8303229-64-Buffer-Solutions-v2 - Chem Factsheet...

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Acid Base III: Buffer solutions, pH Curves and Dibasic Acids Number 64 1 C hem F actsheet www.curriculumpress.co.uk To succeed with this topic you need to understand the concepts of pH, K a , buffers and pH curves. (Factsheets 25 and 26). After working through this Factsheet you will: understand the link between buffer solutions and the pH curve for a weak acid with a strong base; be able to use the pH curve for a weak acid with a strong base to find p K a and K a for the weak acid involved; have met the pH ‘problems’ of H 2 SO 4 , a dibasic acid. Buffer solutions and pH curves 0 10 20 30 40 50 2 4 6 8 12 14 pH Volume of NaOH added /cm 3 A B C D H F G E The pH curve for the titration of a weak acid with a strong base has the shape shown below: We need to look in more detail at what chemicals are in the solution mixture at various points on the curve (A-H): A pure weak acid (HA) B weak acid (HA) + some of its salt (A ) because of the neutralisation reaction between the acid and the alkali and more water because: acid + alkali salt + water) C less weak acid (HA) and more salt (A ) D even less weak acid (HA) and even more salt (A ) E no weak acid left, the solution contains only the salt E-F the end-point where one drop of the base (A ) changes the solution from acidic to basic (pH 7 11) F a salt solution + a slight excess of base (NaOH) G + H as an excess of base is added the pH increases to maximum value of about pH = 13. pH curves Buffer solutions 1. A buffer solution is one whose pH hardly changes when small amounts of acid and alkali are added to it. 2. A buffer solution is made by dissolving the salt of a weak acid in the weak acid itself e.g. sodium ethanoate (CH 3 COONa) dissolved in ethanoic acid (CH 3 COOH) 3. The following mathematical equation is used for buffer solutions: pH = - log 10 K a log 10 [acid] [anion] or pH = p K a – log 10 [acid] [anion] N.B. ‘p’ = ‘– log 10 We need to look at the link between this pH curve and buffer solutions.
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8303229-64-Buffer-Solutions-v2 - Chem Factsheet...

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