CHE118 Introduction

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Topics

• Syllabus, experiment schedule, student drawers, CHE118 packet

• Safety items and contract

• Calibration scales

• Calibration Curve and Spectrophotometer

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Safety Concerns

• Additional safety concerns that were not mentioned on the safety contract will be discussed using another PowerPoint file.

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Calibration Scales

• Know how to read the calibration marks on a piece of calibrated glassware and determine:

1.the correct number of significant figures2.the correct volume reading

BCC YouTube video

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Graduated Cylinder• 10 mL and 100 mL sizes are most common• 0.0 mL when empty, 10.00 mL when full (or 100.0 mL when full)In General:1. Find difference between numbered tick marks2. Use the un-numbered tick marks between the two numbered

marks, count the number of jumps from one numbered mark to the other

3. Determine what each un-numbered mark is worth4. Estimate one digit beyond the un-numbered tick marks (this is the

last significant digit for the volume)5. Read the volume, using the correct number of significant figures

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Graduated Cylinder(in student drawer)

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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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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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Calibration Curve and Spectrophotometer

Work in groups of four.

Everyone must make their own calibration curve.

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Calibration Curves

Calibration curves show the linear relationship between absorbance and concentration.

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Absorbance

Concentration, M

Spectrophotometer

An instrument that will indicate the amount of light (of one wavelength) absorbed by a sample

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LED readoutreadout

detectorsamplemonochromatorLight

source

slit

Beer – Lambert Law

A = abcA is absorbance of light by the samplea is the molar extinction coefficientb is the solution path lengthc is the concentration of solute

This is a linear relationship; y = mx + bA = mc + b

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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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Calibration Curve

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Absorbance

Concentration, units

absorbance reading

concentration

Data point

Find the Absorbance of the Unknown Solution

1. Use a disposable transfer pipet (plastic) to almost fill a small test tube with the unknown solution

2. Wipe the outside of the test tube (remove droplets and fingerprints)

3. Place this test tube in the sample compartment of the spectrophotometer (use the same spectrophotometer each time)

4. Record the absorbance readout from the spectrophotometer

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Find the Concentration of the Unknown Solution

1. You will have an absorbance reading for your solution (from the spectrophotometer).

2. Find that absorbance reading on the y axis of the calibration curve.

3. At that point on the y axis, move horizontally across to the calibration curve line (use a ruler)

4. When you reach the line, drop straight down to the x axis

5. The value that you are at on the x axis is the concentration of your solution

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Using the Calibration Curve to Find Concentration of the Unknown Solution

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Absorbance

Concentration, units

Unknown solution absorbance reading

Concentration of unknown solution