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DETAILED SOLUTIONS TO SOME OF GARRETS AND GRISHAMS PROBLEMS

DETAILED SOLUTIONS TO SOME OF GARRETS AND GRISHAMS PROBLEMS...

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DETAILED SOLUTIONS TO SOME OF GARRET = S AND GRISHAM = S PROBLEMS. CHAPTER 2 PROBLEM 7. Given 0.1M solutions of Na 3 PO 4 and H 3 PO 4 , describe the preparation of 1 L of a (0.1M , omitted in the book, but implicit) phosphate buffer at a pH of 7.5. What are the molar concentrations of the ions in the final buffer solution, including Na + and H + ? First, phosphoric acid has three pKa values: 2.15, 7.2 and 12.4. Also, the Henderson-Hasselbalch equation tells us that pH = pK a + Log [A - ]/[AH] where A - = HPO 4 2- and AH = H 2 PO4 - isolating Log [A - ]/[AH] = pH - pK a , or antilog of Log [A - ]/[AH] = [A - ]/[AH] = 10 pH-pKa at pH = 7.5, the pKa that governs this equation is pKa 2 = 7.2, thus [A - ]/[AH] = 10 pH-pKa = 10 7.5-7.2 = 2.0 or [HPO 4 2- ]/[H 2 PO 4 - ] = 2.0 (equation 1) Since the total molarity of phosphate in the buffer is 0.1M, A - + AH = 0.1M or [HPO 4 2- ] + [H 2 PO 4 - ] = 0.1M (Equation 2) combining equations 1 and 2 , we get, [H 2 PO 4 - ] + 2[H 2 PO 4 - ] = 0.1M And [H 2 PO 4 - ] = 0.1M /3 = 0.0333M therefore [HPO 4 2- ] = 0.1M - 0.0333M = 0.0667M Second, the charge balance requires that the concentration of all positive charges be equal in concentration to all negative charges (not ions concentrations, charges concentrations). Since one ion of

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DETAILED SOLUTIONS TO SOME OF GARRETS AND GRISHAMS PROBLEMS...

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