Chem Lab Quizzes
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:
A. 0.019 moles
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
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+
Based on the Beer's Law plot shown, which of the following statements is true?
A. Two of these answers are correct.
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 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.
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?
B. 0.02546 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)
B. 1.86 x 10-4 M
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 intercept of the regression line?
B. 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.
B. 338.57 g/mol
Consider the uncertainties in your kinetics experiment. Which of the following statements is true?
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.
In the titration curve shown, the value of the pKa(s) are best indicated by: (select all that apply.)
B. Region 2
The reults 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?
B. 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.
A cooling curve to determine freezing point is shown in the figure. The behavior in the graph can be explained in the following way
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.
During a titration of KHP with 0.1 M NaOH, when the 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)
B. pH > 7
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 ____________
B. 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.
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] = 0.25 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.
B. y ~ 2 k =3.99 x 104 M-1s-1
A 1.5 x 10-5 M solution of crystal violet has an absorbance value of 0.942. 1000 microliters of this crystal violet solution is added to a cuvette, along with 2500 microliters of distilled water. The concentration of crystal violet in the cuvette would be (i)_________(choose one answer a-d) and the absorbance of the solution in the cuvette should be approximately (ii)____________ (choose one answer e-h).
C. (i) 4.3 x 10-6 M (ii) A=0.27
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?
C. 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.
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.
C. All of these are cases when you should first remove your gloves.
In the Clausius-Clapeyron plot shown, methanol, CH3OH, is represented by the blue points. If the other two molecules samples shown are n-nonane and 3,3-diethyl pentane, 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. Check all correct answers.
C. 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.
Beer's law is useful for the crystal violet kinetics experiment because:
C. The absorbance of the product in the green region of the spectrum is proportional to its concentration.
An experiment is run to test the general rate law of a reaction, R =k [A]x[B]y, where k=0.25 M-2s-1. The results of the experiment gave the best linear fit to the plot of ln[A] vs. time. Given the units on k and the results obtained, which of the following initial concentrations could give the correct result?
C. [A]0 = 0.005 M; [B]0 = 0.500 M
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.
C. in the sink at the front of the lab, next to the white board
The equilibrium concentrations of Fe3+, SCN-, and [FeSCN]2+ are determined experimentally to be 1.43 x 10-3 M, 2.86 x10-4 M, and 5.96x10-5 M, respectively, for the reaction Fe3+ + SCN- —► [FeSCN]2+. What is the experimental value of the equlibrium constant?
D. 146
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:
D. 37.0 kJ/mol
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:
D. 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
To determine the order of a reaction with respect to one reactant, A, experimentally, a useful experiment would be:
D. Hold [B] at 0.25 M and vary [A] between 0.30, 0.25, and 0.20 M.
The end point in the titration of potassium hydrogen phthalate (KHP) with sodium hydroxide (NaOH) is determined to be the point when: I. A plot of pH vs titrant volume has a slope of zero. II. A plot of pH vs titrant volume has a slope approaching infinity. III. The plot pH vs volume of titrant shows the top of a peak. IV. The plot of the change in pH vs volume of titrant shows the top of a peak. V. The color of a phenolphthalein indicator added to the solution turns bright pink-purple. VI. The color of a phenolphthalein indicator added to the solution turns very pale pink, but fades to colorless ~ 30 seconds later. VII. The color of a phenolphthalein indicator is still colorless, but will turn pink with the next addition of titrant.
D. II, IV, and VI.
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?
D. The R2 value indicates that the experimenter mixed up the stock solution and the solution used to dilute it.
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?
D. 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 standardized base is the most accurate measurement in the experiment.
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 _________________________
D. 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.
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:
E. All but A are valid assessments. (A = The calculation was done incorrectly. The value of the density is not needed to correctly calculate R.)
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?
E. Both A and C are sound assessments. (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. 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.)
For which of the following graphs will a linear fit provide the value of R, the ideal gas constant, in standard units?
E. Graph IV
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?
F. C and D are correct. ( 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.)
The qualities of a primary standard in titration include the following: ____________.
I. A, E, and G only. Reactive in air High purity Easily dried
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.
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_________
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.
