Chem 102L Final

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.

- Notify your TA so that he or she can help you., Second step: - Use a pair of leather gloves found in the lab room to prevent nicks or cuts., Third step: - Clean up large pieces and dispose of in the designated glass only waste container., Fourth step: - Use the dustpan and brush found in the lab room to sweep up small shards and chips in the area where the glassware was broken and dispose of these in the designated glass waste.

Which of the following explains why the lab safety policy specifically addresses loose clothing, contact lenses, and large, loose, or dangling jewelry?

-Loose clothing and jewelry is more likely to impede movement, or accidentally knock materials over, causing a spill or other accident. -In the event of a spill or spray of reagent, all of these items are likely to hold the chemical close to the skin, making it harder to remove and therefore increasing the risk of injury. -Contact with chemicals in the lab might damage these items. -Loose clothing or jewelry is more likely to present a fire hazard if open flames are used in the lab for any reason, or if a fire begins accidentally.

To determine the order of a reaction with respect to one reactant, A, experimentally, a useful experiment would be:

. Hold [B] at 0.25 M and vary [A] between 0.30, 0.25, and 0.20 M. C. 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. D. Two of these would work. Correct E. Run a reaction until both [A] and [B] are much less than their initial values. F. 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.

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:

0.019 moles

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 13.17 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)

0.04131 M

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 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] = 0.005 M, which graph would you expect to best fit a straight line that would confirm reaction order?

1/[B] vs time

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

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

In a Clausius-Clapeyron plot of lnP vs 1/T, 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:

37.0 kJ/mol

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 .

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: I. The calculation was done incorrectly. The value of the density is not needed to correctly calculate R. II. 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. III. The experiment would be more accurate if pure nitrogen was used, because its molar mass and density are more accurately known. IV. 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.

All but I. are valid assessments.

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 - Either add all of the water or non-acid component first, or add a significant portion, before adding the acid to the mixture. This helps to minimize the heat generated, which could otherwise create dangerous fumes or reactions.

How many times should you read the label on a reagent container and compare it with the lab manual?

At least twice each time you remove material from the container.

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? I. 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. II. 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. III. 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. IV. If the stopper was inserted too far into the flask, some results could have been affected by the changing volume.

Both I and III are sound assessments.

If you need to work with a flammable or volatile solvent, which piece of lab equipment should you be sure NOT to use?

Bunsen or Meker burner, or anything with an open flame.

For which of the following situations should you be sure to notify your TA and fill out an incident report form?

During check-in you discover a broken funnel in your drawer, and the broken end of the stem causes a small nick through your glove., During lab check-out at the end of the semester, your lab partner accidentally drops a beaker, and a small chip of glass causes a scratch on your ankle because you weren't wearing socks., You accidentally pick up a beaker from the hot plate, not realizing that it was already hot, and the edge of the beaker leaves a small red mark on your thumb., You arrive in lab, but realize that your cold medicine is making you feel groggy., You begin to feel faint and dizzy in lab because you had to skip lunch.

When should you be sure to wear full PPE (proper attire, lab coat, goggles) in the laboratory?

During the lab quiz., Once the experimental work has begun., While cleaning up after experimental work is done., While waiting in the lab room for a friend to finish., As soon as you enter the lab, and until you are ready to walk out the door.

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 bright pink-purple.

False

When should you inspect the glassware in your lab drawer for chips, cracks, or chemical residues?

Glassware should be inspected at all of these times to prevent accidents, injuries, or unwanted complications during experimental work.

For which of the following graphs will a linear fit provide the value of R, the ideal gas constant, in standard units?

Graph IV

For which of the following should gloves be used in the laboratory?

Handling glassware, even without any reagents present., When using reagents, even nonhazardous materials., When cleaning up the bench, after the experiment is over.

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? I. 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. II. Both unknowns have an end point at a volume of ~14.5 mL, which means they must have very similar pKas. III. Both unknowns have an end point at a volume of ~14.5 mL, which means they must have similar molar mass. IV. Unknown 1 appears to be a monoprotic acid, while Unknown 2 is a diprotic acid. V. Unknown 1 has a larger peak at the end point, so it must have twice as much acid in solution. VI. Unknown 2 shows the peak for the end point at ~28 mL, and a peak for the pKa at ~14.5 mL. Unknown 1 was titrated improperly. Either the pKa should have appeared at ~7.25 mL and was missed due to the large volume of base added, or there is a second peak that should appear ~28 mL as with Unknown 2.

IV

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.

When should you immediately turn off the gas valve for a Bunsen or Meker burner?

If the flame suddenly goes out., If you smell gas in the room., If you must leave the open flame unattended and there is no one else to monitor it.

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.

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.

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

Keep her in the eye wash for at least 15 minutes, and then make sure she goes to student health, just to be sure.

Your lab partner accidentally spills some acid on his wrist and watchband

Let the TA inspect his wrist to see if it is okay., Remove the watch and watchband immediately, and rinse his wrist for at least 15 minutes to be sure all hazardous material has been washed away.

Which of the following is a behavior that can pose a safety risk in the laboratory environment?

Looking away from your work to answer a question from your TA or lab partner., Leaving your lab drawer open while you set up your apparatus and obtain your reagents., Moving rapidly around the lab to be sure to finish the experiment in time., Sitting on lab benches, lab floors, or hallway floors, while waiting for lab to begin or for another student to finish., Staying focused on your own experiment and not being distracted by what nearby groups are doing.

When reading the label on a reagent container, what are the three most important pieces of information?

Name, Hazard warning(s), Concentration

While weighing out a reagent for use in an experiment, a student finds he has leftover reagent. The student should:

Place the excess reagent in the appropriate solid or liquid waste container to prevent contamination of the stock reagent.

In the titration curve shown, the value of the equivalence point(s) are best indicated by: (select all that apply.)

Region 3

What is your BEST resource for understanding the nature of the chemical hazards of materials you work with in lab?

Safety Data Sheets (SDS) that should be consulted before coming to lab each week.

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

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.

Which of the following precautions is important when using a hot plate?

Set up your work space so that wires and cables cannot accidentally make contact with the ceramic surface and melt., Set up your work space with the hot plate in a secure location away from the edge of the bench so that you won't accidentally bump it and spill hot or flammable liquids., Set up the work space so that flammable materials - notebooks, paper towels, other reagents, etc. - are far away from the hot plate to prevent them coming in contact with the hot surface., Remember that the ceramic top of the hot plate, and any glassware heated by the hot plate will look the same when hot as cold, and will not cool down until well after the hot plate has been shut off., Never leave the hot plate turned on an unattended.

Which of the following must be completed as soon as you arrive in lab?

Store your backpack, cell phone, jacket, and all other nonessential items in designated storage areas., Put on your lab coat., Put on your goggles., Tie back long hair.

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.

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 34.4%, what is the concentration of the unknown solution?

The correct answer is: 8.05 x 10-5 M [FeSCN]2+

Based on the calibration curve of a crystal violet Beer's Law experiment shown, which of the following statements is true?

The correct answer is: The spacing of points indicates that the colorimeter is very precise, but at least one solution could have been prepared more accurately., The concentration of a solution with an absorbance of 0.731 would be 2.35 x 10-4 M

When should you be sure to use a sealed container and/or secondary containment in the laboratory?

The correct answer is: When transporting materials from one lab room to another.

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. The material must be stirred to encourage crystallization, so that freezing occurs uniformly throughout the material.

In an experiment to determine the Keq associated with a particular reaction, two different solutions are measured 3 times each on a colorimeter with the following results: Cuvet E1, %T Cuvet E2, %T 48.0 15.3 46.1 14.7 46.0 14.7 sigma 1.12 0.357 mean 46.7 14.9 Which of the following statements correctly assesses the data?

The two cuvets gave data of the same precision, because the value of the CV is the same for each.

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.

Suppose a beaker of solid reagent drops onto the bench and cracks. Which of the following represents the correct disposal:

To the extent possible, solid reagent should be added to the solid waste container and broken glass should be added to the glass waste. Hazardous materials should never be added to the glass waste, and broken glass should only be allowed in the solid waste with the TA's approval.

A primary standard material should be unreactive in air, of high purity, and easily dried.

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.

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.

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.

Which of the following items are not allowed in the lab space during lab, but must be stored in appropriate locations?

backpack, cell phone, Water bottle, Food or snacks

Two trials in a kinetics experiment give relative initial rate values of 0.997, and 0.487. 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 = 399 M-1s-1