Chemistry 321 - Answer Key

Chem 321 D. Miller 1. Name Answer Key If you weigh a sample of mercury on an analytical balance, will the apparent mass be less than, equal to or greater than the true mass. Explain your choice with specific comments. (4) The apparent mass of mercury will be greater than the true mass. Since the density of mercury is larger than that of the internal standard mass used to calibrate the balance, the same mass of mercury will not experience as much buoyancy as the internal standard so it appears to be heavier. Describe the proper technique for dispensing a liquid sample using a pipet labeled “TC”. (3) The “TC” means “to contain”, so all of the liquid must be expelled from (blown out or rinsed out) the pipet in order to obtain a reliable volume. For the following, calculate the result and its absolute error and round off the result to the proper number of significant figures. Show all work. (5) 0.00170 ± 0.00008 In a gravimetric analysis, why is an electrolyte added to the solution used to wash the precipitate? Why is HNO 3 a good choice for the electrolyte in a gravimetric chloride analysis? (6) An electrolyte in the wash solution minimizes peptization (break-up of ppt.) and loss of precipitate. HNO3 is a good choice because it is volatile and will be removed in the drying process. Chem 321 D. Miller Name Answer Key 2. For each of the following multiple-choice questions, circle the letter preceding the correct (best) answer. Only one answer will be accepted for each. (3 ea) For a solution with a density near 1.0 g/cm 3, a solute concentration of 0.96 ppm is the same as: A. 0.96 g/L. B. 0.96 g/mL. C. 0.96 mg/L. D. 0.96 :g/L. During a precipitation, crystal growth can be promoted by: A. precipitating from a cool solution. B. precipitating from a dilute solution. C. precipitating from a basic solution. D. all of the above. A 1.344-g sample was analyzed for its phosphate (PO 43-, M = 95.0 g/mol) content by precipitating the phosphate as Mg(NH 4)PO4 @ 6H2O (M = 245.4 g/mol) and converting this to Mg 2P2O7 (M = 222.6 g/mol). If 0.2672 g of Mg2P2O7 was recovered, what was the mass percent phosphate in the sample? A. 16.97% B. 8.48% C. 19.88% D. 57.36% A student reports a value of 41.26% for an unknown for which the “true value” is 41.05%. The relative error of this reported value is: A. 0.21%. B. 5.1%. C. 0.21 ppth. D. 5.1 ppth. Chem 321 D. Miller 3. Name Answer Key The results of an analysis for the mass percent iron in an unknown were calculated to be: 6.9851% 6.1044% 7.3235% 7.0962% 7.2410% What value, with the proper number of significant figures, should be reported for grading? Justify your answer. Assume that the errors associated with the experimental procedure are 3 ppth. (10) check 2nd value with Q test average of other 4 values = 7.16145% since Qcalc > Qtab (=0.64), reject this data point If experimental errors are known to be 3 ppth, then the result should be reported as 7.16%, which implies errors of about 1.4 ppth (1/716). 7.16% On the basis of eight measurements, the ozone concentration in the atmosphere was found to be 0.68 ± 0.08 ppm where the ± refers to two standard deviations. Explain what specific statistical information is conveyed by this ± value. (4) This means that approximately 95% of the values fall within the range of 0.60 - 0.76 ppm. Calculate the 90% confidence interval for these measurements. (5) 0 = 0.68 ppm, s = 0.04 ppm, n = 8 and at the 90% confidence level, with 7 degrees of freedom, t = 1.895 (see table) 0.68 ± 0.03 ppm What is the significance of this confidence interval? (3) Assuming no determinate errors, there is a 90% chance that the true value is in the range 0.65 - 0.71 ppm. Chem 321 D. Miller 4. Name Answer Key An aqueous solution containing 0.020 M Pb 2+ is treated with Cl- to precipitate the Pb2+ as PbCl2. What must be the concentration of Cl- in this solution to ensure that less than 1 ppth Pb 2+ remains unprecipitated? (10) PbCl2(s) init equil at equil, X Pb2+(aq) + 2Cl-(aq) 0.020 M 0 <0.000020 M x Ksp = 1.7 x 10-5 = [Pb2+][Cl-]2 = (0.000020)(x)2 Y x = 0.92 M > 0.92 M 5. In our gravimetric chloride experiment, we dissolve the unknown sample in a large amount of water, add HNO 3 and add a dilute solution of AgNO3 to the sample with stirring. Briefly explain why each of these steps was taken. (4) The unknown is made up in a dilute solution and the precipitating agent is also dilute and is added slowly with stirring all to keep the relative supersaturation low and hence promote crystal growth and the formation of easily filtered precipitate particles. The HNO3 is added to reduce the electric double layer around precipitate particles and permit the particles to coalesce more easily. In a gravimetric analysis, what is meant by digestion? advantage(s) does this technique offer? What (4) Digestion is the process in which a precipitate is kept in the presence of its mother liquor, usually at elevated temperatures. This promotes slow recrystallization, resulting in larger and purer precipitate particles.