1. CHE118 Introduction
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2. Topics Syllabus, experiment schedule, student drawers, CHE118 packet
Safety items and contract
Calibration scales
Calibration Curve and Spectrophotometer
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3. Safety Concerns
Additional safety concerns that are not mentioned on the safety contract will be discussed using another PowerPoint file.
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4. Calibration Scales
Know how to read the calibration marks on a piece of calibrated glassware and determine:
the correct number of significant figures
the correct volume reading
RCBC YouTube video
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5. Graduated Cylinder 10 mL and 100 mL sizes are most common
0.0 mL when empty, mL when full (or mL when full)
In General:
Find difference between numbered tick marks
Use the un-numbered tick marks between the two numbered marks, count the number of jumps from one numbered mark to the other
Determine what each un-numbered mark is worth
Estimate one digit beyond the un-numbered tick marks (this is the last significant digit for the volume)
Read the volume, using the correct number of significant figures
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6. Graduated Cylinder (in student drawer)
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7. SF and Volume Reading
Difference between numbered calibration tick marks:
8 mL – 7 mL = 1 mL
There are 9 un-numbered tick marks between the 7 and 8 marks (10 jumps from 7 to 8)
Each un-numbered tick mark is worth:
1 mL / 10 jumps = 0.1 mL per jump
Estimate one digit beyond un-numbered mark
0.1? mL (use 2 digits after decimal point for SF, . _ _ mL)
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8. Read a Volume with Correct SF
10 mL graduated cylinder
Read volume at the bottom of meniscus, at eye level
More than 7, less than 8
7.? ? mL
7.5 ? mL
7.53 mL
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9. Calibration Curve and Spectrophotometer
Work in groups of four. Everyone must make their own calibration curve.
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10. Calibration Curves
Calibration curves show the linear relationship between absorbance and concentration.
Absorbance
Concentration, units
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11. 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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12. 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 A = mc + b
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13. Making the Calibration Curve
You will acquire an absorbance reading for each standard solution provided
Plot the corresponding concentration and absorbance values as data points on a graph to construct the calibration curve
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14. Calibration Curve Absorbance absorbance reading of standard solution
Data point
concentration
Concentration, units
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15. Find the Absorbance of the Unknown Solution
Use a disposable transfer pipet (plastic) to almost fill a small test tube with the unknown 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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16. Find the Concentration of the Unknown Solution
You will have an absorbance reading for your solution (from the spectrophotometer).
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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17. Using the Calibration Curve to Find Concentration of the Unknown Solution
Absorbance
Concentration, units
Unknown solution absorbance reading
Concentration of unknown
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