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    Titration Curve of Weak Acid with Strong Base

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    A 30 mL sample of 0.35 M of lactic acid is titrated with a 0.25 M NaOH solution. Calculate the pH before NaOH is added. After 20 mL of NaOH is added. After 42 mL. After 45 mL. After 60 mL. After 80 mL. Graph the titration curve.

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    Solution Preview

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    This site gives the Ka of lactic acid: http://www.mpcfaculty.net/mark_bishop/weak_acid_equilibrium.htm

    To simplify the reaction, I will use HA for lactic acid:

    HA + NaOH -> H2O + NaA

    This problem will use the Ka expression as in problem 9.

    Initial pH:
    (please see the attached file)

    Now you can use the pH equation:
    (please see the attached file)

    After 42 mL

    When NaOH is added, it neutralizes the acid. You need to know the moles of each:

    0.030 L * 0.35 M = 0.0105 mol lactic acid
    0.020 L * 0.25 M = 0.005 mol NaOH

    0.005 mol of lactic acid will be ...

    Solution Summary

    This solution explains how to calculate the pH of 30 mL of 0.35 M lactic acid solution after titration with varying volumes of 0.25 M NaOH. It also explains how to construct a titration curve.

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