CHEM 102L Quiz Questions (Exam Review) – Flashcards

If lnP is known to 3 significant figures, and 1/T is known to 4 significant figures, what are the correct significant figures for the slope of the regression line?

In the Clausius-Clapeyron plot shown, methanol, CH3OH, is represented by the red points. If the other two molecules samples shown are n-nonane and [straight line] 3,3-diethyl pentane [Kind of an x-boxey shape], both with the formula C9H20, the molecule represented by Sample B, in green triangles, is best explained by which statement: (See the structures of these molecules. Check all correct answers.) Graph: lnP vs Inverse T All downward slopes Sample B on top; methanol in middle, Sample A on bottom Options: A. Sample B could be either n-nonane or 3,3-diethylpentane, because both have a much larger molecular weight than methanol and therefore must have stronger intermolecular forces. B. Sample B is most likely n-nonane, because it has the weakest dispersion forces of the three molecules. C. Sample B is most likely 3,3-diethyl pentane, because it has the strongest dispersion forces of the three molecules. D. Sample B is most likely 3,3-diethyl pentane, because it has the weakest dispersion forces of the three molecules. E. Sample B cannot be either n-nonane or 3,3-diethylpentane, because both have a much larger molecular weight than methanol and therefore must have stronger intermolecular forces. F. Sample B is most likely n-nonane, because it has the strongest dispersion forces of the three molecules.

In a Clausius-Clapeyron plot similar to the one in the preceding question, the regression line for one sample has an equation of y = -4450x + 20.98. If the value of R is 8.3145 J mol-1 K-1, the value of the enthalpy of vaporization for the molecule would be:

After performing this experiment [Exp 12 intermolecular forces], a pair of students discover that their thermometer was miscalibrated and was reading consistently 1.82°C higher than the actual temperature. How will this impact their results? A. They should redo the entire experiment with a correctly calibrated thermometer, because the relative strength of intermolecular forces in the various liquids will be impossible to determine for certain. B. The relative strengths of the intermolecular forces will be the opposite of their true order if the measurements had been made correctly. C. All of the intermolecular forces will appear to be stronger than they actually are because the vapor pressure will appear to be higher at any given temeperature, but the relative strengths of intermolecular forces will be correct. D. All of the intermolecular forces will appear to be weaker than they actually are because the vapor pressure will appear to be lower at any given temeperature, but the relative strengths of intermolecular forces will be correct. E. The magnitude of the heat of vaporization might be off for various results, but the rate of evaporation (change in T vs time) should be correct, so their qualitative ranking of IMF should be correct as well.

Beer's law is useful for the crystal violet kinetics experiment because: A. The absorbance of the product in the violet region of the spectrum is proportional to its concentration. B. The colored product will change the reflectivity of the solution. C. The purple color of the product can be used to determine the energy of the electron transitions in the product. D. The absorbance of the product in the green region of the spectrum is proportional to its concentration.

Before the unknown concentration of the crystal violet can be determined for the kinetics experiment, a calibration curve was constructed using solutions with known concentration. If done correctly, a line fit of calibration curve data should provide: A. A polynomial curve in which the terms represent the molar absorptivity, the path length of the cuvette, and the wavelength of the absorbance. B. A. A straight line with a positive slope, in which the slope represents the product of the molar absorptivity and the path length of the cuvette and the intercept represents absorbance or interferences that are not due to the purple product species C. A straight line whose slope represents the change in energy of the electron as it emits purple light. D. A straight line with a negative slope, in which the slope represents the rate at which the purple product is decreasing in solution and the intercept represents the initial concentration.

A 1.5 x 10-5 M solution of crystal violet has an absorbance value of 0.842. 1600 microliters of this crystal violet solution is added to a cuvette, along with 1900 microliters of distilled water. The concentration of crystal violet in the cuvette would be (i)________ and the absorbance of the solution in the cuvette should be approximately (ii)_________. A. (i) 6.8 x 10-6 M (ii) A=0.709 B. (i) 8.1 x 10-6 M (ii) A=0.709 C. (i) 1.3 x 10-5 M (ii) A=0.457 D. (i) 1.8 x 10-5 M (ii) A=0.457 E. A cannot be determined from the information given in the problem. F. (i) 6.8 x 10-6 M (ii) A=0.385

The calibration curve of a crystal violet Beer's Law experiment gives the regression line: y = 21900x - 0.0028, with R2=0.9979. Which of the following statements is true? Check all that apply. A. The concentration of a solution with an absorbance of 0.731 would be 3.35 x 10-5 M B. The concentration of a solution with an absorbance of 0.731 would be 16000 M. C. The R2 value of 0.9979 indicates that the calibration curve is very precise. D. The y-intercept indicates that there are many interferences in the measurement, e.g. scratches or discoloration on the cuvette walls, or particles in the solution. E. None of the answers here are correct. F. The value of the slope of the line indicates that the crystal violet has a very high concentration. G. The concentration of a solution with an absorbance of 0.731 would be 3.32 x 10-5 M.

Two trials in a kinetics experiment give relative initial rate values of 0.997, and 0.262. The general rate law for the reaction is R=k[A]x[B]y, where [B] = 0.25 M for both trials and [A] = 0.0100 in the first trial and 0.0050 M in the second trial. Based on these results, which of the following would be true? A. y ~ 2 The values of x and k cannot be determined without additional experiments. B. x ~ 1 y ~ 1 k =3.98 x 102 M-2s-1 for trial 1 C. x ~ 1 The values of y and k cannot be determined without additional experiments D. x ~ 2 y ~ 1 k = 3.99 x 104 M-3s-1 for trial 1 E. x ~ 2 The value of y and k cannot be determined without additional experiments F. x ~ 1 y ~ 0 k = 99.7 M-1s-1 for trial 1

Consider the uncertainties in your kinetics experiment. Which of the following statements is true? A. If the colorimeter has an error that causes the absorbance reading to be consistently 0.05 units too high, the error will cause the order of reaction to be calculated incorrectly, but the rate coefficient should be okay. B. If the colorimeter has an error that causes the absorbance reading to be consistently 0.05 units too high, the error will cause the value of the rate coefficient to be calculated incorrectly, but the determination of reaction order should be okay. C. None of these statements is correct. D. If the colorimeter has an error that causes the absorbance reading to be consistently 0.05 units too high, the error will cause both the order and value of the rate coefficient to be calculated incorrectly. E. If the colorimeter has an error that causes the absorbance reading to be consistently 0.05 units too high, the error will not cause any inaccuracy in either the order of reaction or the value of the rate coefficient.

An experiment is run that demonstrates that the rate law of a reaction has the form R =k [A]1[B]2, where k=0.25. If the reaction was run with initial concentrations of [A] = 0.25 M and [B] = 0.0025 M, which graph would you expect to best fit a straight line? A. 1/R vs time B. 1/[B] vs time C. [A] vs time D. [B] vs time E. R vs time F. ln [B] vs time

To determine the order of a reaction with respect to one reactant, A, experimentally, a useful experiment would be: A. Run a reaction with a concentration of [A] = 0.800 M and a concentration of [B] = 0.001 M. Continue the run until [A] is < 0.001 M and [B] ~ 0.001 M. B. Run a reaction until both [A] and [B] are much less than their initial values. C. Hold [B] at 0.25 M and vary [A] between 0.30, 0.25, and 0.20 M. D. Run a reaction for three trials in which [B] has a concentration of 0.800 M and [A] has a concentration of 0.05 M, 0.04M, and 0.025 M, respectively.

The equation of the best fit line for a Beer's law plot for [FeSCN]2+ is determined to be A=(4328)C + 0.115, where A is the absorbance of the solution and C is its concentration. If the transmittance of a solution is measured at 54.4%, what is the concentration of the unknown solution? A. 3.45 x 10-5 M [FeSCN]2+ B. 1.11 x 10-5 M [FeSCN]2+ C. 7.06 x 102 M [FeSCN]2+ D. 8.76 x 10-5 M [FeSCN]2+

The initial concentrations of Fe3+ and SCN- are created to be 0.00149 and 3.46 x 10-4M. The equilibrium concentration [FeSCN]2+ is determined experimentally to be 5.96x10-5 M, for the reaction Fe3+ + SCN- —► [FeSCN]2+. What is the experimental value of the equlibrium constant? A. 116 B. 146 C. 6.86 x 10-3 D. The equlibrium constant cannot be determined experimentally. E. Initial concentrations of Fe3+ and SCN- must be known to calculate the equilibrium constant.

Quiz for Experiment 15: A student uses experimental data to create a Beer's Law plot for a set of standard solutions, the resulting graph is shown below. Based on the results shown, what assessment can be made? %T vs Concentration Graph y=-18000x+66.2 R^2= .9221 Options: A. The experiment has not plotted the correct information for Beer's law. No assessment of the quality of results can be made until the correct information is given on the plot. B. The R2 value indicates that the experimenter mixed up the stock solution and the solution used to dilute it. C. The solution concentrations fell outside the range that can be successfully measured by the colorimeter. D. The distribution of standards around the line indicate that the solutions were not prepared properly with the micropipettes.

A solution was prepared using 3000 microliters of distilled water, 325 microliters of 2.00 x 10-3 M NaSCN, and 175 microliters of 0.201 M Fe(NO3)3•9 H2O. What is the initial concentration of Fe3+ in solution before the reaction begins? (NOTE: NaSCN molar mass is 81.07 g/mol. Fe(NO3)3•9 H2O is 403.9 g/mol) A. 2.17 x 10-4 M B. 1.86 x 10-4 M C. 0.0101 M D. 0.0117 M E. The concentration cannot be determined if the equilibrium constant is not known.

Based on the relative initial amounts of Fe3+ and SCN- in the previous problem, that solution was most likely prepared as a: A. equilibrium solution because the volume of SCN- vastly exceeds that of Fe3+. B. standard solution for Beer's law calibration because the inital concentration of Fe3+ is more than 800 times that of SCN-, forcing the equilibrium nearly to completion. C. standard solution for Beer's Law calibration because the volume of Fe3+ is much less than that of SCN-. D. equilibrium solution because the concentrations of Fe3+ and SCN- are sufficiently close that the equilibrium constant will determine how far the reaction proceeds before the forward and reverse rate are equal.

The pKa of a weak acid in a titration can be determined by: A. Finding the volume of titrant at which an acid-base indicator in solution changes color. B. Finding the point where the slope of a plot of pH vs. titrant added approaches infinity. C. Finding the pH at the volume of titrant added that is half of the volume needed to completely neutralize the weak acid. D. Finding the point where the slope of a plot of pH vs. titrant added approaches zero. E. Both answers A and B would work. F. Finding the pH at the volume of titrant needed to completely neutralize the weak acid. G. None of these. H. Finding the volume of titrant at that is half of that needed to reach the equivalence point.

During a titration of KHP with 0.1 M NaOH, when the half equivalence point is reached, what is the pH of the solution? Molecular equation: KHP(aq) + NaOH (aq) Na+(aq) + K+(aq) + P- (aq) + H2O (aq) Ka for KHP = 3.98 x 10-6 A. pH > 7 B. pH = 7 C. pH < 5.5 D. 5.5 < pH < 7 E. The answer cannot be determined without knowing the mass of KHP in the titration.

During a titration between KHP (204.221 g/mol) and sodium hydroxide (40.01 g/mol), the end point was determined to be at a volume of 21.37 mL of 0.09588 M NaOH titrant for a sample of solid KHP dissolved in 50.00 mL of water. What is the mass of the KHP sample? HINT: The equation for the titration reaction is: KHP (aq) + NaOH (aq) Na+(aq) + K+(aq) + P2-(aq) + H2O(l) A. 0.2315 g B. 0.4184 g C. The mass cannot be determined from the information provided. D. 0.05121 g E. 0.4894 g

Which of the following statements correctly describes the function of each chemical species in the titration? A. None of these is true. B. Phenolphthalein was the primary standard used to determine the end point of the titration against the 0.1 M NaOH titrant. C. KHP was the unknown analyte being titrated by the known 0.1 M NaOH solution. D. The 0.1 M NaOH solution was the primary standard, which was used to determine the molarity of the KHP solution in the flask. E. The NaOH solution was the titrant with an unknown concentration, whose concentration must be determined by standardization against the KHP primary standard.

A titration curve of an unknown diprotic acid should show two equivalence points, but only one is easy to identify from the data. A clear equivalence point occurs at a volume of 12.5 mL of 0.1013 M NaOH added and at a pH = 8.05. Based on that information, the experimenter could look more closely at the data to find the second equivalence point: A. at a volume of 6.25 mL and a pH = 4.03. B. at a volume of 6.25 mL; and a pH of 3.23. C. at a volume of 25.0 mL and a pH = 12.08. D. The volume at the second end point cannot be determined from the information given, but the pKa of the first end point would be at a pH of 4.03, so the second pKa would be at a pH of 12.08. E. at a volume of either 6.25 mL or 25.0 mL. The pH at the end point cannot be determined from the data provided. F. at a volume of 25.0 mL and a pH = 16.10.

Experiment 16.1 Quiz: In the attached titration curve, the same mass of two unknown acids are titrated by the same standardized base. Based on the information in the plot, which of the following statements is true? Curve: Unknown 1 starts at pH of 2.3 and goes to 12, with straight line at 15mL Unknown 2 starts at pH 2.5 and goes to 14 with straight line at 15mL and 30mL Options: A. Both unknowns have an end point at a volume of ~14.5 mL, which means it is likely that Unknown 1 and 2 are the same acid, but Unknown 1 wasn't titrated far enough to see the second end point. B. Both unknowns have an end point at a volume of ~14.5 mL, which means they must have very similar pKas. C. Both unknowns have an end point at a volume of ~14.5 mL, which means they must have similar molar mass. D. Unknown 1 appears to be a monoprotic acid, while Unknown 2 is a diprotic acid. E. Unknown 1 has a lower pH when no base was added, so it must be the stronger of the two acids. F. Unknown 1 has a pKa of ~ 4, while Unknown 2 has a pKa of ~3.5, so Unknown 2 would be the stronger acid. G. C, D, and E are all correct. H. B and D are both correct.

An unknown diprotic acid reached its second equivalence point at a volume of 21.39 mL of 0.1013 M NaOH, with a general chemical equation in the form: H2A + 2 NaOH 2 H2O + Na2A If the mass of the unknown acid sample was 0.180 g, the molecular weight of the unknown acid would be: A. 166 g/mol B. 83.1 g/mol C. 41.5 g/mol D. 12.0 g/mol

Exp 16.2 Quiz In the titration curve shown, horizontal sections indicated by Region 2 and Region 4 indicate what? Curve: 1. Start 2. horizontal 3. verticle 4. horizontal A. The substance being titrated has two regions of relatively stable pH. Therefore the substance must be a strong acid. B. The substance being titrated is a weak acid, with two endpoints. Therefore it is a weak, diprotic acid. C. The substance being titrated is a weak base, with two endpoints. Therefore the substance is weak and dibasic. D. None of these.

In an experiment, a container with a volume of 269 mL was filled with air and sealed. Pressure was allowed to vary with temperature in the container. Based on the best fit line, the number of moles of air in the container would be: P vs T: y=.0057x+.9547 A. 19 moles B. 0.019 moles C. 0.13 moles D. 0.0017 moles

Exp 11 In the plot of pressure vs. temperature shown, the data appear somewhat scattered around the regression line. Which of the following statements offers a well-reasoned assessment of the data? P vs T: R^2= .93 A. The regression line likely offers the best value for the relationship between pressure and temperature, because individual points are more likely to have a higher error or uncertainty. B. Some of the data shown in the plot should be eliminated if they don't fall on the regression line to improve the accuracy and precision of the results. C. If the container was imperfectly sealed, "burping" of the stopper could cause some results to be inconsistent because the number of moles of air were allowed to change during the experiment. D. If the container was imperfectly sealed, some results could have been affected by the changing volume. E. Both A and C are sound assessments. F. Both A and D are sound assessments. G. All of these statements are sound assessments. H. None of these statements is a sound assessment of the data.

In a gas law experiment, an experimenter uses the volume of a syringe, 5.00 mL and the density of dry air, 0.001185 g/mL, to calculate the mass of air in the syringe. A value of 28.96 g/mol is then used as the molar mass of air to calculated the number of moles of air in the syringe. This value, in turn, is used to calculate the value of the ideal gas constant, R. One valid criticism of this approach would be: A. The calculation was done incorrectly. The value of the density is not needed to correctly calculate R. B. The calculation was done correctly, however the value 28.96 g/mol is not a true molar mass because air is a mixture of gases. C. The experiment would be more accurate if pure nitrogen was used, because its molar mass and density are more accurately known. D. The experiment would be more accurate if the humidity of the air was included in the calculation, because this would tend to reduce the effective molar mass when it was included in the weighted average. E. All but A are valid assessments. F. All but C are valid assessments.

Which of the following graphs will a linear fit provide the value of R, the ideal gas constant, in standard units? I. kPA vs mL II. atm vs L III. kPa vs 1/mL IV. atm vs 1/L

A solution of 0.613 g erucic acid in 8.115 g stearic acid is measured to have a freezing point of 68.60 ºC. What is the molar mass of erucic acid? HINT: MW stearic acid is 284.48 g/mol. Tf (stearic acid) = 69.60 ºC. Kf (stearic acid) = 4.50 ºC/m. Density (stearic acid) = 847 kg/m3. A. 0.340 g/mol B. 315 g/mol C. 96.8 g/mol D. 338.57 g/mol E. None of these answers is correct.

Exp 13 A cooling curve to determine freezing point is shown in the figure. The behavior in the graph can be explained in the following way: Curve: The curve goes down rapidly at first but then curves to level off a bit (temp vs time) A. None of these explanations is correct. B. When the heat source is removed, the molten substance cools. However, when the substance begins to freeze, the change to solid phase releases energy that allows it to maintain a stable temperature until the freezing is complete. C. When the heat source is removed, the molten substance cools. However, when the substance begins to freeze, the change to solid phase absorbs energy that allows it to maintain a stable temperature until the freezing is complete. D. The process of freezing is an endothermic process; therefore, the substance cools until the freezing process is complete. Once all of the material is in the solid phase, the temperature stops decreasing.

In an experiment studying colligative properties, a solid reagent was heated until it was fully molten, then allowed to cool to determine the freezing point. For this experiment to work, which of the following precautions must be taken to prevent supercooling? A. Temperature must be monitored to the nearest hundredth decimal place. B. The thermometer must be kept as still as possible near the middle of the material to avoid temperature fluctuations. C. The test tube holding the reagent must be insulated to minimize temperature gradients within the material. D. The material must be stirred to encourage crystallization, so that freezing occurs uniformly throughout the material. E. A and B are correct. F. C and D are correct.

The results of an experiment with a solution of an unknown fatty acid in stearic acid are shown in the table (quiz for exp 13). Unfortunately, the value of the molar mass of the unknown is significantly different in each calculation. What is a reasonable explanation for this result? A. The calculation of molar mass was done using the mass of each portion of the unknown acid added, but it should have been calculated for the total mass of unknown acid. When this calculation error is corrected, the calculated molar mass should be consistent for each trial. B. There is not enough information provided to determine the nature of the problem. C. The experiment wasn't carried out correctly. The dramatic increase in the temperature change indicates that the steps needed to prevent supercooling were not followed. D. The experiment should have been carried out for the same mass of unknown in each trial. The differences in the mass of unknown leads to the different molar masses calculated.

You arrive to Morehead Laboratories two minutes after your lab has officially started and you do not want to miss the quiz. As you enter the building you notice that the fire alarm is sounding. The best way to be sure you will be able to take the lab quiz is to _________________________ A. wait at the door of your laboratory room so that you are present when the TA returns. B. meet at the correct designated rally point for your lab section and look for your TA and classmates there, where you will receive additional instructions. C. cross the bridge into Kenan Laboratories and find a place to open your laptop and take the quiz. D. wait just inside the door to the building to see if anyone else enters or leaves the building, so that you can catch your TA and take the quiz.

Your lab partner gets a chemical in his eye during lab, and is in great pain, needing help to get to the eyewash. It is your responsibility to get him to the eyewash which is located ____________________ and to help hold his eyes open in the eyewash for 15 minutes. A. in the sinks in each lab bench B. just inside the door to the hallway C. in the sink at the front of the lab, next to the white board D. inside the fume hood

While carrying a vial of an organic liquid from the hood, it drops on the bench and shatters, soaking the front of your lab coat and shirt. You should immediately ____________ A. proceed to the rest room on the same floor as your lab room where you can remove the lab coat and rinse the chemical from your shirt. B. Notify the TA that you have spilled a chemical and assist with cleanup. Then obtain a new lab coat and continue with the experiment. C. proceed to the safety shower located just inside the door to the hallway, remove both the lab coat and shirt, and rinse your skin for 15 minutes. D. Transfer the broken glass to the glass waste, use a sponge to wipe up the organic liquid, then rinse off the lab coat in the sink.

In the event the fire alarm for Morehead Laboratory sounds, you must evacuate the building. Assuming the emergency does not interfere with your best route, you should__________ A. exit the lab and proceed to the stairs nearest the Venable parking lot and Columbia Street. After exiting on the ground floor you should meet your TA in the driveway to the Venable parking lot just off of South Road so that you can be included in a head count of those safely evacuated. B. exit the lab and proceed to the stairs nearest the elevator. After exiting on the ground floor you should meet your TA on the loading dock between Kenan Laboratories and Morehead Laboratory so that you can be included in a head count of those safely evacuated. C. exit the lab and proceed to the stairs nearest the Venable parking lot and Columbia Street. After exiting on the ground floor you should meet your TA on the far side of the driveway to the parking lot so that you can be included in a head count of those safely evacuated, keeping the drive clear for emergency vehicles. D. exit the lab and proceed to the stairs nearest the elevator. After exiting on the ground floor you should meet your TA in front of the sign for Kenan Laboratories just off of South Road so that you can be included in a head count of those safely evacuated.

When working in lab, you should always remove your gloves when: I. Typing on your laptop keyboard II. Leaving the laboratory III. Picking up your pen to record observations in the lab notebook. IV. Using the doorknob V. Using the laboratory phone VI. Adjusting your hat/hair clip VII. Scratching your nose. A. Only I. and II. are correct B. All but III. are correct. C. Only II. is correct D. All of these are cases when you should first remove your gloves.