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Chem 14BL- Week 4 Notes

by: Daniel Ochs

Chem 14BL- Week 4 Notes CHEM 14BL

Daniel Ochs

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Week 4 Notes
General and Organic Chemistry Laboratory I
Class Notes
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This 11 page Class Notes was uploaded by Daniel Ochs on Friday May 13, 2016. The Class Notes belongs to CHEM 14BL at University of California - Los Angeles taught by Casey in Spring 2016. Since its upload, it has received 9 views. For similar materials see General and Organic Chemistry Laboratory I in Chemistry and Biochemistry at University of California - Los Angeles.

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Date Created: 05/13/16
Week 4 Synthesis of Aspirn Lab Salicylic Acid + Acetic Anhydride ——> Asprin + Acetic Acid In Asprin synthesis analysis: • Titration • Melting Point • No Beer’s Law • No Mass Spectroscopy Analysis of Purity of Caffeine Titrimetric analysis of the amount of acid in pure aspirin product (Titration) • Determine the actual moles of acid in product and compare to theoretical moles of aspirin expected in sample based on stoichiometry Analysis by melting point determination of aspirin product • Melting point temperature and range of a compound can be used for identification and purity analysis Basics of Acid-Base Reactions Acid – a substance that produces H (HO ) i3 an aqueous solution; a proton donor [Acids have pH < 7] Examples: HCl, CHCOOH3 CHCOOH 6 5 Base – a substance that produces OH in an aqueous solution; a proton acceptor [Acids have pH > 7] Examples: NaOH, NH CHN 3,5 5 pH = -log [H O3] pH + pOH = 14 Neutral pH = 7 Acid-Base Reaction (Neutralization Reaction) HCl(aq) + NaOH(aq) → H O(l)2+ NaCl(aq) Basics of Acid-Base Reactions We can use the principles of acid-base neutralization to describe a titration
 In a titration, one substance of known concentration is reacted with another substances in a solution of unknown concentration H + OH→H O 2 The titration is complete when you reach the equivalence point (moles of OH added are equal to moles of H in the original solution) Ma Va = Mb Vb How to Read a Buret 1.  Graduations go down 
 2.  Read the position of the meniscus to the hundredths place (two decimal places) 
 3.  Never try to “set” the volume to a specific value 
 4.  Hold a card with a black line behind the buret to help you read the meniscus Observing the Equivalence Point Equivalence point is determined by monitoring the pH of the solution or by using an indicator – a dye whose color depends on the acidity of the solution With an indica, rely on the the point where the indicator changes color (i.e. the endpoint) to determine the equivalence point Titrimetirc Analysis in Aspirin Lab Recrystallized aspirin solution (with unknown concentration) will be titrated with a standard NaOH with known concentration Phenolphthalein will be the indicator − ed % purity = (actual moles of acid) / (theoretical moles ofaspirin in sample) ×100% What are Melting Points? Molecules within crystals (solid phase of matter) are aligned in a structure that is held together by intermolecular forces
 To break this ordered structure requires the input of energy (such as thermal energy when you increase the temperature) Principles of Analysis Using Melting Point Determination 1.  The freezing or melting point of a compound is a physical characteristic that can be used in identification 
 2.  Pure compounds melt within a narrow temperature range (~1-2 degrees) 
 3.  Impure compounds melt over a much larger temperature range 
 Identity: Results of melting point analysis do not confirm identity, but can provide evidence of a hypothesized identity Purity: The sharpness of the range of the melting point indicates purity of the sample (pure compounds have a narrow range while impure compounds have a large range) Effects of Impuritites on Melting Point Melting Point Depression
 The impure solid will begin melting at a temperature lower than the literature melting point for the pure solid 
 Melting Point Range Broadening
 The melting point range (difference between the initial melting temperature and the temperature after complete melting) will broaden 


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