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lecture+4.105 - Biological Sciences 105 Lecture 4 LAST TIME...

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Biological Sciences 105 Lecture 4, January 13, 2011 Copyright Steven M. Theg, 2011. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 1 LAST TIME WE ENDED WITH THE AN EXAMPLE PROBLEM QUIZ, THEN SOME QUICK PROBLEMS DON’T DO THESE IN CLASS UNLESS THERE ARE SPECIFIC QUESTIONS. Example #1 : In a 1.0 L solution of 0.1 M Tris at pH 7.5, what is the concentration of protonated and unprotonated Tris? The solution comes from these steps: 1. Determine [A - ]/[HA] from the H-H equation 2. Express [A - ] as x [HA] 3. C T = [A - ] + [HA] = x [HA] + [HA] = (1+x)[HA] 4. Plug in numbers 5. Check that it makes sense Use Henderson-Hasselbalch equation pH = pK a + log{[A - ]/[HA]} 7.5 = 8.1 + log{[A - ]/[HA]} log{[A - ]/[HA]} = 7.5 - 8.1 = -0.6 [A - ]/[HA] = 10^(-0.6) = 10 -0.6 = 0.25 [A - ] = 0.25[HA] C T = total concentration = [A - ] + [HA] 0.1 = 0.25[HA] + [HA] = 1.25[HA] [HA] = 0.1/1.25 = 0.08 M [A - ] = 0.1 - 0.08 = 0.02 M This answer makes sense since pH < pK a , and so should have more protonated than unprotonated buffer. Example #2 : How many moles of Tris base and HCl are required to make a 1.0 L solution of 0.1 M Tris-HCl at pH 7.8?
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