Chem 102L

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. The gas constant is 0.08206 (L•atm)/(mol•K). Based on the best fit line, the number of moles of air in the container would be: Graph of temp v presssure - y=0.0057x-0.9547 Select one: A. 0.0017 moles B. 0.019 moles C. 0.13 moles D. 19 moles

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) Select one: A. 0.3375 M B. The concentration cannot be determined from the information provided. C. 0.2108 M D. 0.007165 M E. 0.04131 M

0.04131 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? Select one: A. [B] vs time B. 1/[A] vs time C. ln [B] vs time D. 1/[B] vs time

1/[B] vs time

Graph of Concentration of Fe(NCS) [M} vs Absorance at 460: y=3958x-0.01151 The literature value for the molar absorptivity coefficient at 460 nm is 4680 M-1cm-1. What is the percent error in the measured molar absorptivity coefficient? Select one: A. 15.43 % B. 18.24 % C. Cannot be determined because the data were acquired at a different wavelength than the literature value.

15.43 %

The equation of the best fit line for a Beer's Law calibration curve of [Fe(NCS)]2+ was determined to be A=(4328)C + 0.115, where A is the absorbance of the solution and C is its concentration. If the absorbance of an equilibrium solution was measured at 0.264, what is the concentration of [Fe(NCS)]2+? Select one: A. 8.76 x 10-5 M [FeSCN]2+ B. 1.11 x 10-5 M C. 7.06 x 102 M D. The answer cannot be calculated from the information given. E. 3.44 x 10-5 M

3.44 x 10-5 M

The equilibrium concentrations of Fe3+, SCN-, and [Fe(NCS)]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- —► [Fe(NCS)]2+. Assuming there was no [Fe(NCS)]2+ at the beginning of the reaction, what was the initial concentration of SCN-? Select one: A. 3.46 x 10-4 M B. 1.49 x 10-3 M C. Initial concentrations of Fe3+ and SCN- can't be calculated until the equilibrium constant is known. D. 146 M

3.46 x 10-4 M

Two trials in a kinetics experiment give relative initial rate values of 0.997 M/s, and 0.262 M/s. 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 M 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. Select one: A. 2.51 x 10-5 M-2s-1 B. 3.99 x 104 M-2s-1 C. 2.51 x 10-3 M-1s-1 D. 399 M-1s-1

3.99 x 104 M-2s-1

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. Select one: A. 0.340 g/mol B. 315 g/mol C. 339 g/mol D. None of these answers is correct. E. 96.8 g/mol

339 g/mol

Before an unknown concentration of the [Fe(NCS)]2+ can be determined for the equilibrium experiment (experiment 15.2), a Beer's law calibration curve was constructed using solutions with known concentrations of [Fe(NCS)]2+. If done correctly, a fit of calibration curve data should provide: Select one: A. A straight line with a negative slope, in which the slope represents the rate at which the red product is decreasing in solution and the intercept represents the initial concentration. B. A straight line with a positive slope, in which the slope represents the product of the molar absorptivity coefficient and the path length of the cuvette and the intercept represents absorbances or interference that are not due to the red product species . C. A polynomial curve in which the terms represent the molar absorptivity, the path length of the cuvette, and the wavelength of the absorbance. D. A straight line whose slope represents the change in energy of the electron as it emits red light.

A straight line with a positive slope, in which the slope represents the product of the molar absorptivity coefficient and the path length of the cuvette and the intercept represents absorbances or interference that are not due to the red product species .

It is often advised that you check the tire pressure on your car in the late fall to winter when the temperatures fall. Why is this? Which gas law governs this observation? Select one: a. As the temperature drops, the gas pressure in the tire increases. Running on over-inflated tires can damage them. The tire pressure is governed by Gay-Lussac's Law. b. As the temperature drops, the gas pressure in the tire decreases. Running on low tire pressure can damage the tire. The tire pressure is governed by Boyle's Law. c. As the temperature drops, the volume of the tire decreases. Running on misshapen tires can damage them. The tire pressure is governed by Boyle's Law. d. As the temperature drops, the gas pressure in the tire decreases. Running on low tire pressure can damage the tire. The tire pressure is governed by Gay-Lussac's Law.

As the temperature drops, the gas pressure in the tire decreases. Running on low tire pressure can damage the tire. The tire pressure is governed by Gay-Lussac's Law.

Which of the following statements is true in regards to a chemical system at equilibrium and the equilibrium constant, Keq? Select one: a. At equilibrium, all of the limiting reactant has been consumed. b. The Keq changes as the initial concentrations of the reactants change. c. Equilibrium is a static process and no chemical reactions occur once equilibrium has been achieved. d. At equilibrium, the ratio of the concentrations of the reactant(s) and products(s) is constant.

At equilibrium, the ratio of the concentrations of the reactant(s) and products(s) is constant.

Which of the following statements accurately describes the relationship between the enthalpy of vaporization (ΔHvap) and the initial rate of evaporation? Select one: A. Molecules with a high enthalpy of vaporization have a fast initial evaporation rate. B. Molecules with a high enthalpy of vaporization have a slow initial evaporation rate. C. There is no relationship between the enthalpy of vaporization and evaporation rate.

Molecules with a high enthalpy of vaporization have a slow initial evaporation rate.

Which answer choice below correctly matches the chemical to its role in this experiment? Select one: a. Potassium hydrogen phthalate (KHP) = Colorimetric Indicator ; Sodium hydroxide (NaOH) = Colorimetric Indicator b. Potassium hydrogen phthalate (KHP) = Primary Standard ; Phenolphthalein = Colorimetric Indicator c. Potassium hydrogen phthalate (KHP) = Colorimetric Indicator ; Phenolphthalein = Primary Standard d. Potassium hydrogen phthalate (KHP) = Primary Standard ; Sodium hydroxide (NaOH) = Colorimetric Indicator

Potassium hydrogen phthalate (KHP) = Primary Standard ; Phenolphthalein = Colorimetric Indicator

Which of the plots listed below would yield a straight line in accordance with Gay-Lussac's Law? Select one: A. Pressure vs Inverse Volume B. Pressure vs Temperature C. Pressure vs Inverse Temperature D. Pressure vs Volume

Pressure vs Temperature

To determine the order of a reaction with respect to one reactant, B, experimentally, a useful experiment would be: Select one: A. 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, 0.04 M, and 0.025 M, respectively. B. Run a reaction with a concentration of [A] = 0.800 M and a concentration of [B] = 0.001 M. Continue the run until [B] is <<0.001 M and [A] ~ 0.790 M. C. Hold [B] at 0.25 M and vary [A] between 0.30, 0.25, and 0.20 M. D. Run a reaction until both [A] and [B] are much less than their initial values.

Run a reaction with a concentration of [A] = 0.800 M and a concentration of [B] = 0.001 M. Continue the run until [B] is <<0.001 M and [A] ~ 0.790 M.

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? 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. 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. 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.

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.

Why was it important to record the wavelength of maximum absorbance (λmax)? Select one: A. The spectrophotometer was calibrated to for the λmax. Thus, measuring at other wavelengths would introduce additional error into the experiment. B. There is no scientific reason to record the λmax. It is good to know for reporting data. C. The molar absorptivity coefficient (ε) is a wavelength-dependent variable. Thus, to determine ε and to use it to determine concentration, all absorbance values must be measured at the same λmax.

The molar absorptivity coefficient (ε) is a wavelength-dependent variable. Thus, to determine ε and to use it to determine concentration, all absorbance values must be measured at the same λmax.

A cooling curve to determine freezing point is shown in the figure. The behavior in the graph can be explained in the following way: A. 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. B. None of these explanations is correct. C. 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. D. 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.

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 DOES NOT signal an end point or equivalence point to the titration? Select one: a. When the solution turns a faint shade of pink during the colorimetric titration. b. When the pH of the titration reached 11. c. At the steepest point, or inflection point, on the pH vs volume of titrant plot. d. When there is a peak in the ΔpH/ΔV vs volume of titrant plot.

When the pH of the titration reached 11.

Based on the results of the experiment 12, which is the strongest intermolecular force?

hydrogen bonding

hydrogen bonding _______ the relative strength of IMF

increases

In exp. 12: vaporization &IMF, how were you able to determine which compound has molecules escaping into the gas phase most rapidly on a time v temp graph

steepest slope= fastest

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? Select one: 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 and the inner test tube needs to be insulated. C. The test tube holding the reagent must be insulated to minimize temperature gradients within the material and the molten fatty acids need to be stirred to ensure the temperature is uniform and encourage crystallization D. None of the above.

the test tube holding the reagent must be insulated to minimize temperature gradients within the material and the molten fatty acids need to be stirred to ensure the temperature is uniform and encourage crystallization

In the colorimetric titration, the end point of the titration is signaled by the change in color of the indicator in solution and marks the stopping point in the titration. The equivalence point occurs ar the exact volume of titrant at which the number of moles of acid (KHP) is equal to the number of moles of base (NaOH). Select one: True False

true

To ensure that the concentration of [Fe(NCS)]2+ is known when forming your Beer's Law calibration curve, you added a large excess of Fe(NO3)3 • 9H2O relative to NaSCN. In accordance with Le Chatelier's principle, this forced the reaction to favor the formation of products which essentially converted all of the NaSCN to the red product [Fe(NCS)]2+. True or False

true