Midterm1StudyGuideKey-W07

# Midterm1StudyGuideKey-W07 - Chemistry 20L Midterm Study...

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Chemistry 20L Midterm Study Guide Answer Key Please see the Study Guide for the experiment-related topics that you need to know. Open lab notebook, lecture guides and hand-written notes. No lab manuals. Bring a calculator and an extra pencil and eraser. Error Analysis 1. Calculate the percent error for the following quantities (a) (20.54 ± 0.02)(0.254 ± 0.003) 3.21 ± 0.05 percent error = ( 0.02 20.54 + 0.003 0.254 + 0.05 3.21 ) X 100 = 2.8% ~ 3% (b) (30.078 ± 0.003) - (20.174 ± 0.001) + (9.813 ± 005) Absolute error = (30.078 - 20.174 + 9.813) ± (0.003 + 0.001 + 0.005) = 19.717 ±0.009 % error = 0.009 19.717 x 100 = 0.046% ~ 0.05% (c) [(0.642 ± 0.002) (2.413 ± 0.004)] - (0.501 ± 0.002) absolute error = [( 0.002 0.642 + 0.004 2.413 ) X 1.549 ] + 0.002 = 0.007 + 0.002 = 0.009 % error = ( 0.009 (0.642)(2.413) 0.501 ) X 100 = 0.009 1.048 X 100 = 0.86% ~ 0.9% (d) 12.635 ± 0.005 (5.967 ± 0.003) + (0.478 ± 0.004) 12.635 ± 0.005 (5.967 ± 0.003) + (0.478 ± 0.004) = 12.635 ± 0.005 6.445 ± 0.007 % error = ( 0.005 12.635 + 0.007 6.445 ) X 100 = 0.148% ~ 0.1% 2. Calculate the absolute errors for the quantities in question 1. Answers: (a) 1.625 X 2.8% = ±0.045 or 1.625 X 3% = ± (b) ± 0.009 (c) ± 0.009 (d) 12.635 6.445 x0.148% 0.003 or 12.635 6.445 x0.1% 0.002

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3. Calculate the percent relative average deviation for the following set of data: 20.96, 20.85, 20.89, 20.92 average = 20.905; average deviation = 0.035; relative average deviation = 0.17% (You should carry the extra digit in the average while you are calculating the relative average deviation. You will not round to 20.91 until you report your answer) 4. Define the concentration terms molarity, normality, weight percent (w/w and w/v), PPM, pH. Molarity = moles solute/liter solution Normality = equivalents of solute/liter solution Weight percent (w/w) = grams solute/100g solvent Weight percent (w/v) = grams solute/100mL solvent ppm = parts per million pH = -log[H + ] 5. When would it be appropriate to use each of the concentrations in question 1. What limitations are there with any of these concentrations? Molarity and normality are used to relate laboratory practice with theoretical and experimental chemical reactions. Since the relationship between chemicals in reactions is on a molecular (or mole basis), these units efficiently convey the arithmetic of the reactions. Molarity requires a knowledge of the molecular formula of the reagents; normality requires only a knowledge of the weight of the reagent that reacts with or furnishes a mole of protons (or electrons, in a redox reaction.) Normality is only used in experimental situations. Weight percent (either w/w or w/v) are useful recipes. These concentrations can be reproduced precisely by someone who has no knowledge of chemistry or of the reaction that the solutions might be used for. The weight/volume concentration is very easy to prepare as it is easier to measure the volume of a solution than its weight. When the
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## Midterm1StudyGuideKey-W07 - Chemistry 20L Midterm Study...

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