CHE314_Homework assignment 1-F23 (3)

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Jan 9, 2024

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Name: ____________________________________ _____ UB#: ________________________ HW Assignment #1 CHE-314: Analytical Chemistry Directions: Answer all questions below. SHOW YOUR WORK CLEARLY. Illegible work will not be graded. Draw a BOX around your final answer, use correct significant figures, and include units. If you use EXCEL, provide a MAXIMUM 1-page printout of your calculations for each question to show your work. This assignment is intended as independent work. Copying or sharing of work will result in penalties as per the syllabus. Question 1 (10pts): Potassium hydrogen phthalate (KHP) is commonly used as a primary standard to measure [OH - ] in basic solutions. KHP (at right) dissociates into K + and HP - upon dissolution. HP - was used to determine pOH of a unknown sample. a) First, you need to make a stock solution for the primary standard, KHP. To do this, you weigh 0.7636 (± 0.0001) g of certified reference standard KHP (C 8 H 5 KO 4 ) on a balance and then dissolve in 25.00-mL of water using a volumetric flask with an uncertainty of ±0.01mL. Calculate the molarity (with uncertainty) of your stock solution. b) Next, you will titrate the unknown sample to determine [OH - ]. To do this, you fill a buret with the KHP stock solution to 50.01 ± 0.01 mL. Then you titrate the unknown by adding KHP stock solution to the unknown [OH - ] sample, using a color indicator to determine the point at which all [OH - ] was consumed in the chemical reaction. The final buret reading was 35.62 (± 0.01) mL. What volume of KHP stock solution (with uncertainty) was used to complete the titration? c) How many moles of HP - were used to complete the titration? (with uncertainty) d) Based on the reaction above, what was the molar concentration (with uncertainty) of the unknown [OH - ] sample if the volume of unknown solution before the titration began was 10.00±0.03mL? e) What was the pOH (with uncertainty) of the original unknown sample [OH-] solution (before the titration)? f) Give two examples of mathematical operations for which the uncertainty formula is calculated as a relative uncertainty . How do you convert relative uncertainty to absolute uncertainty? Note: If you are having trouble knowing how to calculate uncertainty, please refer to Table 3-1 in section 3.4 of your textbook. Example problems also appear in your lecture notes. HP - + OH - → H 2 O + P 2- KHP

Name: ____________________________________ _____ UB#: ________________________

Name: ____________________________________ _____ UB#: ________________________ Question 2 (10pts): The Environmental Protection Agency’s (EPA’s) action level for lead drinking water is 15 ppb. Two different laboratories were tasked with reporting the concentration of lead in a drinking water sample from the same community. Their measurement results are reported below in units of ppb. a) What is 15 ppb expressed in mass per volume units? b) Use a statistical test to determine if the precision of the two sets data is equal at the 95% confidence level. c) Is there a significant difference between the means of the two measurements? Provide a statistical basis for your decision (i.e. support your decision according to the outcome of a statistical test). What statistical test did you use? How many degrees of freedom were used for this test? Analyst 1 Result (ppb) Analyst 2 Result (ppb) 5.2 6.1 5.7 6.2 5.4 5.4 5.3 6.2 5.5 5.7

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Name: ____________________________________ _____ UB#: ________________________ Question 3 (10pts): The isotopes of Uranium and their abundance are of much interest because U is used in nuclear energy and weaponry. Uranium with a high 235 U content is known as ‘enriched uranium’ because it contains a higher percentage of 235 U than found in nature (0.7%). Uranium used in reactors contains 235 U content of 10-20% of the total mass. Uranium used in weapons contains 50-80% 235 U as a mass percent (wt%). Uranium ore found in nature contains 0.7% 235 U. Depleted uranium as used in ammunition has a low 235 U concentration of 0.2% by mass. An agent of the International Atomic Energy Agency (IAEA) makes several replicate measurements of uranium isotopes composition at various sites where ore is prevalent. Site A 235 U (wt%) Site B 235 U (wt%) Site C 235 U (wt%) Site D 235 U (wt%) 1.60 1.71 1.73 0.72 1.42 1.72 1.70 0.72 1.55 1.71 1.71 0.70 1.45 1.77 1.70 0.71 3.23 1.70 1.72 0.70 a) Examine the data. Evaluate the data statistically to determine if there any outliers in each set. b) Choose and apply the appropriate statistical method(s) to determine whether or not there is a significant difference between these sampling sites. Provide the results. c) Can you make a conclusion about the sites based on this data?

Name: ____________________________________ _____ UB#: ________________________ Question 4 (20pts): The concentration of magnesium ion was measured in an experiment using a colorimetric indicator and UV-VIS absorbance spectroscopy (Recall: Beer’s Law: A=  bc). Here, Mg 2+ complexed with the indicator to become colored in solution, and standard solutions were created and analyzed to produce the calibration data below. a) Construct a calibration curve using the data. Include uncertainty. Attach the graph to this problem set. b) What is the sensitivity of the calibration curve? c) What is the limit of detection of this method? d) What is the limit of quantitation? e) An unknown sample was analyzed by the same experiment and provided the following replicate measurements: 0.75 AU, 0.79 AU, 0.77 AU. Using the calibration curve, determine the concentration of Mg 2+ in the unknown sample. Include a a 95% confidence limit. Absorbance (AU) Mg 2+ Concentration (mg/mL) Replicate 1 Replicate 2 Replicate 3 0.01 0.10 0.09 0.11 0.05 0.21 0.19 0.2 0.1 0.33 0.31 0.32 0.5 1.5 1.49 1.49 1 3.10 3.12 3.11 BLANK 0.001 0.004 0.002