Solution%20Assn%203

Solution%20Assn%203 - Solutions for Assignment 3 - Segel...

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Solutions for Assignment 3 - Segel Book: Page 93 Problem 48 In solving this problem it is best to sketch out the different ionic species of amino acid that one can have. For example, since glycine has only one COOH and one amino group you can simply do this as follows. NH 3 + COOH COO - NH 3 + +1 0- COO - NH 2 1 Underneath each sketch you put the net charge on the species. You can thus see which form of glycine is isoelectric (the middle form). Look up the pKas for glycine in Appendx 7 of your Segel book. They are 2.36 and 9.6 and are on each side of the isoelectric form. You are beginning with 480 ml of .02 M isoelectric glycine. 480 x .02 = 9.6 millimoles of total isoelectric glycine at beginning. You will add 20 mls of .2 M base to this solution. 20 x .2 = 4 millimoles of base added.
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+1 0 -1 0 9.6 0 At beginning: 0 5.6 4 After add 4 mmole base pH = pKa + log [species -1]/[species 0] pH = 9.6 + log 4/5.6 pH = 9.6 + log .71 pH = 9.6 - .15 = 9.45
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Problem 49 Part a) Must first identify the various ionic species of leucine and look up the pKa values.
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Solution%20Assn%203 - Solutions for Assignment 3 - Segel...

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