Ionic Eqm Question
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hey guys would like to ask ur help for how to do this qn on ionic eqm... not sure how to start though... and promo is like on fri. -
(i) [HCl] = [CH3N2], since : 1) 25cm3 of base required 25cm3 of acid for equivalence point. 2) HCl is monoprotic and CH3N2 is monobasic. (ii) Kb = ( [OH-] x [conjugate acid] ) / [conjugate base] Since at initial, [OH-] = [conjugate acid], you get Kb = [OH-]^2 / [conjugate base] Hence you can find [OH-], hence pOH, hence pH. (iii) pOH = pKb for maximum buffer capacity. Notice that the question was much too generous in reminding you it was "a solution of maximum bufferring capacity". It was actually sufficient to simply ask, "Calculate the pH of the solution obtained when 12.5cm3 of HCl was added during the titration". Comments : This question is too easy for 'A' levels exam standard (and your Promos, and your Prelims, etc). Expect much tougher questions for the actual 'A' levels (especially considering the recent trend that papers are getting tougher every year - the 2008 exam papers for Chemistry was much tougher than the 2007 exam papers. Brace yourself for 2009 and 2010).
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thanks man! btw its a past year prelim paper from AJC prelim 2007
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It's unrealistically easy. You want to prepare yourself better than this. For a proper question on acid-base equilibria, do these two questions :
Qn 1 (Phosphorous acid-base Equilibria Qn)
25 cm3 of the diprotic acid phosphorous acid a.k.a. phosphoric(III) acid, H3PO3, of initial molarity 0.1mol/dm3 was titrated against NaOH, of molarity 0.1mol/dm3.
Given the following equations and corresponding Ka values:-
H3PO3 --> H2PO3- + H+ Ka1 = 1x 10-2
H2PO3- --> HPO32- + H+ Ka2 = 2.5 x10-7
Sketch the shape of the pH curve during this titration, labelling all significant pH values (there are 6 of these) and the relevant volumes of NaOH, on the axes of the curve.
Qn 2 (Ammonia acid-base Equilibria Qn) Click here for Qn -
Ultimaonline,
can u post the answers? wanna check if i got it right
thx
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Answers to Phosphorous acid Qn Relevant pH values (at various volumes of NaOH added) : 1.5 (0 cm3) 2.0 (12.5cm3) 3.95 (25cm3) 6.6 (37.5cm3) 9.5 (50cm3) approaches 13.0 (at large excess of NaOH) Answers to Ammonia Qn : Mass of ammonium chloride added = 23.41g Change in pH (upon adding acid) from 8.9 to 8.868 is minimal due to buffer action. Significant pH values of graph : 11.27 (0cm3), 9.24 (25cm3), 5.12(50cm3), approaches 0.699 (large excess of HCl).
