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Chemistry! Please Help!

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What mass of propanoic acid, CH3CH2COOH, pKa= 4.87, should be dissolved in water to 2.00L volume in order to produce a pH of 2.50?



  • Chemistry! Please Help! -

    Let's call propionic acid HP, then it will ionize as
    HP ==> H^+ + P^-

    First convert pH = 2.50 to (H^+) by
    pH = -log(H^+).
    Then set up the expression for Ka.
    Ka = (H^+)(P^-)/(HP).
    You know (H^+) now from the pH calculation. You know (P^-) because it equals the same as (H^+). (HP) = y-(H^+).
    Solve for y which = (HP).
    Then M x molar mass propanoic acid = grams propanoic acid.
    Post your work if you get stuck.

  • Jude -

    When you do the -log of 2.50 you get a negative number, so I am not really sure what you do with that, do you not use the pKa for anything?

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