ws5s08key - CH302 Spring 2008 Worksheet 5 Answer Key 14...

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CH302 Spring 2008 Worksheet 5 Answer Key 14 questions involving simple water equilbria and the approximations that make them simple. 1. The only water equilibrium for which we make no approximations is the case of pure water (amazing how simple something is when you don’t add anything to it.) What is the most common approximation made when we add acids or bases to water? Answer: The most common assumption is that the H + or OH - contribution from water dissociation isn’t significant. You know this assumption has occurred because you rarely see a calculation of pH for an acid or base that includes a K w term. 2. For each of the compounds and concentrations shown below, tell what kind of compound it is (strong acid, weak acid, strong base, weak base, salt) and write down the equation you would use to solve the problem. Answer: compound type equation 0.2 M HClO 4 strong acid [H + ] = C a 0.1 M NH 4 Cl weak acid [H + ] = (K a C a ) 0.5 0.03 M Ba(OH) 2 strong base [OH - ] = C b 0.2 M CH 3 NH 2 weak base [OH - ] = (K b C b ) 0.5 0.2 M lithium acetate weak base [OH - ] = (K b C b ) 0.5 0.1 M lactic acid weak acid [H + ] = (K a C a ) 0.5 3-9. For the next six problems, find either the pH or the molar solubility of the compound for the compounds in Table 2. Assume a simple equilibrium in each case. You will note that the total time and uncertainty involved in solving these six problems will probably be less than the time and doubt involved in completing Table 2. This should be a heads up to you that the biggest reason people struggle in this section of material is that they don’t know what kind of problem they are working—and we haven’t even started dumping multiple compounds into solution!! 3.
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This note was uploaded on 03/22/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas.

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ws5s08key - CH302 Spring 2008 Worksheet 5 Answer Key 14...

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