AP Chemistry Unit 1

Which of the following numerical expressions gives the number of moles in 5.0g of CaO? A 5.0g×56g/mol B 5.0g56g/mol C 56g/mol5.0g D 15.0g×156g/mol

B 5.0g56g/mol Dividing the mass of the sample by the molar mass of CaO gives the number of moles of CaO in the sample.

1s2 2s2 2p6 3s2 3p6 How many unpaired electrons are in the atom represented by the electron configuration above? A 0 B 1 C 2 D 3

A 0 The 1s,2s,2p,3s, and 3p orbitals are all full, and all the electrons are paired.

In a lab a student is given a 21g sample of pure Cu metal. Which of the following pieces of information is most useful for determining the number of Cu atoms in the sample? Assume that the pressure and temperature in the lab are 1.0atm and 25°C. A The molar mass of Cu B The density of Cu at 25°C C The volume of the Cu sample D The ratio of the two main isotopes found in pure Cu

A The molar mass of Cu By dividing the mass of the sample by the molar mass of Cu, the number of moles of Cu in the sample can be calculated. A simple multiplication of that number with Avogadro's number will provide the desired information.

A vessel contains a mixture of gases. The mass of each gas used to make the mixture is known. Which of the following information is needed to determine the mole fraction of each gas in the mixture? A The molar mass of each gas B The density of the gases in the vessel C The total pressure of the gases in the vessel D The number of atoms per molecule for each gas

A The molar mass of each gas The mole fraction of each gas is equal to the number of moles of the gas divided by the total number of moles of gases in the mixture. Given the mass of each gas, the molar mass of each gas is needed to determine the number of moles for each gas and the total number of moles in the mixture.

The figure presents a graph of a mass spectrum with five vertical lines. The horizontal axis is labeled Atomic Mass, in a m u, and the numbers 88 through 98, in increments of 2, are indicated. The vertical axis is labeled Percent Abundance, and the numbers 0, 50, and 100 are indicated. The five vertical lines from left to right are as follows. Note all values are approximate. The first line begins on the horizontal axis at 90 a m u, and extends to 52 percent. The second line begins on the horizontal axis at 91 a m u, and extends to 11 percent. The third line begins on the horizontal axis at 92 a m u, and extends to 17 percent. The fourth line begins on the horizontal axis at 94 a m u, and extends to 17 percent. The fifth line begins on the horizontal axis at 96 a m u, and extends to 3 percent. The mass spectrum for an unknown element is shown above. According to the information in the spectrum, the atomic mass of the unknown element is closest to A 90amu B 91amu C 93amu D 94amu

B 91amu By estimation, the weighted average of the isotope masses would be closer to 91 than 90.

Rb reacts with O in a mole ratio of 2 to 1, forming the ionic compound Rb2O. Which of the following elements will react with O in a mole ratio of 2 to 1, forming an ionic compound, and why? A S, because it is in the same group as O. B Cs, because it is in the same group as Rb. C Sr, because it is in the same period as Rb. D Br, because the atomic mass of Br is similar to that of Rb.

B Cs, because it is in the same group as Rb. Cs and Rb have the same number of valence electrons because they are in the same group in the periodic table. Both of these elements can transfer one electron to a nonmetal when forming ionic compounds.

The figure presents a graph of a mass spectrum with six vertical lines. The horizontal axis is labeled Atomic Mass, in a m u, and the numbers 152 through 162, in increments of 1, are indicated. The vertical axis is labeled Relative Abundance, and no values are indicated. The six vertical lines from left to right are as follows. Note all values are approximate. The first line begins on the horizontal axis at 154 a m u, and extends a small way up the vertical axis. The second line begins on the horizontal axis at 155 a m u, and extends halfway up the vertical axis. The third line begins on the horizontal axis at 156 a m u, and extends slightly more than halfway up the vertical axis. The fourth line begins on the horizontal axis at 157 a m u, and extends halfway up the vertical axis, but slightly more than the second line. The fifth line begins on the horizontal axis at 158 a m u, and extends high up the vertical axis. The sixth line begins on the horizontal axis at 160 a m u, and extends high up the vertical axis, but less than the fifth line. The mass spectrum represented above is most consistent with which of the following elements? A Eu B Gd C Tb D Dy

B Gd The average atomic mass of Tb is close to 159amu. The relative abundance and masses of the isotopes shown in the mass spectrum predict an average atomic mass close to 157amu, not 159amu.

Ba2+(aq)+SO42−(aq)→BaSO4(s) A student obtains a 10.0g sample of a white powder labeled as BaCl2. After completely dissolving the powder in 50.0mL of distilled water, the student adds excess Na2SO4(s), which causes a precipitate of BaSO4(s) to form, as represented by the equation above. The student filters the BaSO4(s), rinses it, and dries it until its mass is constant. Which of the following scientific questions could best be answered based on the results of the experiment? A Is the Na2SO4(s) used in the experiment pure? B Is the BaCl2(s) used in the experiment pure? C What is the molar solubility of BaCl2 in water? D What is the molar solubility of BaSO4 in water?

B Is the BaCl2(s) used in the experiment pure? The results cannot be used to answer the question. However, the calculated number of moles of Ba in the precipitate can be compared with the expected number of moles of Ba in 10.0g of BaCl2. The presence of fewer moles of Ba in the precipitate would imply that the BaCl2 was not pure.

Mass Mass Percent of Na Density Color 27.3g 35.2% 2.05g cm−3 White A jar labeled NaCl contains a powder. The table above contains information determined by analyzing a sample of the powder in the laboratory. What information in the table is the most helpful in determining whether the powder is pure NaCl? A Mass B Mass percent of Na C Density D Color

B Mass percent of Na Percent by mass is an intensive property that is determined by the composition of a substance and therefore is helpful for determining the purity of the powder.

The figure presents a photoelectron spectrum. The horizontal axis is labeled Binding Energy, in megajoules per mole, and the numbers 7 through 0, in increments of 1, are indicated from left to right. The vertical axis is labeled Relative Number of Electrons and no values are indicated. The line begins on the vertical axis and moves horizontally to the right. It reaches a peak at 5.5 megajoules per mole which is labeled X. The line moves back down, and then moves horizontally to the right from approximately 5 to 1 megajoules per mole. The line reaches another, shorter, peak at 0.5 megajoules per mole, which is labeled Y. Peak Y is half as tall as peak X. The complete photoelectron spectrum of an element is given above. Which labeled peak corresponds to the 1s electrons and why? A Peak X, because 1s electrons are the easiest to remove from the atom. B Peak X, because 1s electrons have the strongest attractions to the nucleus. C Peak Y, because electrons in the 1s sublevel are the farthest from the nucleus. D Peak Y, because there are fewer electrons in an s sublevel than in a p sublevel.

B Peak X, because 1s electrons have the strongest attractions to the nucleus. Greater binding energies correlate to stronger attractions to the nucleus.

A student obtains a mixture of the chlorides of two unknown metals, X and Z. The percent by mass of X and the percent by mass of Z in the mixture is known. Which of the following additional information is most helpful in calculating the mole percent of XCl(s) and of ZCl(s) in the mixture? A The number of isotopes of Cl B The molar masses of X and Z C The density of either XCl(s) or ZCl(s) D The percent by mass of Cl in the mixture

B The molar masses of X and Z The molar mass (mass/moles) is the quantity that establishes the ratio of the mass of a pure substance in a mixture to its number of moles. This ratio can be used to calculate the mole percent of either XCl or ZCl in the mixture.

The figure presents a photoelectron spectrum. The horizontal axis is labeled Binding Energy, in megajoules per mole, and the numbers 100, 10, 1, and 0.1 are indicated at equal intervals. The vertical axis is labeled Relative Number of Electrons and no values are indicated. The line moves horizontally and to the right along the horizontal axis and has 3 peaks. The data for the 3 peaks are as follows. Note that all values are approximate. Peak 1, 30 megajoules per mole. Peak 2, 2 megajoules per mole. Peak 3, 0.7 megajoules per mole. Peaks 1 and 2 are equal in height, and peak 3 is half as tall. The photoelectron spectrum for the element boron is represented above. Which of the following best explains how the spectrum is consistent with the electron shell model of the atom? A The spectrum shows an odd number electrons. B The spectrum shows a single electron in the 2p subshell. C The spectrum shows equal numbers of electrons in the first and second electron shells. D The spectrum shows three electrons with the same binding energy in the second electron shell.

B The spectrum shows a single electron in the 2p subshell. The spectrum is consistent with the electron configuration for boron: 1s22s22p1. The leftmost peak represents the two electrons in the filled 1s subshell. The two peaks on the right represent the two electrons in the filled 2s subshell and the single electron in the 2p subshell.

The figure presents a photoelectron spectrum. The horizontal axis is labeled Binding Energy, in megajoules per mole, and the numbers 100, 10, 1, and 0.1 are indicated at equal intervals. The vertical axis is labeled Relative Number of Electrons and no values are indicated. The line moves horizontally to the right along the horizontal axis and has 3 peaks. The data of the 3 peaks are as follows. Note that all values are approximate. Peak 1, 50 megajoules per mole. Peak 2, 4 megajoules per mole. Peak 3, 2 megajoules per mole. The three peaks are equal in height. The complete photoelectron spectrum of the element carbon is represented above. Which of the following best explains how the spectrum is consistent with the electron shell model of the atom? A The spectrum shows four electrons in the inner electron shell. B The spectrum shows equal numbers of electrons in the three occupied electron subshells. C The spectrum shows that all the electrons in the valence shell have the same binding energy. D The spectrum shows more electrons in the inner electron shell than in the outer electron shell.

B The spectrum shows equal numbers of electrons in the three occupied electron subshells. The peak on the left represents two 1s electrons, and the two peaks on the right represent two 2s electrons and two 2p electrons, respectively.

A student measures the mass of a sample of a metallic element, M. Then the student heats the sample in air, where it completely reacts to form the compound MO. The student measures the mass of the compound that was formed. Which of the following questions can be answered from the results of the experiment? A What is the density of M? B What is the molar mass of M? C What is the melting point of M? D What is the melting point of MO?

B What is the molar mass of M? The number of moles of O in the compound is equal to the number of moles of M and can be calculated using the difference in the masses of the sample and the compound. The molar mass of M is calculated by dividing the mass of the sample of M by the number of moles of M.

What charge does Al typically have in ionic compounds, and why? A +1, because in the ground state it has one unpaired electron. B +2, because it has two electrons in the 2s subshell. C +3, because it has three valence electrons. D +4, because it is in the fourth row of the periodic table.

C +3, because it has three valence electrons. The valence electrons of metal atoms are the farthest from the nucleus and are the most easily removed.

The figure presents a graph of a mass spectrum with two vertical lines. The horizontal axis is labeled Atomic Mass, in a m u, and the numbers 183 through 189, in increments of 1, are indicated. The vertical axis is labeled Relative Abundance, in percent, and the numbers 0 through 60, in increments of 20, are indicated. The two vertical lines from left to right are as follows. Note all values are approximate. The first line begins on the horizontal axis at 185 a m u, and extends to 37 percent. The second line begins on the horizontal axis at 187 a m u, and extends to 63 percent. Based on the mass spectrum of a pure element represented above, the average atomic mass of the element is closest to which of the following? A 185.7amu B 186.0amu C 186.3amu D 186.9amu

C 186.3amu Based on the mass spectrum, the 185amu isotope has a relative abundance close to 40% and the 187amu isotope has a relative abundance close to 60%. This predicts an average atomic mass slightly higher than 186amu.

Which of the following is the correct electron configuration for a ground-state atom of magnesium (atomic number 12) ? A 1s2 2s2 2p8 B 1s2 2s2 3s2 3p6 C 1s2 2s2 2p6 3s2 D 1s2 2s2 3s4 3p4

C 1s2 2s2 2p6 3s2 This electron configuration has the proper sequence of orbitals filled with the correct number of electrons.

Element N P As Sb Bi Atomic Radius (picometers) 65 100 115 145 160 The atomic radii of the elements in the nitrogen group in the periodic table are given in the table above. Which of the following best helps explain the trend of increasing atomic radius from N to Bi? A The number of particles in the nucleus of the atom increases. B The number of electrons in the outermost shell of the atom increases. C The attractive force between the valence electrons and the nuclei of the atoms decreases. D The repulsive force between the valence electrons and the electrons in the inner shells decreases.

C The attractive force between the valence electrons and the nuclei of the atoms decreases. The nucleus occupies only a tiny fraction of the volume of an atom and thus does not affect atomic radius. However, going down the group, the net attraction of the valence electrons for the nucleus decreases; thus the valence electrons maintain a greater average distance from the nucleus.

Which of the following represents the electron configuration of an oxygen atom in the ground state? A The figure presents an electron configuration. There are 5 horizontal lines. The first line is labeled 1 s, the second line is labeled 2 s, and the third, fourth, and fifth lines are labeled 2 p. The first line has one arrow pointing upward and one arrow pointing downward. The second line has one arrow pointing upward and one arrow pointing downward. The third line has one arrow pointing upward. The fourth line has one arrow pointing upward. There are no arrows for the fifth line. B The figure presents an electron configuration. There are 5 horizontal lines. The first line is labeled 1 s, the second line is labeled 2 s, and the third, fourth, and fifth lines are labeled 2 p. The first line has one arrow pointing upward and one arrow pointing downward. The second line has one arrow pointing upward and one arrow pointing downward. The third line has one arrow pointing upward. The fourth line has one arrow pointing upward. There are no arrows for the fifth line. C The figure presents an electron configuration. There are 5 horizontal lines. The first line is labeled 1 s, the second line is labeled 2 s, and the third, fourth, and fifth lines are labeled 2 p. The first line has one arrow pointing upward and one arrow pointing downward. The second line has one arrow pointing upward and one arrow pointing downward. The third line has one arrow pointing upward. The fourth line has one arrow pointing upward. There are no arrows for the fifth line. D The figure presents an electron configuration. There are 5 horizontal lines. The first line is labeled 1 s, the second line is labeled 2 s, and the third, fourth, and fifth lines are labeled 2 p. The first line has one arrow pointing upward and one arrow pointing downward. The second line has one arrow pointing upward and one arrow pointing downward. The third line has one arrow pointing upward. The fourth line has one arrow pointing upward. There are no arrows for the fifth line.

C The figure presents an electron configuration. There are 5 horizontal lines. The first line is labeled 1 s, the second line is labeled 2 s, and the third, fourth, and fifth lines are labeled 2 p. The first line has one arrow pointing upward and one arrow pointing downward. The second line has one arrow pointing upward and one arrow pointing downward. The third line has one arrow pointing upward. The fourth line has one arrow pointing upward. There are no arrows for the fifth line. This configuration contains eight electrons. According to Hund's rule, each orbital in the last subshell to have electrons placed in it (in this case the 2p subshell) has one electron placed in it before pairing of electrons in orbitals occurs.

Which of the following best helps explain why the first ionization energy of K is less than that of Ca? A The electronegativity of K is greater than that of Ca. B The atomic radius of the K atom is less than that of the Ca atom. C The valence electron of K experiences a lower effective nuclear charge than the valence electrons of Ca. D The nucleus of the K atom has fewer neutrons, on average, than the nucleus of the Ca atom has.

C The valence electron of K experiences a lower effective nuclear charge than the valence electrons of Ca. The amount of energy required to remove the electron (IE1) will depend on the strength of the attractive force between the electron and the nucleus, as predicted by Coulomb's law.

Which of the following best helps explain why an atom of Rb gas more easily loses an electron in a chemical reaction than an atom of Li gas? A Rb has a higher electronegativity than Li has. B The Rb atom has a greater number of valence electrons than the Li atom has. C The nucleus of the Rb atom has a greater number of protons and neutrons than the nucleus of the Li atom has. D In the Rb atom the valence electron is farther from its nucleus than the valence electron of Li is from its nucleus.

D In the Rb atom the valence electron is farther from its nucleus than the valence electron of Li is from its nucleus. Because the Rb electron is farther from its nucleus, there is a weaker attraction and it is easier to lose.

A 1.0mol sample of which of the following compounds has the greatest mass? A NO B NO2 C N2O D N2O5

D N2O5 The mass of 1.0mol of NO is less than the mass of 1.0mol of N2O5 because it has fewer N and O atoms and thus a smaller molar mass.

Which of the following best helps to explain why the atomic radius of K is greater than that of Br? A The first ionization energy of K is higher than that of Br. B The valence electrons in K are in a higher principal energy level than those of Br. C In the ground state, an atom of K has fewer unpaired electrons than an atom of Br has. D The effective nuclear charge experienced by valence electrons is smaller for K than for Br.

D The effective nuclear charge experienced by valence electrons is smaller for K than for Br. Because the effective nuclear charge of K is smaller than that of Br, the attraction between its valence electron and the nucleus is less than the attraction between the valence electrons in Br and the nucleus.

A 42.0g sample of compound containing only C and H was analyzed. The results showed that the sample contained 36.0g of C and 6.0g of H. Which of the following questions about the compound can be answered using the results of the analysis? A What was the volume of the sample? B What is the molar mass of the compound? C What is the chemical stability of the compound? D What is the empirical formula of the compound?

D What is the empirical formula of the compound? The number of moles of C and H is calculated by dividing the masses of each element in the compound and their respective molar masses. The ratio of the number of moles of C to H can be used to determine the empirical formula of the compound.