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Chemistry LibreTexts

5: Acid Strength

  • Page ID
    204129
  • Objectives

    • To determine the difference between a strong and a concentrated acid.
    • To determine the difference between a weak and a dilute acid.
    • To determine the relationship between ionization and pH.

    While pure acids are molecular compounds, when they are placed in water the molecules dissociate and form ionic solutions composed of hydronium ions \(\ce{H_{3}O^{+}}\) and the counter anion \(\ce{A^{-}}\).

    \[\ce{HA} (aq) + \ce{H_{2}O} (l) \ce{<=>} \ce {H_{3}O^{+}} (aq) + \ce {A^{-}} (aq) \label {1} \]

    The presence of the hydronium ion is the basis for determining the pH of the solution.

    \[pH=-\log[\ce{H_{3}O^{+}}]\label{2}\]

    Using the apparatus with the pH sensor attached will allow you to collect data for the value of the pH in each solution. This will give you information on the concentration of hydronium ions in solution.

    Conductivity measures the ability of current to be conducted by a solution. Because it is the ions in the solution that carry current, the more ions there are, the higher the conductivity will be. Thus, conductivity is a measure of the number of ions in a solution.

    You will be preparing 0.10 M, 0.010 M, 0.0010 M, and 0.00010 M solutions of hydrochloric acid \(\ce{HCl}\), acetic acid \(\ce{C_{2}H_{4}O_{2}}\), and mandelic acid \(\ce{C_{8}H_{8}O_{3}}\). 


    Pre Lab Video


    Procedure

    Safety And Waste Disposal

    • The solutions will be sufficiently dilute and as such can be disposed of in the sink. 

    Part I:  Preparing Solutions
    Step 1

    Acids Part I Step 1.jpgLabel 4 small beakers (100 or 150 mL) 0.10 M (Beaker 1), 0.010M (Beaker 2), 0.0010 M (Beaker 3), and 0.00010 M (Beaker 4). Be sure to identify the formula or name of the acid on each label.


    Step 2

    Acids Part I Step 2a.jpgPlace 50.0 mL of one of your 0.10 M acid solutions into Beaker 1.


    Step 3

    Acids Part I Step 3.jpgInto each of the three remaining Beakers (2 – 4) add 45 mL of deionized water.


    Step 4

    Acids Part I Step 4.jpgAcids Part I Step 4b.jpgUsing a graduated cylinder, add 5.0 mL of the acid in Beaker 1 into Beaker 2 and mix THOROUGHLY.


    Step 5

    Using a graduated cylinder, add 5.0 mL of the acid in Beaker 2 into Beaker 3 and mix THOROUGHLY.


    Step 6

    Using a graduated cylinder, add 5.0 mL of the acid in Beaker 3 into Beaker 4 and mix THOROUGHLY.


    Part II:  Measuring pH

    Note

    • The pH electrode bulbs are made of glass. They are very fragile. Use extreme care.
    • Measure the pH of each of your acids three times rinsing the sensor between readings.
    Step 1

    Acids Part II Step 1.JPGConnect the pH Sensor to LabQuest in Channel 1 (CH1).


    Step 2

    Acids Part II Step 2.jpgUse about 100 mL of distilled water in a 250 mL beaker to rinse the electrode.


    Step 3

    Acids Part II Step 3.jpgStarting with the 0.00010 M solution, pour about 10 mL of the test solution into a clean 50 mL beaker and use it to thoroughly rinse the electrode.


    Step 4

    Acids Part II Step 4.jpgPlace the pH electrode in the remaining amount of acid in the beaker. Swirl the solution vigorously. Readings may drift without proper swirling! When the pH reading stabilizes, record the value in your notebook. 


    Step 5

    Repeat with the remaining solutions to determine pH working from the most dilute to the most concentrated beaker. 


    Step 6

    Acids Part II Step 6.JPGWhen you are finished, rinse the sensor with distilled water and return it to the sensor soaking solution.


    Part III:  Measuring Conductivity

    Note

    You cannot use the conductivity sensor and pH sensor at the same time.

    Step 1

    Acids Part III Step 1a.JPGAcids Part III Step 1b.JPGAttach the conductivity sensor to the LabQuest in Channel 2 (CH2). Move the range selector switch on the conductivity sensor box to “0-20000 \(\mu\)S/cm” (the largest range).


    Step 2

    Acids Part III Step 2.JPGRinse the sensor with distilled water and pat dry. Do not push the paper towel through the cavity in the probe, but gently tap to remove any droplets of water.


    Step 3

    Acids Part III Step 3.jpgTest only the 0.1 M solutions of each acid. Dip the probe into the solution and swirl the solution. Wait 5 seconds for the conductivity to stabilize. Record the value in your notebook.


    Step 4

    Acids Part III Step 4.JPGRinse and dry the sensor.


     

    Calculations

    Step 1

    Calculate the average values for the pH of all the acid solutions at each concentration. Record the values on your notebook


    Step 2

    Open Windows Excel and enter the data on initial concentration and pH for the three acids.


    Step 3

    Plot pH vs –log (HA)o for each acid (three plots). Use a linear trend line and add the equation along with the \(R^{2}\) value, to the graph. Print these plots.


    Step 4

    Calculated the experimental \([\ce{H_{3}O^{+}}]\) for all the solutions.


    Step 5

    Calculate % ionization

    \[ \%\: ionization=\frac{[\ce{H_{3}O^{+}}]}{(HA)_{o}}\label{3}\]


    Step 6

    Calculate, using the appropriate equations, the theoretical \([\ce{H_{3}O^{+}}]\) in each solution. For the weak acids, you will need the values of \(K_{a}\) you found in prelab. 


    Step 7

    Calculate the theoretical pH based on the theoretical \([\ce{H_{3}O^{+}}]\)

     

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