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Homework Help: Tonicity and Osmoticity of saline with weak acid

  1. Jul 27, 2017 #1
    Hi all

    I got an exercise:
    Cells were floated in a mass in a solution with 150mM of NaCl (isotonic conditions). A weak acid was added to the solution with a pK of 7.0 and concentration of 50mM in the solution.
    What is the tonicity and osmoticity of the new solution if PH=4

    I know that I should use the following equation:
    log([A-]/[HA]) = PH - pKa

    But It seem that I am getting the wrong result.
    Can somebody help?
  2. jcsd
  3. Jul 27, 2017 #2

    jim mcnamara

    User Avatar

    Staff: Mentor

    Thread moved to the homework forums. No template.
  4. Jul 31, 2017 #3
    The answer in case somebody else will interest:

    Need to understand that only the fraction of the drug that is uncharged can diffuse across membranes.
    So in case PH=4 and pKa=7 the solution will be acidic so the weak acid remain almost in the same concentration (50mM)

    so the solution is isotonic and hyperosmotic
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