smooth sides. Slide the cuvette down into the holder; and, slide the lid shut again.
b. Turn the knob on the “spec” to “”0%T. Then, on the top toolbar: From the “experiment” window, click on “Calibrate: “LabPro1 Ch1: Colorimeter”, then click “calibrate now”. Type, “0” in the reading 1 box (underneath “enter value in data units”). When the displayed numeric reading stabilizes, click “keep”.
c. Turn the knob on the spec to “blue 470”. Type “100” in the reading 2 box (underneath “enter value in data units”. When the displayed reading 2 stabilizes, click “keep”, then click “done”.
6. Procedure: Preparing Kool-Aid Solution and Dilutions:
a. Obtain about 14.50 grams of grape Kool-Aid by using a plastic weighing boat. Record the exact mass of the Kool-Aid. ___________________________This sample contains 0.0400 moles of sugar (C12H22O11).
b. Fold the weighing boat opposite corner to opposite corner, and using one corner as a spout, carefully transfer all of the sample into a 100 mL volumetric flask. Use tap water to fill most of the bulb of the flask (but don’t go into the neck yet). Swirl the flask until the Kool-Aid dissolves. Then (use care), use the squirt bottle of distilled water to slowly add water up the neck, until the meniscus lies just above the engraved line. Use your finger as a cap; then, iInvert fully 10 times in order to mix the solution (wait for the bubble to go all the way up, and all the way down each time). Then, pour the solution into your beaker. Someone should now rinse the volumetric flask out with DISTILLED water, and set it aside so it doesn’t get broken.
NOTE – you don’t have any extra of this solution, and you won’t be given a second batch. If you make a mistake below, you’ll have to ignore it. So, try not to add water (when going up the neck) too quickly. Don’t go over the line
c. Use the graduated pipet to measure 40.0 mL of the Kool-Aid solution. Transfer it into a 50-mL volumetric flask. Use the distilled water squirt bottle to bring the solution “to the volume mark” (as above; meniscus just above the line). Use your finger as a cap; then invert fully 10 times in order to mix it thoroughly. Leave it sit.
d. With a different 50-mL volumetric flask, prepare a dilution using 20.0 mL of the Kool-Aid solution. Transfer it to a second 50-mL volumetric flask. Use tap water to fill the bulb, and then distilled water to slowly fill the neck, bringing the solution “to the volume mark”. As before, invert fully 10 times in order to mix it. Leave it sit.
e. Similarly, using a 100-mL volumetric flask, use 20.0 mL Kool-Aid solution to prepare a dilution to the mark.
f. Last, you should have exactly 20.0 mL Kool Aid solution remaining… no need to measure it…. Just pour it all into a 250-mL volumetric flask, dilute up to the mark. Invert and mix thoroughly as before.
g. Transfer your dilutions into test tubes: Pour each dilution into a separate test tube (fill the tube about 3/4ths of the way up). Use the peg rack to hold the 4 test tubes.
h. Calculating Molarity: Before you take measurements using the spectrophotometer, you need to calculate the molarity for the original sample, as well as for each dilution. Proceed as indicated below:
Original sample: 0.0400 moles C12H22O11 Volume = 100.0 mL
Molarity = ______________________
i. Dilution (20.0 mL / 250.0 mL)
1. M1 =
2. V1 = V2 =
3. Solve for M2 (show work to the left) Molarity = _______________________
ii. Dilution (20.0 mL / 100.0 mL)
1. M1= same as previous M1
2. V1 = V2 =
3. Solve for M2 (show work to the left) Molarity= _______________________
iii. Dilution (20.0 mL / 50.0 mL)
1. M1=same as previous M1
2. V1 = V2 =
3. Solve for M2 (show work to the left) Molarity= _______________________
iv. Dilution (40.0 mL / 50.0 mL)
1. M1= same as previous M1
2. V1 = V2 =
3. Solve for M2 (show work to the left) Molarity= _______________________
7. 7. Collecting absorbance data:
a. On the computer screen, click “collect”.
b. (Make sure that the cuvette with distilled water is still in the spec, and the lid is closed): Click “keep” Then, type, “0” in the edit box. Click “enter”. You should see the data record show up on the computer screen.
c. Swing open the lid, remove the cuvette, discard the water. Then, carefully (so as not to overflow), fill the cuvette about 3/4ths full with your MOST DILUTE (least color) dilution by carefully pouring from your test tube…. PLEASE DON’T DO THIS OVER THE OPEN SPECTROPHOTOMETER CHAMBER. Use a towel or tissue to wipe the outside of the cuvette (and inside the chamber if you spilled into it). Put the cuvette into the chamber; swing shut the lid. When the reading on the computer stabilizes, click “keep”. Then, type in the Molarity that you have calculated for the 250 mL dilution. Press “enter”. Check the data record. Ask for help if you made a mistake.
d. Continue on, obtaining the absorbance measurements for the remaining dilutions… As long as you go in order from least color to most color, you don’t have to rinse out the cuvette between measurements! If I’ve given you an “unknown” sample, please obtain the absorbance reading from the computer and write it here: ___________
e. Click “Stop”. Then,click, “File(Print” and use default printer to print a graph for each person in your group. Tell me; I’ll go get them.
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At any time when you see a pop-up saying “sensor confirmation”, click on the button that says “connect”.
[pic]
M1V1 = M2V2
M1[pic]=M2
This answer is the “M1” to use FOR EVERY CALCULATION
Turn spec to OFF;
then, clean up.
Rinse cuvette with distilled water.
Dump all flasks, beakers, test tubes; then, rinse all with distilled water.
Towel dry spills.
SAVE and LogOff.
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