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-   -   Titration, equivilence points. (https://www.askmehelpdesk.com/showthread.php?t=363552)

  • Jun 10, 2009, 03:18 PM
    Khouuu
    Titration, equivilence points.
    In a titration of 100 mL 0.10M nitrous acid HNO2 against 0.10M sodium hydroxide, calculate the pH at the following stages:
    K(a) for the nitrous acid = 4.5 x 10-4

    a) before any base has been added to reach the acid solution.

    b) after 25% of the base necessary to reach the equivalence point has been added.

    c) at the equivalence point.
  • Jun 10, 2009, 08:21 PM
    Perito
    Quote:

    In a titration of 100 mL 0.10M nitrous acid HNO2 against 0.10M sodium hydroxide, calculate the pH at the following stages:
    K(a) for the nitrous acid = 4.5 x 10-4

    a) before any base has been added to reach the acid solution.
    a. Calculate the concentration of H+



    Take the negative log of that concentration to find the pH.

    0.1M HNO2. You immediately know the concentration.

    (Equation 1)

    where x is the amount that's dissociated.





    the negative root makes no chemical sense so



    x is both the hydrogen ion concentration and the nitrite ion concentration.



    Quote:

    b) after 25% of the base necessary to reach the equivalence point has been added.
    b. Add the base. Figure out the concentration of NO2 -- don't forget to add the volume of base that's added. Assume that the volumes add. Then, do the same calculation as in #1.

    Quote:

    In a titration of 100 mL 0.10M nitrous acid HNO2 against 0.10M sodium hydroxide, calculate the pH at the following stages:
    K(a) for the nitrous acid = 4.5 x 10-4

    c) at the equivalence point.
    c. Here the amount of acid equals the amount of base. You have only sodium nitrite (NaNO2). Nitrous acid is a weak acid:



    This is really not all that different from Equation 1 since

    Figure out the concentration of Nitrite. Create an equation like I did in problem #1. In this case, the conc of HNO2 equals the conc of OH-

    (Equation 1b)

    if C is the concentration of HNO2 before dissociation

    (Equation 1b)

    Solve for x and calculate the pH.

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