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anonymous

  • one year ago

The compound para–aminobenzoic acid (PABA) is a powerful sun–screening agent whose salts were once used widely in sun tanning and screening lotions. The parent acid, which we may symbolize as H–Paba, is a weak acid with a pKa of 4.92 (at 25 °C). What are the [H+] and pH of a 0.030 M solution of this acid? Please help!!

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  1. aaronq
    • one year ago
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    So write an equation for the dissociation of the acid, then use it to write an equilibrium expression. \(\sf pK_A=-log(K_A)\)

  2. anonymous
    • one year ago
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    i thought that you would find H+ by multiplying 4.92 and 0.030 and then for the ph you would solve for x?

  3. aaronq
    • one year ago
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    it's not too far off. multiply the Ka by 0.03 then take the squared root, then you have [H+]. Next take the negative logarithm of [H^+] then you have the pH

  4. anonymous
    • one year ago
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    i got -.021 for H+? is that right?

  5. aaronq
    • one year ago
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    no it can't be negative

  6. anonymous
    • one year ago
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    okay so would it be .021 then?

  7. anonymous
    • one year ago
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    i did the -log of 4.92 and then i multiplied it by the0.030

  8. aaronq
    • one year ago
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    the first step is wrong, the formula is \(\sf pK_A=-log(K_A)\), so then \(\sf K_A=10^{-pK_A}\)

  9. anonymous
    • one year ago
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    okay i got 3.61?

  10. aaronq
    • one year ago
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    what is that?

  11. anonymous
    • one year ago
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    for H?

  12. aaronq
    • one year ago
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    Does that make sense to you? the initial concentration of the acid is 0.030 M

  13. anonymous
    • one year ago
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    yes

  14. aaronq
    • one year ago
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    no it can't be higher than the concentration of the acid

  15. aaronq
    • one year ago
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    did you do what i said previously? "multiply the Ka by 0.03 then take the squared root, then you have [H+]"

  16. anonymous
    • one year ago
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    yeah you said ka was equal to 10^-4.92? and then i multiplied it by 0.030? and then i took the square root of it and got 6.00x10^-4

  17. aaronq
    • one year ago
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    yeah, that answer is right

  18. aaronq
    • one year ago
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    it's not what you wrote before though

  19. anonymous
    • one year ago
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    yeah i forgot to take the square root

  20. anonymous
    • one year ago
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    and then for the ph i get 3.22?

  21. aaronq
    • one year ago
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    yep that's right

  22. anonymous
    • one year ago
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    yay! thanks!!

  23. aaronq
    • one year ago
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    no problem !

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