Worksheet 05 Key - CH302 Spring 2007 Worksheet 5 Answer Key...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
CH302 Spring 2007 Worksheet 5 Answer Key 20 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 HgI 2 salt [ Hg ++ ] = (K sp /4 ) 1/3 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 AgI salt [ Ag + ] = K sp 0.5 3-11. For the next eight 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 eight 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. What is the pH of 0.2 M HClO 4 ? Answer: pH = 0.69 Comment: Straight ahead strong acid calculation. The answer makes sense, a really low pH. 4. What is the pH of 0.1 M NH 4 Cl? The pK b of NH 3 is 4.75. Answer: pH = 5.1 Comment: Tricky one. First you have to convert from pK b to pK a by subtracting from 14 so that you are working in acid terrain. Then use the 10x function on your calculator and perform a simple weak acid calculation. The answer makes sense, a little acidic as weak acids should be. 5.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 4

Worksheet 05 Key - CH302 Spring 2007 Worksheet 5 Answer Key...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online