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Chemistry help

  1. Jul 12, 2005 #1
    Im am working on my grade 12 chemistry course ALMOST DONE YEAHH!
    I am stuck ona few questions, and Im hoping to get some help even if you just tell me where to begin on solving these.
    1) If the [tex]Ka=1.8 * 10^{5}[/tex] for acetic acid ( ie. [tex]CH_{3}COOH[/tex]), what is the [tex]H^{+}[/tex] ion concentration in a solution of this acid, if 1.2 grams of acid are dissolved in 1.0L of solution.
    Is there a formula i should be using to solve this question I cant seem to figure out what I am supposed to be doing.

    I know Im going ot get told, I have to show my work so far but thats just the problem I have no clue where to begin, so if someone could just help me with that, I am not asking for you to do the question for me.

    2. An electrochemical cell is constructed by placing a nickel electrode into a 1.0 M [tex]NiSO_{4}[/tex] solution, and a silver electrode into a 1.0 M [tex]AgNO_{3}[/tex] solution, and then joining them with a salt bridge to complete the circuit.
    a) give the anode half reaction and the E[tex]^{o}[/tex] value.
    b) give the cathode half reaction and the E[tex]^{o}[/tex] value.
    c) Give the net reactiona nd its E[tex]^{o}[/tex] value.
    What I am able to find out is The Eo value of Ag+ is +0.80 ans the Eo value of Ni2+ is -0.24> But other than this I am completely clueless as where to solve the question
  2. jcsd
  3. Jul 13, 2005 #2


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    This is a really basic question in ionic equilibirium. This is an example of dissociation of an acid in water.
    Let me give you the general case.
    Consider an acid HA which when dissolved in water partially ionizes into H+ and A- ions. There are H+ ions, A- ions and unionized HA in equilibirium in the solution.
    Apply the law of mass action to this equilibirium.
    You get,
    [tex] K_a = \frac {[H+][A-]}{[HA]} [/tex]

    Where [H+], [A-], [HA] are the equilibirium concentrations and [tex] K_a [/tex] is the dissociation constant of the acid HA.
    Now, how do you find the H+ concentration from this?

    You know the initial concentration of the acid because it is given in the question. Let the degree of dissociation of the acid be [tex] \alpha [/tex].

    The degree of dissociation is a fraction of the total number of moles of an acid or base or electrolyte that dissociates into ions in an aqueous solution when equilibrium is reached.

    Here, the total moles of the acid present initially is known. Call it as [itex] c [/itex]
    So, from the above definition, the number of moles of ions of H+ present is [itex] c\alpha [/itex]. Similarly the number of moles of ions of A- present is also [itex] c\alpha [/itex].
    So how many moles of unionized HA are present at equilibirium? From this, can you find the concentration of H+? Can you also post and show the general forumla to find the concentration of H+ and the value of alpha in terms of 'c' and 'Ka'?

    Note: The value of Ka for Acetic acid is 1.8 x 10^(-5) and not 1.8 x 10^(+5)
    Last edited: Jul 13, 2005
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