Section%20B%20Midterm%201%20Key

# Section%20B%20Midterm%201%20Key - BIBC 103 Section B...

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BIBC 103 ANSWER KEY Section B Midterm Exam 1 (150 pts.) Lab Section___________ 1) (20 pts.) Your first day on the job at the trendy, new biotech company, Spliceogen, you are asked to make a buffer solution for an enzymatic assay. You want to make 500 ml of 10 mM tris (pH 8.5), 25 mM MgCl 2 , and 0.1% tween-20 (a detergent). Looking on the shelves in the lab you find MgCl 2 powder (MW 95.3 g/mol), 0.5 M tris stock solution at pH 8.5, and 50% tween-20 stock. How will you make the buffer solution? Show all your work. MgCl 2 : M = n/V .025 M = n/0.5 L n = (.025 mols/L).5 L = .0125 mols (.0125 mols)95.3 g/mol = 1.19 grams of MgCl 2 powder. Tris: .5 M/.01 M = 50 = df 500 ml/50 = 10 ml of the 0.5 M stock solution Tween-20: 50%/0.1% = 500 = df 500 ml/500 = 1 ml of the 50% stock solution Mix all reagents with 489 ml water. 2) (5 pts.) If a 5 M solution is diluted to 0.5 M, what is the dilution factor? 5 M/.5 M = 10 = df 3) (10 pts.) For the buffering compound HEPES that has a pKa of 7.55, what would be the ratio of the proton acceptor form to the proton donor form of the compound in a solution where the pH is 9.55? Show how you plug the numbers into the Henderson- Hasselbalch equation to solve this. pH = pKa + log A - /HA plugging in, 9.55 = 7.55 + log A - /HA 2 = log A - /HA A - /HA = 100 or 100 A - to 1 HA

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4) (10 pts.) A solution has exactly equal amounts of acetic acid, and its conjugate base, acetate, and the pH of this solution is 4.8. What is the pKa of acetic acid? 4.8
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Section%20B%20Midterm%201%20Key - BIBC 103 Section B...

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