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Organic: Acid/Base Practice Problems

  • Page ID
    9372
  • Rank these compounds in terms of increasing acidity:

    HNO2 H2SO4 HF H2CO3

    Solution

    NH3 CH4 H2O HCl

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    Rank these molecules in terms of decreasing basicity:

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    What is the most acidic group of protons on methyl acetate? (This is a flammable liquid with a smell reminiscent of some glues and nail polish.)

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    From the structures below, what is the strongest acid? What is the strongest base?

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    Show the structures of species X and Y in the following acid-base reactions.

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    Classify the solutions as acidic or basic.

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    Rank these acids from weakest to strongest:

    CF3COOH HCO3- H2PO4- CH3COOH

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    What structural features of a molecule may effect the pKa of an acid?

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    Which of the following is the strongest acid?

    CH3OH CH3SH CH3PH2

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    Which of the following is the strongest base?

    CH3NH- CH3O- F-

    Solution

    Heartburn is caused by a buildup of excessive amounts of stomach acid, particularily HCl. This acid is used to digest the food we eat, but it can often back up into the esophagus causing that burning sensation many of us are familiar with. The symptoms of heartburn can be treated with a mild base, which acts to neutralize the excess HCl. For example, Tums is a very commonly used antacid that can be bought over the counter. Other antacids, such as Alka Seltzer, work in similar ways. Below, show the mechanism and products for reaction in which calcium carbonate (the active ingredient in Tums) neutralizes HCl in a simple proton transfer reaction. Following this acid-base reaction, carbonic acid quickly degrades into CO2 and H2O.

    Solution

    Identify the more acidic compound in the following pairs:

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    Using the concept of resonance delocalization of charge, find which compound is the stronger base.

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    Using a pKa table, identify the stronger base in each pair of compounds.

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    Determine which proton is more acidic.

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    Determine which proton is more acidic using resonance arguments.

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    Determine which proton is more acidic and explain how you arrived at your answer.

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    List the following carboxylic acids in order of decreasing acidity. carb acids 1.png

    a. How does the presence of an electronegative substituent such as Cl affect the acidity of a carboxylic acid?

    b. How does the location of the substituent affect the acidity of the carboxylic acid?

    Solution

    For each of the following compounds, draw the form in which it will predominate at pH =3, pH =6, pH =10, and pH =14.

    pKacompounds2.png

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    Which compound has the lower pKa?compoundlowerpKa.png

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    For each pair of compounds below, indicate which is more acidic.

    compoundsmoreacidic.png

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    Write the formula for the conjugate base of each of the following acids:

    a. sulfurous acid, H2SO3 b. chloric acid, HClO3 c. hydrogen sulfide, H2S

    d. dimethyloxonium, (CH3)2OH+ e. hydrogen sulfate, HSO4-

    Solution

    Write the formula for the conjugate acid of each of the following bases:

    a. dimethylamide, (CH3)2N- b. sulfide, S2- c. ammonia, NH3

    d. acetone, (CH3)2NH e. 2,2,2-triflouroethoxidev, CF3CH2O-

    Solution

    Which is the stronger acid, nitrous (HNO2 pKa = 3.3) or phosphorous acid (H3PO3 pKa = 1.3)?

    Calculate the Ka for each.

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    Show, using resonance, why acetic acid is more acidic than methanol.

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    For each of the following compounds, shown in their acidic forms, write the form that will predominate in a solution of pH = 5.5.

    Screen Shot 2014-04-25 at 10.50.07 AM.png

    Solution

    A naturally occuring amino acid such as alanine has both a carboxylic acid group (pKa = 2.34) and an amine group (pKa = 9.69).

    Screen Shot 2014-04-25 at 10.53.53 AM.png

    a. Draw the structure of alanine in a solution at physiological pH (pH = 7.3).

    b. Is there a pH at which neither functional group will have a formal charge?

    c. At what pH will alanine have no net charge ( the amount of negative charge will be the same as the amount of positive charge)?

    Solution

    Indentify the strongest acid and the weakest acid in each row:

    Screen Shot 2014-05-01 at 7.42.38 PM.png

    Screen Shot 2014-05-01 at 7.43.52 PM.png

    Screen Shot 2014-05-01 at 7.44.43 PM.png

    Screen Shot 2014-05-01 at 7.45.56 PM.png

    Solution

    Indicate the order of acid strength of these compounds by ranking 1-4 (1 being the strongest acid):

    *note: C6H5 - phenyl group

    Screen Shot 2014-05-01 at 8.30.48 PM.png

    Screen Shot 2014-05-01 at 8.31.33 PM.png

    Screen Shot 2014-05-01 at 8.32.17 PM.png

    Screen Shot 2014-05-01 at 8.33.38 PM.png

    Solution

    To the following compounds:

    a) Indicate the order of acid strength by ranking 1-4 (1 being the strongest acid):

    b) Find the most acidic C-H proton of each:

    *note: C6H5 - phenyl group

    Screen Shot 2014-05-01 at 8.59.28 PM.png

    Screen Shot 2014-05-01 at 9.01.07 PM.png

    Solution

    Indicate the order of base strength of these compounds by ranking 1-4 (1 being the strongest base):

    Screen Shot 2014-05-01 at 9.27.49 PM.png

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    Determine the structure(s) of the most favored conjugate base.

    Screen Shot 2014-05-01 at 9.45.56 PM.png

    Solution