Lecture 20

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Unformatted text preview: red each answer on the scantron sheet. Go back and reread the exam strategies in L ecture 7 . 9 Friday, March 1, 13 Where were we? For very dilute solutions (8 > pH > 6), the number of H3O+ ions formed from water may be comparable to or larger than the number formed from the original acid, which means they need to be included in pH calculations. For very dilute strong acids, [H3O+] can be calculated using this formula: [H3O+]2 – [A–][H3O+] – Kw = 0 Calculations for very dilute weak acids are not included in this course. A shift in equilibrium position due to the addition of an ion involved in the equilibrium is known as the common ion effect. A solution of a weak acid and its salt, or a weak base and its salt, is known as a buffer, and has the very useful function of resisting pH change. The general equation for buffers, which is based on the assumptions of negligibly small changes in concentration, is called the HendersonHasselbalch equation, often abbreviated to HH. [base]in pH = pKa + log [acid]in 10 Calculations Aqueous solutions that contain one or more chemicals that are water soluble may simply be neutral, where the ions do not react with water (e.g., NaCl), or else one or more ions react with water to affect the pH of the solution. Ions may also react with each other, and these reactions may go to completion, or may react to an equilibrium position. To perform calculations on such a system: 1. Deal with any reactions that go to completion (e.g., dissolving salts, neutralizations). Perform stoichiometric calculations to determine the concentrations of components after these reactions. 2. Given the major species left after step 1, determine the dominant equilibrium; i.e., highest K 3. Make an ICE table and determine component concentrations. 11 The best, most accurate, way to do an ICE table, is to make no assumptions and always solve the quadratic (or higher power equation). It is usually faster to make the assumption of small change (x), and then once x is calculated, check to ensure t...
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This note was uploaded on 04/09/2013 for the course CHEM 302 taught by Professor Mccord during the Spring '10 term at University of Texas at Austin.

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