1. Experiment : Solutions Preparation, Part 2
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2. Purpose of This Experiment
To learn how to prepare solutions from a premade, concentrated solution
To practice making a calibration curve
Work in groups of 4 (each person must prepare their own calibration curve
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3. Preparing a Solution by Dilution
(Mc) (Vc) = (Md) (Vd) Mc is the molarity of the concentrated solution Vc is the volume of the concentrated solution used Md is the molarity of the diluted or final solution Vd is the volume of the diluted solution
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4. Preparing a Solution by Dilution
To prepare a solution using a concentrated stock solution:
You need to know what molarity the new solution will be (Md)
You need to know the volume the new solution will be (Vd)
You need to know the concentration of the concentrated solution (Mc)
You need to calculate the volume needed of the concentrated solution (Vc). This is the volume you will pipet.
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5. Remember….
Do not put any pipet or other piece of equipment into the stock bottle (reagent bottle).
Pour some of the stock solution into a clean, dry beaker and use the stock solution from the beaker.
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6. Find the Absorbance of Your Prepared Solution
Use a disposable transfer pipet (plastic) to almost fill a small test tube with your solution
Wipe the outside of the test tube (remove droplets and fingerprints)
Place this test tube in the sample compartment of the spectrophotometer (use the same spectrophotometer each time)
Record the absorbance readout from the spectrophotometer
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7. Spectrophotometer
An instrument that will indicate the amount of light (of one wavelength) absorbed by a sample
Readout
detector
sample
monochromator
Light source
slit
LED readout
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8. Beer – Lambert Law
A = abc A is absorbance of light by the sample a is the molar extinction coefficient b is the solution path length c is the concentration of solute This is a linear relationship; y = mx + b y is absorbance, x is concentration
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9. Calibration Curves
Calibration curves show the linear relationship between absorbance and concentration.
Absorbance
Concentration, M
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10. Making the Calibration Curve
You will have an absorbance reading for each standard solution (a different concentration from each person in the group)
Plot the corresponding concentration and absorbance values as data points on a graph
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11. Absorbance
Concentration, M
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12. Using the Calibration Curve
You will have an absorbance reading for your unknown solution.
Find that absorbance reading on the y-axis of the calibration curve.
At that point on the y-axis, move horizontally across to the calibration curve line (use a ruler)
When you reach the line, drop straight down to the x-axis
The value that you are at on the-x axis is the concentration of your solution
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13. Calibration Curve Absorbance Unknown solution absorbance reading
Concentration, (units)
Unknown solution absorbance reading
Your concentration
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