All Homework

A student adds 50.0g of liquid water at 25.0°C to an insulated container fitted with a temperature probe. The student then adds 10.0g of ice at 0.0°C, closes the container, and measures the temperature at different intervals. Part of the data is shown in the graph above. The student predicts that the temperature will continue to decrease then level out to a constant temperature. Which of the following best explains why the student's prediction is correct? A The H2OH2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy. B The transfer of energy between the H2OH2O molecules in the ice and liquid water stops once all the molecules are in the liquid phase. C Once all of the H2OH2O molecules are in the liquid phase, the individual molecular speeds either increase or decrease until all the particles have the same speed. D Once all of the H2OH2O molecules are in the liquid phase, collisions between them virtually stop as they reach an equilibrium distance from their neighboring molecules.

A

AgNO3(aq) + NaC1(aq)→ AgC1(s) + NaNO3(aq)AgNO3(aq) + NaC1(aq)→ AgC1(s) + NaNO3(aq) In an experiment a student mixes a 50.0 mL sample of 0.100 M AgNO3(aq) with a 50.0 mL sample of 0.100 M NaCl(aq) at 20.0°C in a coffee-cup calorimeter. Which of the following is the enthalpy change of the precipitation reaction represented above if the final temperature of the mixture is 21.0°C? (Assume that the total mass of the mixture is 100. g and that the specific heat capacity of the mixture is 4.2 J/(g °C).) A −84 kJ/molrxn B −0.42 kJ/molrxn C 0.42 kJ/molrxn D 84 kJ/molrxn

A

Mass of water-50.003g Temperature of water24.95° CSpecific heat capacity for water4.184J/g°C Mass of metal63.546g Temperature of metal99.95°C Specific heat capacity for metal? Final temperature32.80°C In an experiment to determine the specific heat of a metal, a student transferred a sample of the metal that was heated in boiling water into room-temperature water in an insulated cup. The student recorded the temperature of the water after thermal equilibrium was reached. The data are shown in the table above. Based on the data, what is the calculated heat q absorbed by the water reported with the appropriate number of significant figures? A 1600J B 1640J C 1642J D 1642.3J

A

The student made the standard curve above. Which of the following most likely caused the error in the point the student plotted at 0.050MCo2+(aq) ? A) There was distilled water in the cuvette when the student put the standard solution in it. B There were a few drops of the 0.100MCo2+(aq)0.100MCo2+(aq) standard solution in the cuvette when the student put the 0.050 M0.050 ⁢M standard solution in it. C The student used a cuvette with a longer path length than the cuvette used for the other standard solutions. D The student did not run a blank between the 0.050MCo2+(aq)0.050MCo2+(aq) solution and the one before it.

A

Which statement correctly compares what occurs when molecules absorb photons in the microwave region with what occurs when molecules absorb photons in the infrared region? A Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause the molecules to increase their vibrational energy states. B Microwave photons cause electrons in the molecules to increase their electronic energy states, whereas infrared photons cause the molecules to increase their rotational energy states. C Microwave photons cause the molecules to increase their vibrational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states. D Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

A

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH°=−57.1kJ/molrxn The chemical equation above represents the reaction between HCl(aq) and NaOH(aq). When equal volumes of 1.00MHCl(aq) and 1.00MNaOH(aq) are mixed, 57.1kJ of heat is released. If the experiment is repeated with 2.00MHCl(aq), how much heat would be released? A 28.6kJ B 57.1kJ C 85.7kJ D 114kJ

B

In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C. When the forecast calls for air temperatures to be below −5°C for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes. Which of the following is a correct scientific justification for spraying water on the blossoms to protect them from temperatures below −2°C? A Water on the blossoms will not freeze unless the air temperature falls significantly below −5°C−5°C. B Water is a good thermal conductor that transfers heat from the cold air to the blossoms, keeping the blossoms from going below −2°C−2°C. C The freezing of water is an endothermic process; thus, water that freezes on the blossoms absorbs heat from the atmosphere, which in turn keeps the blossoms above 0°C0°C. D The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C−2°C.

D

MgCl2(aq)+2NaOH(aq)→2NaCl(aq)+Mg(OH)2(s) A 100mL sample of 0.1MMgCl2(aq) and a 100mL sample of 0.2MNaOH(aq) were combined, and Mg(OH)2(s) precipitated, as shown by the equation above. If the experiment is repeated using solutions of the same molarity, which of the following changes in volume will double the amount of Mg(OH)2(s) produced? A Using the same volume of MgCl2(aq)MgCl2(aq) but twice the volume of NaOH(aq)NaOH(aq) B Using twice the volume of MgCl2(aq)MgCl2(aq) but half the volume of NaOH(aq)NaOH(aq) C Using twice the volume of MgCl2(aq)MgCl2(aq) but the same volume of NaOH(aq)NaOH(aq) D Using twice the volume of MgCl2(aq)MgCl2(aq) and twice the volume of NaOH(aq)

D

Which of the following phase changes involves the transfer of heat from the surroundings to the system? CH4(g)→CH4(l)CH4(g)→CH4(l), because CH4CH4 molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the solid state. B CO2(g)→CO2(s)CO2(g)→CO2(s), because CO2CO2 molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the liquid state. C H2O(l)→H2O(s)H2O(l)→H2O(s), because H2OH2O molecules in the liquid phase must absorb energy in order to create a crystalline structure with strong intermolecular attractions in the solid state. D NH3(l)→NH3(g)NH3(l)→NH3(g), because NH3NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.

D

Infrared spectroscopy is a useful tool for scientists who want to investigate the structure of certain molecules. Which of the following best explains what can occur as the result of a molecule absorbing a photon of infrared radiation? A The energies of infrared photons are in the same range as the energies associated with changes between different electronic energy states in atoms and molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of electronic transitions within the molecules. B The energies of infrared photons are in the same range as the energies associated with different vibrational states of chemical bonds. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the types and strengths of different bonds in the molecules. C The energies of infrared photons are in the same range as the energies associated with different rotational states of molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of transition between different rotational energy states of the molecules. D The energies of infrared photons are in the same range as the total bond energies of bonds within molecules. Chemical bonds can be completely broken as they absorb infrared photons of characteristic wavelengths, thus revealing the energies of the bonds within the molecules.

B

Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) The chemical equation shown above represents the reaction between Mg(s) and HCl(aq). When 12.15g of Mg(s) is added to 500.0mL of 4.0MHCl(aq), 95kJ of heat is released. The experiment is repeated with 24.30g of Mg(s) and 500.0mL of 4.0MHCl(aq). Which of the following gives the correct value for the amount of heat released by the reaction? A 380kJ B 190kJ C 95kJ D 48kJ

B

Q29 Moles Al2O3 yielded from 0.40mol Al and 0.40mol O2 5.F 0/1 MC point What is the maximum number of moles of Al2O3 that can be produced by the reaction of 0.40 mol of Al with 0.40 mol of O2 ? A 0.10 mol B 0.20 mol C 0.27 mol D 0.33 mol E 0.40 mol

B

2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)ΔH°=−1120kJ/molrxn Based on the reaction represented by the chemical equation shown above, what is the amount of heat released when 4.00mol of H2S(g) reacts with 9.00mol of O2(g)? A −560kJ B −1120kJ C −2240kJ D −3360kJ

C

What is the empirical formula of an oxide of chromium that is 48 percent oxygen by mass? A CrO B CrO2 C CrO3 D Cr2O E Cr2O3

C

2 NO2(g) ⇄ N2O4(g) dark brown colorless The dimerization of NO2(g) , an exothermic process, is represented by the equation above. Which of the following experimental techniques will allow the most accurate determination of the concentration of NO2(g) at equilibrium? A Paper chromatography B Gravimetric analysis C Titration D Spectrophotometry

D

A piece of Fe(s) at 25°C is placed into H2O(l) at 75°C in an insulated container. A student predicts that when thermal equilibrium is reached, the Fe atoms, being more massive than the H2O molecules, will have a higher average kinetic energy than the H2O molecules. Which of the following best explains why the student's prediction is incorrect? A At thermal equilibrium, the less massive H2OH2O molecules would have a higher average kinetic energy than the FeFe atoms because they are more free to move than are the FeFe atoms. B At thermal equilibrium, the collisions between the FeFe atoms and the H2OH2O molecules would cease because the average kinetic energies of their particles would have become the same. C At thermal equilibrium, the movement of both the FeFe atoms and the H2OH2O molecules would cease; thus, the average kinetic energy of their particles would have to be the same. D At thermal equilibrium, the average kinetic energy of the FeFe atoms cannot be greater than that of the H2OH2O molecules; the average kinetic energies must be the same according to the definition of thermal equilibrium.

D

A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration. A wavelength of 510nm corresponds to an approximate frequency of 6×1014s−1. What is the approximate energy of one photon of this light? A 9×10^47J B 3×10^17J C 5×10^−7J D 4×10^−19J

D

For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students' observation? A An exothermic chemical change occurred because heat flowed from the contents of the beaker to the room. B An exothermic physical change occurred because heat flowed from the contents of the beaker and the water on the board to the room. C An endothermic physical change occurred because the freezing of water is an endothermic process. D An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.

D

For an experiment, 50.0g of H2O was added to a coffee-cup calorimeter, as shown in the diagram above. The initial temperature of the H2O was 22.0°C, and it absorbed 300.J of heat from an object that was carefully placed inside the calorimeter. Assuming no heat is transferred to the surroundings, which of the following was the approximate temperature of the H2O after thermal equilibrium was reached? Assume that the specific heat capacity of H2O is 4.2J/(g⋅K). A 21.3°C B 22.0°C C 22.7°C D 23.4°C

D