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
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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