Midterm II Practice Version
Midterm II Practice Version
Information you may (or may not) find useful
Constants constant Planck’s constant (h) Rydberg constant (RH ) speed of light Avogadro’s number gas constant (R) gas constant (R) electron charge (e) electron mass Proton mass Neutron mass
value 6.626 × 10−34 2.178 × 10−18 2.997924×108 6.022 × 1023 8.31451 0.08206 1.6022 × 10−19 9.1094 × 10−31 1.67252 × 10−27 1.67495 × 10−27
1
unit J ·s J m/s J/K · mol L· atm/K · mol C kg kg kg
1
Molarity and molality
In the following questions, we consider several aqueous solutions. The density of pure water is 1.00 kg/L. a You added 5.0 g of acetone (C3 H6 O) to 2.00 L of water. How many grams of acetone do you have to add in addition to make a 0.100 m aqueous solution of acetone?
Answer: b The density of a 1.80 M aqueous MgCl2 solution is 1.055 g/mL. Calculate the molality of the solution.
Answer: c An amount of 0.100 L of methanol (CH4 O, density is 0.792 g/mL) is added to 0.900 L of pure water. Calculate the molality and molarity of methanol after mixing.
molality: molarity:
2
2
Solubility a Given are formaldehyde and ethylene, two small and neutral molecules. Which compound shows a higher solubility in water and why?
Answer: b A lot of ionic compounds dissolve well in water, forming strong electrolyte solutions. However, some ionic compounds are insoluble in water. Explain in terms of intermolecular forces why some ionic compounds are unable to form strong electrolyte solutions.
Answer: c The solubility of KNO3 increases with temperature. An aqueous solution of KNO3 is prepared at 60o C. Enough KNO3 (s) is added up to the point that the maximum amount of KNO3 (aq) is dissolved and the solution is fully saturated. We then lower the temperature to 20o C. Describes what happens. Explain the process in terms of intermolecular forces and the kinetic energy properties of the molecules.
Answer:
3
3
Vapor pressure
The vapor pressure of water at 20 o C is 17.54 mmHg. The density of water is 1.00 kg/L. a Calculate the vapor pressure in mmHg of an aqueous solution composed of 300. g sorbitol (C6 H14 O6 , nonvolatile) and 1.00 L water.
Answer: b Use Raoult’s law to calculate the total vapor pressure of a mixture of 200. g of propanol (C3 H8 O) and 200. g of water. Propanol is a volatile liquid with a vapor pressure of 14.93 mmHg and a density of 0.8034 g/mL.
Answer: c The interactions between water molecules is stronger than between water and propanol molecules. Do you expect the total vapor pressure of the solution to be more or less than the pressure predicted by Raoult’s law for volatile mixtures? Explain why.
Answer:
4
4
Colligative properties a The freezing temperature of butanol is 25.5 o C and the density is 0.810 g/cm3 . Given that 100. mL of acetone (C3 H6 O, density is 0.791 g/cm3 ) is added to 2.00 L of butanol (Kf =9.12 o C/m), what is the new freezing point of the solution?
Answer: b Given are a 1.00 M aqueous solution of Na2 SO4 and a 1.00 M solution of propylene glycol (C3 H8 O2 ). Which solution gives rise to a higher osmotic pressure?
Answer: c A tank contains an aqueous 0.1 M KCl solution. A tube is inserted in the tank that has a wall which is semi-permeable to all neutral compounds and blocks all charged compounds. When a 1.0 × 10−2 M aqueous solution of KCl is constantly flowing through the tube, will water level in the tank rise or fall?
Answer:
5
5
Entropy and free energy a Use the table of standard free energies of formation below to calculate ∆Go in kJ/mol of the following reaction:
2C2 H6 (g) + 7O2 (g) → 4CO2 (g) + 6H2 O(l) Substance H2 O(l) CO2 (g) C2 H6 (g)
∆Gof (kJ/mol) -237.2 -394.4 -32.89
∆Hfo (kJ/mol) -285.8 -393.5 -84.7
Answer: b Is the reaction spontaneous (yes/no)?
Answer: c A given chemical reaction has a ∆Ssys < 0. Under what conditions is this process spontaneous?
Answer:
6
7
Information you may (or may not) find useful
Constants constant Planck’s constant (h) Rydberg constant (RH ) speed of light Avogadro’s number gas constant (R) gas constant (R) electron charge (e) electron mass Proton mass Neutron mass
value 6.626 × 10−34 2.178 × 10−18 2.997924×108 6.022 × 1023 8.31451 0.08206 1.6022 × 10−19 9.1094 × 10−31 1.67252 × 10−27 1.67495 × 10−27
1
unit J ·s J m/s J/K · mol L· atm/K · mol C kg kg kg
1
Molarity and molality
In the following questions, we consider several aqueous solutions. The density of pure water is 1.00 kg/L. a You added 5.0 g of acetone (C3 H6 O) to 2.00 L of water. How many grams of acetone do you have to add in addition to make a 0.100 m aqueous solution of acetone?
Answer: b The density of a 1.80 M aqueous MgCl2 solution is 1.055 g/mL. Calculate the molality of the solution.
Answer: c An amount of 0.100 L of methanol (CH4 O, density is 0.792 g/mL) is added to 0.900 L of pure water. Calculate the molality and molarity of methanol after mixing.
molality: molarity:
2
2
Solubility a Given are formaldehyde and ethylene, two small and neutral molecules. Which compound shows a higher solubility in water and why?
Answer: b A lot of ionic compounds dissolve well in water, forming strong electrolyte solutions. However, some ionic compounds are insoluble in water. Explain in terms of intermolecular forces why some ionic compounds are unable to form strong electrolyte solutions.
Answer: c The solubility of KNO3 increases with temperature. An aqueous solution of KNO3 is prepared at 60o C. Enough KNO3 (s) is added up to the point that the maximum amount of KNO3 (aq) is dissolved and the solution is fully saturated. We then lower the temperature to 20o C. Describes what happens. Explain the process in terms of intermolecular forces and the kinetic energy properties of the molecules.
Answer:
3
3
Vapor pressure
The vapor pressure of water at 20 o C is 17.54 mmHg. The density of water is 1.00 kg/L. a Calculate the vapor pressure in mmHg of an aqueous solution composed of 300. g sorbitol (C6 H14 O6 , nonvolatile) and 1.00 L water.
Answer: b Use Raoult’s law to calculate the total vapor pressure of a mixture of 200. g of propanol (C3 H8 O) and 200. g of water. Propanol is a volatile liquid with a vapor pressure of 14.93 mmHg and a density of 0.8034 g/mL.
Answer: c The interactions between water molecules is stronger than between water and propanol molecules. Do you expect the total vapor pressure of the solution to be more or less than the pressure predicted by Raoult’s law for volatile mixtures? Explain why.
Answer:
4
4
Colligative properties a The freezing temperature of butanol is 25.5 o C and the density is 0.810 g/cm3 . Given that 100. mL of acetone (C3 H6 O, density is 0.791 g/cm3 ) is added to 2.00 L of butanol (Kf =9.12 o C/m), what is the new freezing point of the solution?
Answer: b Given are a 1.00 M aqueous solution of Na2 SO4 and a 1.00 M solution of propylene glycol (C3 H8 O2 ). Which solution gives rise to a higher osmotic pressure?
Answer: c A tank contains an aqueous 0.1 M KCl solution. A tube is inserted in the tank that has a wall which is semi-permeable to all neutral compounds and blocks all charged compounds. When a 1.0 × 10−2 M aqueous solution of KCl is constantly flowing through the tube, will water level in the tank rise or fall?
Answer:
5
5
Entropy and free energy a Use the table of standard free energies of formation below to calculate ∆Go in kJ/mol of the following reaction:
2C2 H6 (g) + 7O2 (g) → 4CO2 (g) + 6H2 O(l) Substance H2 O(l) CO2 (g) C2 H6 (g)
∆Gof (kJ/mol) -237.2 -394.4 -32.89
∆Hfo (kJ/mol) -285.8 -393.5 -84.7
Answer: b Is the reaction spontaneous (yes/no)?
Answer: c A given chemical reaction has a ∆Ssys < 0. Under what conditions is this process spontaneous?
Answer:
6
7
