REVIEW FOR MIDYEAR EXAM

*Open ended questions Thursday, January 7

- Long Free Response — 3 Questions

- Short Free Response — 3 Questions (actually 4)

*Multiple Choice questions Friday, January 8

- 60 Questions

Problem #1

The diagram shows the first ionization energies for the elements Li to Ne. Briefly, explain each

of the following in terms of atomic structure.

a) In general, there is an increase in the first ionization energy from Li to Ne.

b) The first ionization energy of B is lower than that of Be.

c) The first ionization energy of O is lower than that of N.

d) Predict how the first ionization energy of Na compares to those of Li and of Ne. Explai

1a) Across the period from Li to Ne the number of protons is increasing in the nucleus hence

the nuclear charge is increasing with a consequently stronger attraction for electrons and an

increase in I.E.

1b) The e- ionized in the case of Be is a 2s electron whereas in the case of B it is a 2p electron.

2p electrons are higher in energy than 2s electrons because 2p electrons penetrate the core to

a lesser degree

1c) The e- ionized in O is paired with another electron in the same orbital, whereas in N the

electron comes from a singly—occupied orbital. The ionization energy of the O electron is less

because of the repulsion

1d) The ionization energy of Na will be less than those of both Li and Ne because the electron

removed comes from an orbital which is farther from the nucleus, therefore less tightly hel

Problem #2

An unknown compound contains only the three elements C,H, and O. A pure sample of the

compound is analyzed and found to be 65.60 percent C and 9.44 percent H by mass.

(a) Determine the empirical formula of the compound.

(b) When 1.570 grams of the compound is vaporized at 300 °C and 1.00 atmosphere, the gas

occupies a volume of 577 milliliters. What is the molar mass of the compound based on

this result? And what is the molecular formula

(c) Write the balanced equation for the combustion of this unknown substance.

(d) If 25.0 g of this unknown is combusted with 25.0 L of oxygen at STP. How many molecules

of gaseous water will be produced?

Problem #3

Elemental analysis of an unknown pure substance indicated that the percent composition by mass

is as follows.

Element Percent by Mass

Carbon 49.02%

Hydrogen 2.743%

Chlorine 48.23%

(a) Determine the empirical formula of the unknown substance.

(b) If the molar mass of the compound is 294 g/mol, what is the molecular formula of the

unknown substance?

Problem #4

Propane, C3H8, is a hydrocarbon that is commonly used as fuel for cooking.

(a) Write a balanced equation for the complete combustion of propane gas, which yields CO2(g)

and H2O(l).

(b) Calculate the volume of air at 30°C and 1.00 atmosphere that is needed to burn completely

10.0 grams of propane. Assume that air is 21.0 percent O2 by volume.

(c) The enthalpy of combustion of propane is –2,220.1 kJ/mol. Calculate the enthalpy of

formation, Hof for propane given that Ho

f H2O(l) = –285.3 kJ/mol and Hof of

CO2(g) = –393.5 kJ/mol.

(d) Assuming that all of the heat evolved in burning 30.0 grams of propane is transferred to 8.00

kilograms of water (specific heat = 4.18 J/g. K), calculate the increase in temperature of

water.

Problem #5

A 5.00 gram sample of a dry mixture of potassium hydroxide, potassium carbonate, and

potassium chloride is reacted with 0.100 liter of 2.00 molar HCl solution

(a) A 249 milliliter sample of dry CO2 gas, measured at 22°C and 740 torr, is obtained from

this reaction. What is the percentage of potassium carbonate in the mixture?

(b) The excess HCl is found by titration to be chemically equivalent to 86.6 milliliters of

1.50 molar NaOH. Calculate the percentages of potassium hydroxide and of potassium

chloride in the original mixture.

Problem # 6

An experiment is performed to determine the empirical formula of a copper iodide formed by

direct combination of elements. A clean strip of copper metal is weighed accurately. It is

suspended in a test tube containing iodine vapor generated by heating solid iodine. A white

compound forms on the strip of copper, coating it uniformly. The strip with the adhering

compound is weighed. Finally, the compound is washed completely from the surface of the metal

and the clean strip is dried and reweighed

mass of clean copper strip 1.2789 g

mass of copper strip and compound 1.2874 g

mass of copper strip after washing 1.2748 g

(a) State how you would use the data above to determine each of the following.

(Calculations not required.)

1. The number of moles of iodine that reacted.

I would subtract the mass of the clean copper strip from mass of the copper strip and compound (1.2874g - 1.2789g), then divide this answer by the gram atomic mass of I, 126.91g.

2. The number of moles of copper that reacted.

I would subtract the mass of the copper strip after washing from the mass of the clean copper strip (1.2789g - 1.2748g). Then I would divide this answer by the molar mass of copper, 63.55g.

(b) Explain how you would determine the empirical formula of the copper iodide.

I would find the least whole number ratio of moles of Cu and I by dividing the larger of the two by the smaller and finding the whole number ratio equal to that ratio.

(c) Explain how each of the following would affect the empirical formula that would be

calculated.

1. Some unreacted iodine condensed on the strip.

If some unreacted iodine vapor condensed on the strip then the mass and moles of I calculated would be too high thereby resulting in an empirical formula with too much I in it.

2. A small amount of the white compound flaked off before weighing.

If some of the white copper iodide compound

flaked off before weighing then the mass and moles of

I calculated would be too small resulting in an

empirical formula with too little I in it.

Problem #7

An experiment is to be performed to determine the mass percent of sulfate in an unknown

soluble sulfate salt. The equiptment available is: a scale, a funnel, a grad cylinder, a ringstand, a

beaker, a stirring rod .20 M BaCl2, Unknown sulfate salt, and a drying oven.

(a) Explain a procedure to carry this out.

Mix unknown and BaCl2 as reactants

Collect precipitate/set up filtration

(b) What experimental data needs to be collected to calculate the mass percent of sulfate in the

unknown

Mass of unknown salt as reactant

(sulfate = “salt” = unknown salt, unless otherwise

specified)

Mass BaSO4 (must be specified) as dried

precipitate/product

Note: “dried” must appear to earn all 4 points for A

and B

(c) List the calculations necessary to determine the mass percent of sulfate in the unknown

Mass BaSO4 moles SO4-2 mass SO4

-2

Then take mass SO4-2/mass unknown

Notes: A list alone is acceptable. Method, if correct,

is acceptable as list. Response must clearly

distinguish between SO4-2, BaSO4, and unknown

sulfate. Only one of two points earned is mass of

SO4-2 incorrect but fraction for percent clearly

indicates part (of original salt)/whole (of original

salt)

(d) Would .2 M MgCl2 be an acceptable substitute for the BaCl2 solution provided for this

experiment? Explain.

MgCl2 is NOT an acceptable substitute for BaCl2

MgCl2 is to soluble

Problem # 8

Describe a laboratory procedure needed to carry out each of the following:

(a) Separate a mixture of powdered CaCl2 from CaCO3.

Add water to mixture

CaCO3 does not dissolve in water whereas CaCl2 does

Filter the solution. The aqueous CaCl2 solution passes

through the filter paper and the CaCO3 is collected on

the paper

(b) Determine the concentration of solute in an aqueous solution of sodium chloride (give the

concentration units your method provides).

Pipet an aliquot of known volume into a flask

Add excess AgNO3 to precipitate AgCl

Filter, dry and weigh AgCl

wt AgCl/MW of AgCl=# moles AgCl = # moles NaCl

M = moles AgCl (or NaCl)/L of aliquot

Or

Take a known volume of solution. Evaporate solution

to dryness and weigh the NaCl residue

wt NaCl/MW of NaCl = # moles NaCl

M = moles NaCl/L solution

Full credit for this problem could also be achieved by using a weight measurement of the

solution (and obtaining the weight of the solvent), evaporating the solvent, weighing the

residue and calculating the molality. Through the proper application of colligative properties, it

is possible to obtain full credit

(c) Separate a mixture of two volatile liquids.

Fractional distillation

(ordinary distillation receives 1 pt)

Problem # 9

Consider five unlabeled bottles, each containing 5.0 g of one of the following pure salts:

AgCl BaCl2 CoCl2 NaCl NH4Cl

For all of the following questions, describe the observation that supports your identification.

(a) Identify the salt that can be distinguished by appearance alone.

CoCl2 is blue or red depending on whether water is present.

(b) Identify the salt that can be distinguished by adding 10 mL of water to a small sample of

each of the remaining unidentified salts.

Of the remaining compounds AgCl is insoluble in water

(c) Identify the salt that can be distinguished by adding 1.0 M Na2SO4 to a small sample of

each of the remaining unidentified salts.

When BaCl2 is added to 1.0 M Na2SO4 a white

precipitate of BaSO4 is formed.

BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq)

(d) Identify the salt that can be distinguished by adding 1.0 M NaOH to a small sample of each

of the remaining unidentified salts.

NH4Cl reacts with NaOH to release NH3 which has a distinct odor.

Problem #10

Represented above are five identical balloons, each filled to the same volume at 25 degrees

Celsius and 1.0 atmosphere pressure with the pure gases indicated.

(a) Which balloon contains the greatest mass of gas? explain

CO2

Because all the balloons contain the same number of

molecules (moles) and CO2 molecules are the

heaviest.

(b) Compare the average kinetic energies of the gas molecules in the balloons. explain.

All are equal

Because same temperature, therefore same average kinetic energy

Note: restatement of “same conditions” does not earn

(c) Which balloon contains the gas that would be expected to deviate most from the behavior of

an ideal gas? explain.

CO2

Either:

It has more e-, hence it is most polarizable

It has the strongest intermolecular (London) forces

Note: -also allowable are “polar bonds” inelastic

collisions”-claiming larger size or larger volume does not earn 2 points

(d) 12 hours after being filled, all the balloons have decreased in size. Predict which balloon will

be the smallest

He

-greatest movement through balloon wall

-smallest size

-greatest molecular speed

-most rapid effusion (Graham’s Law)

CO2 O2 He N2 CH4

Problem # 11

Observations about real gases can be explained at the molecular level according to the kinetic

molecular theory of gases and ideas about intermolecular forces. Explain how each of the

following observations can be interpreted according to these concepts, including how the

observation supports the correctness of these theories.

(a) When a gas-filled balloon is cooled, it shrinks in volume; this occurs no matter what gas is

originally placed in the balloon.

(b) When the balloon described in (a) is cooled further, the volume does not become zero; rather,

the gas becomes a liquid or solid.

(c) When NH3 gas is introduced at one end of a long tube while HCl gas is introduced

simultaneously at the other end, a ring of white ammonium chloride is observed to form

in the tube after a few minutes. This ring is closer to the HCl end of the tube than the

NH3 end.

-2 pts

Reducing temperature of a gas reduces the average kinetic energy (or velocity) of the

gas molecules. This would reduce the number (or frequency) of collisions of gas

molecules with the surface of the balloon (or decrease the momentum change that

occurs when the gas molecules strike the balloon surface). In order to

maintain a constant pressure vs the external pressure, the volume must decrease.

-2 pts

The molecules of the gas do have volume (1 pt for this) when they are cooled

sufficiently, the forces of attraction that exist between them cause them to liquify or

solidify (2nd pt)

-2 pts

The molecules of gas are in constant motion, so the HCl and NH3 diffuse along the tube.

Where they meet NH4Cl is formed. Since HCl has a higher molar mass, its velocity (ave)

is lower. Therefore, it doesn’t diffuse as fast as NH3.

Problem # 12

A student collected a sample of hydrogen gas by the displacement of water as shown by the

diagram above. The relevant data are given in the following table.

(a) Calculate the number of moles of hydrogen gas collected.

(b) Calculate the number of molecules of water vapor in the sample of gas.

(c) Calculate the ratio of the average speed of the hydrogen molecules to the average speed of the

water vapor molecules in the sample.

(d) Which of the two gases, H2 or H2O, deviates more from ideal behavior? Explain your answer.