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    hi guys ,

    i need help, i need to find the pKa of citric acid

    pH =2.3

    concentration = 1 mol dm-3
    volume = 50 cm3 = 0.05
    i find the number of moles
    1 *0.05 = .05 moles

    [H+] = 10^-2.3
    [H+} = 5.01*10^-2.3
    because its a one proton acid it dissociates as shown below
    HA<--> H+ + A-

    the concentration of the [A-] = H+

    how do i calculate the concentration of non dissociated ions ?

    then i know that i insert the data

    ka= [A-][H+]
    --------
    [HA]

    then pKa=-log Ka


    thanks a lot
    i really appreciate it
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    What level is that?
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    what do you mean ? higher level?
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    What is the question? I see a lot of working out but I don't follow it.
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    how do i calculate the concentration of non dissociated ions ?

    i need help to get to
    ka= [A-][H+]
    --------
    [HA]

    cheers
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    I don't follow your working. Remember this is about concentrations, so amounts are irrelevant-

    [H+] = 10^-2.3 looks correct

    [H+} = 5.01*10^-2.3 looks incorrect

    Citric acid is a tribasic acid (3 acidic protons) but assume you are only dissociating the first one, ie the reaction is H3A ----> H+ + H2A-

    If you have generated 10^-2.3 M H+, then you must have also generated 10^-2.3 M H2A-

    Original conc of acid was 1 M, so undissociated acid conc must be (1 - 10^-2.3)

    So now you have top and bottom lines of the Ka calculation

    Hope this helps

    C
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    Charco and Eier FTW
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    i meant with [H+} = 5.01*10^-2.3 is that
    that pH = 10^ -pH
    10^-2.3 gives the result of 5.01*10^-3

    is therefore dissociated acid conc 1- 5.01*10^-3
    Are there any numbers squared ?

    ka= (5.01*10^-3)^2 / 1- 5.01*10^-3 ??

    thanks a lot and thumbs up for you

    sorry but [H+} = 5.01*10^-2.3 was a typo
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    (Original post by INDIGO123456789)
    hi guys ,

    i need help, i need to find the pKa of citric acid

    pH =2.3

    concentration = 1 mol dm-3
    volume = 50 cm3 = 0.05
    i find the number of moles
    1 *0.05 = .05 moles

    [H+] = 10^-2.3
    [H+} = 5.01*10^-2.3
    because its a one proton acid it dissociates as shown below
    HA<--> H+ + A-

    the concentration of the [A-] = H+

    how do i calculate the concentration of non dissociated ions ?

    then i know that i insert the data

    ka= [A-][H+]
    --------
    [HA]

    then pKa=-log Ka


    thanks a lot
    i really appreciate it
    Furrowing through the chaff..

    if the pH = 2.3 then

    [H+] = 0.00501

    Assuming you are dealing with the first dissociation

    HA <==> H+ + A-

    so that [H+] = [A-]

    Then:

    Ka = [H+]2/[HA]

    You are told that [HA] = 1.0 mol dm-3

    Therefore: Ka = [H+]2 = 2.5 x 10-5

    so pKa = 4.6

    ---------------------------------------------------------

    NOTE: This is well off the accepted pKa value of either the first or second dissociation of citric acid (3.13 & 4.76)
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    iif [HA] equals the mol dm^-3 why do we find the undissociated acid conc? which would be 1- 10^-2.3 ?

    Ka=2.5 x 10^-5 / 1- 10^-2.3?
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    (Original post by INDIGO123456789)
    iif [HA] equals the mol dm^-3

    why do we find the undissociated acid conc??
    These two phrases are contradictory.

    [HA] is the undissociated acid concentration.
 
 
 
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