CHEM 102L FINAL
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 NaOH titrant for a 0.1111-g sample of solid KHP dissolved in 50.00 mL of water. What is the concentration of the NaOH titrant? HINT: The equation for the titration reaction is: KHP (aq) + NaOH (aq) Na+(aq) + K+(aq) + P2-(aq) + H2O(l)
. 0.02546 M
In an experiment, a container with a volume of 283 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:
0.020 moles
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 SCN- 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. Molarity is measured in moles of solute per liter of solution.)
1.86 x 10^-4 M
An unknown diprotic acid reached its second equivalence point at a volume of 12.93 mL of 0.1301 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 0.118 g, the molecular weight of the unknown acid would be:
140 g/mol
The equilibrium concentrations of Fe3+, SCN-, and [FeSCN]2+ are determined experimentally to be 1.43 x 10-3 M, 2.86 x 10-4 M, and 5.96 x 10-5 M, respectively, for the reaction Fe3+ + SCN- —► [FeSCN]2+. What is the experimental value of the equlibrium constant?
146
If lnP is known to 4 significant figures, and 1/T is known to 3 significant figures, what are the correct significant figures for the slope of the regression line?
3
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?
3.45 x 10-5 M [FeSCN]2+
In a Clausius-Clapeyron plot of lnP vs. 1/T, the regression line for one sample has an equation of y = -3750x + 23.89. If the value of R = 8.3145 J mol-1 K-1, the value of ΔHvap for the molecule would be:
31.2 kJ/mol
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.
338.57 g/mol
When should you inspect the glassware in your lab drawer?
During check-in, before beginning every, and during an experiment
For which of the following graphs will a linear fit provide the value of R, the ideal gas constant, in standard units?
Graph IV
When working in the 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
In Experiment 15, two sets of cuvets were prepared in which the same equilibrium reaction occurred. One set was used to define the Beer's Law relationship between Absorbance and the Concentration of the product ion. In the other set, the concentration of the product ion at equlibrium was treated as an unknown, and was used to solve for the value of Keq. What is the principal difference between these two sets of solutions that allow them to be used in different ways?
In one set of cuvets, LeChatelier's principle was used to drive the equilibrium so nearly to completion that the equilibrium concentration could be assumed
Ideal Gaw Law Relationships *(Assume variables are constant when not mentioned)
P/V: indirectly proportional P/T: directly proportional V/T: directly proportional V/n: directly proportional p/n: directly proportional T/n: indirectly proportional
What is your BEST resource for understanding the nature of the chemical hazards of materials you work with in lab?
SDS: safety data sheet
In the Clausius-Clapeyron plot shown, methanol, CH3OH, is represented by the red points. If the other two molecules shown are n-nonane and 3,3-diethylpentane, both with the formula C9H20, the molecules represented by Sample A and Sample B can best be identified by which statement: (See the structures of these molecules.)
Sample B is most likely 3,3-diethyl pentane, because it is branched and has the weakest dispersion forces. Sample A is most likely n-nonane, which has stronger dispersion forces due to its straight chain and high molecular weight, giving it slightly stronger IMF than methanol.
In some circumstances, using the fire extinguisher found in the laboratory may be necessary. Which of the following scenarios represents an appropriate use of a fire extinguisher?
a handful of paper towels in the garbage can has caught fire. The fire is small and relatively contained within the trash can
Before the unknown concentration of the iron thiocyanate can be determined for the equilibrium experiment, a calibration curve was constructed using solutions with known concentration. If done correctly, a line fit of calibration curve data should provide:
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 red product species
Which of the following precautions is important when using a Bunsen burner or Meker burner?
a. Set up your work space so that wires and cables cannot accidentally make contact with the flame or hot glassware and melt b. Set up your work space with the burner in a secure location away from the edge of the bench c. Set up the work space so that flammable materials are away from the burner d. Never leave an open flame unattended e. Remember that any glassware heated by the burner will look the same when hot as cold, and it will take a while to cool f. Always tie back hair
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 elevator, meet your TA in front of the sign for Kenan Laboratories just off of South Road b. exit the lab and proceed to the stairs nearest the Venable parking lot and Columbia Street, meet your TA on the far side of the driveway
For which of the following situations should you be sure to notify your TA and fill out an incident report form?
a. you arrive in lab, but realize that your cold medicine is making you feel groggy b. your lab partner accidentally drops a beaker, and a small chip of glass causes a scratch on your ankle c. you pick up a beaker from the hot plate and the edge of the beaker leaves a small red mark on your thumb d. a broken funnel in your drawer causes a small nick through your glove e. you begin to feel faint and dizzy in lab
When the procedure calls for making a more dilute solution of an acid, or mixing an acid with other solutions, what is the correct order of steps?
always add acid
How many times should you read the label on a reagent container?
at least twice each time you remove material from the container
Which of the following items are not allowed in the lab space during lab, but must be stored in appropriate locations?
backpack, food/snacks, cellphone, and water
Which of the following is the reason backpacks must be stored during lab?
backpacks that are exposed are tripping hazards that may cause accidents and spills, they also pose risk to being exposed to and spreading hazardous materials
If you need to work with a flammable or volatile solvent, which piece of lab equipment should you be sure NOT to use?
bunsen/merker burner or any open flame
When dealing with hazardous materials generated in the lab, the two most important guidelines are:
collect ALL hazardous materials for proper disposal and organize the experiment to generate the least amount of hazardous materials possible
When reading the label on a reagent container, what are the three most important pieces of information?
concentration, name, hazard warning
colligative properties
describe the behavior of a solution
If you need to work with a flammable or volatile solvent, which piece of lab equipment should you be sure to use?
fume good with good ventilation
Consider the uncertainties in your kinetics experiment. Which of the following statements is true?
if the colorimeter has an error that causes the absorbance reading to be consistently 0.05 higher than when the calibration curve was prepared, the error will cause the value of the rate coefficient to be calculated incorrectly, but the determination of reaction order should be okay
After performing this experiment, 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?
if the temperature reading is artificially high, the vapor pressure will appear to be lower than the true vapor pressure at that temperature. This will cause all of the intermolecular forces to appear stronger than they actually are, but the relative strengths of intermolecular forces will be correct
During lab, a beaker containing a solid reagent cracks. To be safe you should ____________
immediately dispose of the solid reagent into a solid waste container in the hood, then dispose of the beaker in the glass waste container, and begin the experiment again
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 [A] = 0.250 M for both trials and [B] = 0.0100 in the first trial and 0.0050 M in the second trial. Assuming the value of x = 1, round the value of y determined in the experiment above to calculate the value of the rate coefficient, k, in the first trial.
k =3.99 x 104 M-2s-1
Your lab partner accidentally gets a mist of your solution in her eye. You immediately help her to the eye wash to rinse it. After about a minute, she feels better and is ready to get back to work. You should:
keep her in the eye wash for at least 15 minutes, and then make sure she goes to student health
supercooling
liquid molecules don't have time to arrange themselves so they stay liquid past the freezing point temperature
An experiment is run that demonstrates that the rate law of a reaction has the form R =k [A]2[B]1, where k=0.25. If the reaction was run with initial concentrations of [A] = 0.25 M and [B] = 0.005 M, which graph would you expect to best fit a straight line that would confirm reaction order?
ln[B] vs time
Which of the following is a behavior that can pose a safety risk in the laboratory environment?
looking away from your work , moving rapidly around the lab , sitting on lab benches, lab floors, or hallway floors, staying focused on your own experiment, and leaving your lab drawer open
Which of the following explains why the lab safety policy specifically addresses loose clothing, contact lenses, and large, loose, or dangling jewelry?
loose clothing or jewelry is more likely to present a fire hazard, contact with chemicals in the lab might damage these items, in the event of a spill all of these items are likely to hold the chemical close to the skin, and loose clothing and jewelry is more likely to impede movement or cause an accident
solvent
major component of solution; stearic acid
solute
minor component of solution that affects physical properties; unknown fatty acids
K(f)
molal freezing point depression constant
molality
moles per kilogram of solvent
When should you immediately turn off the gas valve for a Bunsen or Meker burner?
no one to monitor it, if you smell gas in the room, and if the flame suddenly goes out
When should you be in full PPE?
once the experiment has begun, while cleaning up after experimental, during the lab quiz, while waiting for a friend to finish, and as soon as you enter the lab, and until you are ready to walk out the door
Gay-Lussac's Law
pressure and temperature are directly proportional
Boyle's Law
pressure and volume are indirectly proportional
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 ____________
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
Which of the following must be completed before taking the quiz?
put on lab coat, put on goggles, tie back long hair, and store backpack
Your lab partner accidentally trips and douses the front of your lab coat and shirt with a hazardous material. The TA immediately takes you to the safety shower, since the spill has soaked into your clothing. What do you need to do at this point?
remove clothing and stay under safety shower for 15 min
To determine the order of a reaction with respect to one reactant, A, experimentally, a useful experiment would be:
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 or 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 correct process for disposing of liquid waste containing hazardous material would be:
step 1: bring the waste to liquid waste bottle in fume hood step 2: check the liquid level in waste bottle; don't overfill (2") step 3: Read label on waste bottle, make sure its correct step 4: open latch, pour into funnel step 5: close and snap latch shut
Order the following steps you would take if you find broken glassware in your lab drawer, or if a piece of glassware breaks during the lab.
step 1: notify TA step 2: use leather gloves to pick up step 3: clean up large pieces and dispose of in proper container step 4: use dustpan/brush to clean up smaller pieces and properly dispose of
prevent supercooling
stirring and insulation
Which of the following must be completed as soon as you arrive in lab?
store your backpack, cell phone, jacket, etc; tie back long hair, put on your lab coat, and put on your goggles
Beer's law is useful for the [Fe(SCN)]2+ equilibrium experiment because:
the absorbance of the iron thiocyanate in the blue region of the spectrum is proportional to its concentration.
During a titration of KHP with 0.1 M NaOH, before the equivalence point is reached, what is the pH of the solution? Molecular equation: KHP(aq) + NaOH (aq) →NaKP (aq) + H2O (aq)
the answer cannot be determined without knowing the Ka value for KHP and the concentration and volume of NaOH added.
The results of an experiment with a solution of an unknown fatty acid in stearic acid are shown in the table. Unfortunately, the value of the molar mass of the unknown is significantly different in each calculation. What is a reasonable explanation for this result?
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.
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:
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, the experiment would be more accurate if pure nitrogen was used, because its molar mass and density are more accurately known, and 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.
In the titration curve shown, the value of the pKa(s) are best indicated by
the half-equivalence point
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?
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 and 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.
Based on the calibration curve of a crystal violet Beer's Law experiment shown, which of the following statements is true?
the spacing of points for a single concentration, and the R2 value of 0.9854 both indicate that the colorimeter was not as precise as it could be and the concentration of a solution with an absorbance of 0.731 would be 2.31 x 10-4 M
Consider the following scenario: A student has prepared a series of analytical solutions to be run on an instrument in another room. The student has worn gloves throughout the preparation of the solutions, and has wiped up all drips and spills that occurred during that process. The student removes her gloves before leaving the lab room and carries her solutions in scintillation vials in a large clean beaker to the room where the instrument is located. Once in the new room, the student obtains a new pair of gloves before dispensing the solutions for analysis. Which of the following changes could improve the student's procedure?
the student could have worn a single glove on the hand she used for transporting the solutions to the instrument room, and used her non-gloved hand for opening doors and pushing elevator buttons, the student could have worked with her lab partner so that one of them could carry the materials with gloved hands, while the other opened doors and pressed elevator buttons with non-gloved hands, or the student could have chosen a plastic bin, or other less breakable container to carry her solutions to the instrument so that she wasn't carrying glassware with her bare hands
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?
the test tube holding the reagent must be insulated to minimize temperature gradients within the material and the material must be stirred to encourage crystallization, so that freezing occurs uniformly throughout the material.
A titration curve of an unknown diprotic acid has a first end point corresponding to 15.23 mL of 0.1094 M NaOH. The molar mass calculated from this end point is 154.32 g/mol. The curve shows two additional endpoints, giving pKa values of 3.80, 8.04, and 11.33. When compared with a chart of unknowns, the experimenter finds a triprotic acid with a mass of 155.16 g/mol but the pKas are very different. There is a second triprotic acid with pKa values of 3.96, 7.54, and 11.14. Which statement expresses the most likely identity of the unknown?
the unknown that matches closest in molar mass is most likely the correct one, because the rapid change in pH identifies the end point, and the volume of standardized base is the most accurate measurement in the experiment
A primary standard material should be unreactive in air, of high purity, and easily dried.
true
The end point in the titration of potassium hydrogen phthalate (KHP) with sodium hydroxide (NaOH) is determined to be the point when the color of a phenolphthalein indicator added to the solution turns very pale pink, but fades to colorless a few minutes later.
true
In some circumstances, materials that are being heated in the lab could catch fire. If a flammable substance in a beaker should catch fire while you are working but the flames are relatively contained, what is a simple method for extinguishing the flame?
turn off the heat source immediately, and use a watch glass to cover the beaker and minimize the oxygen around the flame
In the attached titration curve, two unknown acids are titrated by the same standardized base. Based on the information in the plot, which of the following statements is true?
unknown 1 appears to be a monoprotic acid, while unknown 2 is a diprotic acid.
What is the most important consideration to preserve safety when it is necessary to feed glass tubing, thermometers, or other apparatus through a rubber stopper?
using the correct hand position so that if something goes wrong you can avoid coming into contact with broken or sharp ends
colligative properties examples
vapor pressure depression, freezing point depression, and boiling-point elevation
Charle's Law
volume and temperature are directly proportional
A cooling curve to determine freezing point is shown in the figure. The behavior in the graph can be explained in the following way:
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
When should you be sure to use a sealed container and/or secondary containment in the laboratory?
when transporting materials from one lab room to another
For which of the following should gloves be used in the laboratory?
when using nonhazardous or hazardous reagents, when cleaning up, and when handling glassware
