Acids and Bases questions

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This topic contains 1 reply, has 0 voices, and was last updated by  Ethon 12th May 2018 at 8:57 am.

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    Ethon
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    Hello,

    There are 2 questions I don’t understand, would love to get some help:

    First question:
    0.6 grams of a base (MOH) with Molar mass of 34 grams/mole has been dissolved in 100 mL of water. Pkb=9.5.
    What is the pH of the solution?

    Second question:
    Usually, in aquatic solutions of acids and bases, you can omit the concentration of H3O+ and OH- (of the water). Under which of the following circumstances you must not omit the contribution of the water?
    1) Anytime when an acid ionization constant (Ka) is greater than Kw.
    2)A solution containing a weak diluted acid.
    3)A solution where there is an equilibrium between a weak acid and its conjugate base.
    4)A mixture of strong acid and a weak acid.
    5)A solution containing a strong diluted acid.

    Sorry if the English is bad, I translate it from my language.

    What I tried to do:
    First question: I found the concentration of the base MOH to be (0.6/34)/0.1 = 0.176 Molar.
    The reaction should be : MOH —> M + OH.
    Then I know that in the reaction x molar reacted, so thats 0.176-x for MOH and in the reaction you get x for M and x for OH.
    so you have this equation:
    (x^2)/(0.176-x)=9.5, you can omit the x, so thats (x^2)/0.176=9.5, from here you get that x=1.293. x is the concentration of OH, so let’s find pOH, and then do (14-pOH) to get pH.
    pOH is:
    -log(OH)=-log(1.293) = -0.11 < = I get a negative pOH. Where is my mistake?
    About the second question : I really don’t know what to do so I didn’t really try, if you find the answer, can you please explain why it is the answer?

    Thank you

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