Exam 3 practice problems KEY

# Exam 3 practice problems KEY - 1 How would you prepare 750...

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1. How would you prepare 750 mL of a solution consisting of 50 mM NaCl, 100 mM Tris, 15 mM EDTA, and 20 μ g/mL RNaseA? You have available to you stock solutions of 5 M NaCl, 2 M Tris, 1 M EDTA, and 50 μ g/ μ L RNaseA. Express your answers in the simplest form with the most logical units, and show all work for full credit. Solution C 1 C 2 V 2 V 1 calculation Final V 1 NaCl 5 M 50 mM 750 mL 0.5M * 750 mL/5M 7.5 mL Tris 2 M 100 mM 750 mL 0.1 M * 750 mL/2 M 37.5 mL EDTA 1 M 15 mM 750 mL 0.015 M * 750 mL/1M 11.25 mL RNase 50 μ g/ μ L 20 μ g/mL 750 mL 0.02 μ g/ μ L * 750 mL/ 50 μ g/ μ L 300 μ L Add the four ingredients to some water, mix, and adjust the volume to 750 mL. (Be careful with units, they were all converted before proceeding with calculation!) 2. You have run out of some DNA that you often use, so you go to the freezer to get your stock tube of nice, clean maxiprep DNA. Unfortunately, some of your label has rubbed off, and you can’t read the concentration. You remember that the concentration was relatively high, so you dilute 10 μ L of this DNA with 90 μ L of water, and then you take 5 μ L of this into 995 μ L of water in a quartz cuvet, and read the absorbance of this solution at 260 nm. You read an absorbance of 0.12.

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