MH Sp 24 14BL MT practice problems & Answer

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University of California, Los Angeles *

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14BL

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Chemistry

Date

Apr 28, 2024

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pdf

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5

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Chem 14BL Spring 2024 Practice Problems for Midterm Midterm – April 30 (Tuesday)- Lecture 1: 8:00 am – 8:50 am in CS 24 Lecture 2: 9:00 am – 9:50 am in CS 24 Exam MUST be taken during the lecture in which you are officially enrolled. DON’T BE LATE – NO EXTRA TIME WILL BE GIVEN IF YOU ARE LATE TO THE EXAM The midterm will include topics such as error analysis, solutions concentrations, Beer’s Law and acid- base titration experiment. Error Analysis 1. Calculate the percent error for the following quantities: (a) [(20.54 + 0.02) x (0.254 + 0.003)] / (3.21 + 0.05) (b) [(30.078 + 0.003) – ( 20.174 + 0.001) + (9.813 + 0.005)] (c) [(0.642 + 0.002) X (2.413 + 0.004)] – (0.501 + 0.002) (d) [(12.635 +0.005) / (5.967 + 0.003)] + (0.478 + 0.004) 2. Calculate the absolute errors for the quantities in question 1. 3. Calculate the percent relative average deviation for the following set of data. 20.96, 20.85, 20.89, 20.92 Concentration Units 4. Define the concentration terms molarity, normality, weight percent(w/v%) , ppm 5. A phosphoric acid (H 3 PO 4 ) solution was prepared by transferring 0.0196 g to a 1-L volumetric flask and diluting the solution to the mark. Calculate the concentration of the H 3 PO 4 solution in molarity, normality, ppm, and weight percent (w/v % and w/w % ) (Atomic mass H=1.0000 g/ mol, P = 31.00 g/mol, O = 16.00 g/mol)
Practice Problems (Continued) 6. A sodium hydroxide solution was prepared by transferring 0.4063g to a 100-mL volumetric flask and diluting the solution to the mark. A 10-mL aliquot of this solution was transferred to a 50-mL volumetric flask and the solution we diluted to the mark. Calculate the concentration of the final sodium hydroxide solution in molarity and w/v% units. (Atomic mass H=1.0000 g/ mol, Na = 23.0000 g/mol, O = 16.0000 g/mol) 7. A student made a standard solution by transferring 0.0400 g of NaCl into a 50-ml volumetric flask, dissolving the salt and diluting to the mark. She then withdrew 5-ml aliquot with a volumetric pipet and transferred to a 50 ml volumetric flask. The solution was diluted to the mark. Calculate the concentration of the sodium in parts per million. Assume that 1 ml = 1g. (Atomic mass Na = 23.0000 g/ mol, Cl = 35.4500 g/mol) Beer’s law 8. (a) Beer’s Law is summarized by the equation: A = e bc Name and define each of the symbols in this equation. (b) Calculate the percent transmittance of a solution if its absorbance is 0.352 (9) (a) Calculate the absorbance of the solution if its concentration is 0.0278 M and its molar extinction coefficient is 35.9 L/(mol cm). The depth of the cell is 5 mm. (b) What is the %T? (10) Calculate the absorbance of the solution if the transmitted light intensity is 70% of the initial light beam intensity. (11) What would happen to the unknown absorbance if you forgot to zero the absorbance with the blank in the aspirin experiment? (12) The following set of data was obtained for the calibration curve for a Beer’s Law experiment. Molarity (moles/L) Absorbance blank solution 0.0 9.80 x 10-5 0.240 1.96 x 10-4 0.475 3.10 x 10-4 0.741 (a) Without plotting any graph, calculate the slope of the line. (b) Calculate the concentration of a solution of the same compound if its absorbance were 0.500.
Practice Problems (Continued) (13) (a) Calculate the molarity of the standard base solution if a 5.1362 g sample is used to prepare 100 ml in the volumetric flask. (Molecular Weight – 121.13g). (b) Calculate the Molar concentration of an acid solution, if 20.74 ml were required to titrate a 10.00-ml aliquot of the solution prepared in 14. Recrystallization/Aspirin (Review how to calculate the limiting reagent, % yield) (14) What is the purpose of recrystallization? (15) What is the major difficulty when performing recrystallization? (16) Outline the general procedures in recrystallization. (17) Does polarity play a role when picking a solvent for any given product in recrystallization? (18) (a) Which solvent pairs did we use in recrystallizing the aspirin product? (b) Outline the major steps in using a mixed-solvents technique in recrystallization.
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