CHEM123 Study Guide - Final Guide: Acetic Anhydride, Potassium Dichromate, Diffraction Grating

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Exam Review
Accuracy is how close you get to the actual mass, and precision is if you can get that accurate
mass consistently in many trials
Experiment 1: Synthesis of Acetaminophen
Acetaminophen is formed when the amine group of p-acetaminophen is given an acetyl group
from acetic anhydride to form an amide  product is acetaminophen and by-product is acetic
acid
The crude solid must be separated from the acetic acid through vacuum filtration then purified
through recrystallization by heating it in a minimal amount of solvent
The hot solution must be cooled to room temperature and then put in an ice bath so that the
crystals can solidify
These crystals are more pure than the crude solid because the impurities dissolved in the cold
solution
To measure the melting point of the crude and pure solid, samples were taken from each and
were placed into melting point tubes to be inserted into a MelTemp melting point apparatus
10 ml of water was used to remove impurities for 1 gram of the crude solid
A supersaturated solution is required for recrystallization (more crude solid than water)
Experiment 2: Spectrophotometer Analysis – The Breathalyzer reaction
Beer’s law and spectrophotometric determination: A= Elc
A = absorbance
E = molar absorptivity (M/cm)
l = path of light through the solution (cm)
c = molar concentration of solute (M)
Absorbance is a measure of how much light the solution absorbs
Readings are taken at the wavelength of maximum absorption in order to have maximum
sensitivity
A spectrophotometer selects the desired wavelength of light to measure the absorbance at that
wavelength
White light from a bulb passes through a slit and lenses which hits a diffraction grating that
spreads the light into its spectrum of wavelengths (colours)
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