hw1.pdf

Name : RedID # : Show your work, answers alone cannot receive partial credit. Answers that are to be expressed in units (e.g. grams, moles/liter,?) for which the units are not included will receive a grade of ZERO regardless of the work done in the calculations. Attach this first page to your answers, the pages with the questions do not need to be included. All necessary data tables, equilibrium constants and dissociation constants and other standard measurements can be found in the textbook if they are not given in the problem. Homework assignment #1 page 1 of 3 Due Sept. 14, 2010 at the beginning of class CHEM 251 Question Points Score 1 30 2 14 3 20 4 16 5 10 6 20 7 20 Total 130 1) Convert each given concentration into the format requested. All solutions are aqueous, unless otherwise stated the solutions are a room temperature and a density of 1g/mL can be assumed. 2) Calculate both the molar and molal concentration of water (H2O) in a solution of pure water when the water is at 10°C and 30°C. This is not a trick question, consider H2O to be both the solute and solvent; consult your textbook (chapter 2.9) for the density of water; use an accurate molecular weight for water, not 18 g/mol. 3) 25.06 mL of a 0.188 M solution of NaOH is used to neutralize (bring to pH 7.00) a 10.00 mL sample of hydrochloric acid (HCl). The original concentrated acid had been diluted by aliquoting two 20.00 mL volumes into a 250 mL volumetric flask, which was diluted to volume with water. From this volume, the 10.00 mL sample was taken for analysis by titration. Determine the concentration of the original hydrochloric acid solution. 4) In the problem above we did not account for any uncertainties or calibrations in our measurements. Given the calibrated measurements and uncertainties listed below, what is the new concentration of the hydrochloric acid solution and the absolute and percent relative uncertainty in that concentration? Original 25.06 mL 0.188 M 10.00 mL 250 mL 20.00 mL Calibration 25.062 mL 0.1886 M 10.008 mL 250.00 mL 19.993 mL Uncertainty ± 0.003 mL ± 0.0001 M ± 0.005 mL ± 0.04 mL ± 0.003 mL 5) Calculate the propagation of uncertainty to illustrate why it is favorable to use a calibrated 10 mL pipette to transfer 60 mL of solution instead of an uncalibrated 20 mL pipette. The calibrated pipette has a volume of 10.005 ± 0.006 mL and the uncalibrated pipette has a tolerance of 20.00 mL ± 0.02 mL. (Note the excess volume from the calibrated pipette is acceptable as it can be accounted for in later calculations.) Homework assignment #1 page 2 of 3 Due Sept. 14, 2010 at the beginning of class CHEM 251 Original Concentration Converted Concentration a 86 ppm phosphate mM b 8.93 w/w% fructose, solution density = 1.16 g/mL M c Aqueous solution, pH = 3.07 ppm (of H + ) d A solid mixture of 22% potassium fluoride and 78% magnesium fluoride (no water) w/w% of fluoride 6) 15.0934 ± 0.0001 g of sodium carbonate is diluted to volume in a 125.00 ± 0.01 mL volumetric flask. Two aliquots are taken, with a calibrated pipette (10.021 ± 0.003 mL), and delivered to a 500.00 ± 0.03 mL volumetric flask. What is the final concentration of sodium carbonate in the 500 mL volumetric flask? What is the absolute uncertainty in this value? Remember to account for the uncertainty in molecular weight in you calculations. 7) To prepare an analysis system for use a standard is measured multiple times. To pass the test the the true value for the standard must fall within the 90% confidence interval for the set of measurements. The standard used has a concentration of 1.984 mM; based on the tabulated values below did the instrument pass the test? Trial #1 #2 #3 #4 #5 #6 Conc. (mM) 1.975 1.993 2.000 1.999 2.002 2.004 Homework assignment #1 page 3 of 3 Due Sept. 14, 2010 at the beginning of class CHEM 251 Chris Harrison Homework #1 - CHEM 251