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Tough pH question. PLEASE HELP!

Discussion in 'DAT Discussions' started by Lazerous, Nov 30, 2008.

  1. Lazerous

    7+ Year Member

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    Q: A sample of L-valine hydrochloride (i.e. the totally protonated form of L-Valine) is dissolved into water (250 mL of 0.1M). To this solution is added 40 mL of 0.5 M NaOH. What is the final pH of the solution? (Assume L-valine has the following pKa values: 2.29 and 9.74)

    OK, so I know I have to use the hendersen hasselboch equation (pH=pKa + log ([A-]/[HA]) but I don't know how to to get either [A-] or [HA]. PLease help me on this one.

    The ANSWER is: 2.89
     
  2. Sublimation

    5+ Year Member

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    I havent looked over my Acid base stuff in a while. But this is how i would approach this. First, i wont use that equation, because you introduce a base to neutralize. Now since the acid does not dissociate 100% the fact that they give u the pka you can determine the concentration of H then determine the moles of H by multiplying by the volume. then get the moles of OH and watever is left after neutralizing will determine your pH. remember that Ka=[A-][H+]/[HA] use the information given to determine the [H]. remember that you can use Ka= [H+]^2 / [HA]o - [H+] . Use this to solve i guess. i would do it now but i got a debate to prepare. pm me and ill show u the calculations. From the way im doin it, if you dont have a calc. it will take u forever lol.where did u get the question from.
     

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