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Homework Help: Anticipated pH of Buffer Solution ( !)

  1. Feb 18, 2008 #1
    [SOLVED] Anticipated pH of Buffer Solution (urgent!)

    1. The problem statement, all variables and given/known data
    Can someone verify my process here? i did a lab where i had 100 mL CH3COOH/CH3CO2- buffer solution. I have to find the pH if 100 mL of 0.0983 M HCl is added to it. In the lab i got pH of 1.72

    2. Relevant equations

    [H3O+][CH3CO2-]
    Ka= ---------------------
    [CH3COOH]

    Ka : 1.8x10-5
    initial moles CH3COOH: 0.00996
    Initial moles CH3CO2-: 0.0100
    moles H+ in 100 mL 0.0983 M HCl: 0.00983



    3. The attempt at a solution

    It's my understanding that the mole ratio of H+/CH3CO2- is one to one.
    Next i think i do this:

    moles CH3COOH = 0.00996 + 0.00983 = 0.0198 moles
    moles CH3CO2-= 0.0100- 0.00983= 0.00017 moles

    concentration CH3COOH= 0.0198 moles/(0.200 L) = 0.099 M
    concentration CH3CO2-= 0.00017/(0.200 L) = 0.00085 M

    Then by rearranging the equilibrium expression i can solve for [H3O+].
    [H3O+]=(1.8x10-5)(0.099M) / (0.00085) = 0.00212 M

    pH= -log(0.00212M) = 2.67

    is that how am supposed to go about doing this? because my other lab values are very strange. for instance, i keep getting a pH of 4.7 after adding 5, 10, and 20 mL of HCl to it.
     
  2. jcsd
  3. Feb 18, 2008 #2
    k i finally got the answer thanks anyone who looked at it tho
     
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