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Posted by on Thursday, September 9, 2010 at 7:40am.

A 0.513 M solution of a weak base has a pH of 11.4. What is the Kb of the base?

  • science - , Thursday, September 9, 2010 at 7:41am

    2.64*10^-5

  • science - , Thursday, September 9, 2010 at 8:37am

    We will assume that this is monobasic and the temp is 25C so that Kw=10^-14

    If pH is 11.4 then pOH is 14-11.4 = 2.6

    so [OH-] = 10^-2.6 = 2.512 x 10^-3

    assuming that [weak base] at equilibrium is large compared with [OH-] then

    Kb is approximately [OH-]^2/[weak base]

    Kb=(2.512 x 10^-3)^2/0.513

    =1.23 x 10^-5

    but check my maths.

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