AP Chemistry - AP Classroom Unit 1 Progress Check: MCQ

Which of the following represents the electron configuration of an oxygen atom in the ground state?

(C) WHY? 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 2p2p subshell) has one electron placed in it before pairing of electrons in orbitals occurs.

What charge does Al typically have in ionic compounds, and why?

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

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?

CsCs, because it is in the same group as RbRb. WHY? CsCs and RbRb 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.

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?

The molar mass of each gas WHY? 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.

1s22s22p63s23p6 How many unpaired electrons are in the atom represented by the electron configuration above?

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

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?

186.3 amu WHY? 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) ?

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

Which of the following numerical expressions gives the number of moles in 5.0g of CaO?

5.0g ÷ 56 g/mol WHY? Dividing the mass of the sample by the molar mass of CaO gives the number of moles of CaO in the sample.

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

91 amu WHY? The relative abundances of the isotopes with atomic masses 90 and 92 have a weighted average that would be less than 91. That result with the influence of the mass and abundance of the isotope with atomic mass 94 would result in a weighted average closer to 91 than to 93.

The mass spectrum represented above is most consistent with which of the following elements?

Gd WHY? The average atomic mass of Gd is close to 157amu. The relative abundance and masses of the isotopes shown in the mass spectrum predict an average atomic mass close to 157amu.

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?

In the RbRb atom the valence electron is farther from its nucleus than the valence electron of LiLi is from its nucleus. WHY? Because the RbRb electron is farther from its nucleus, there is a weaker attraction and it is easier to lose.

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?

Is the BaCl2(s)BaCl2(s) used in the experiment pure? WHY? 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 in the precipitate would imply that the BaCl2 was not pure.

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?

Mass percent of Na WHY? 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.

A 1.0mol sample of which of the following compounds has the greatest mass?

N2O5 WHY? The mass of a sample of any compound can be calculated by n×M, where n is the number of moles of molecules in the sample and M is the molar mass (mass per mole) of the compound. The compound with molecules containing the greatest number of atoms of N and O has the greatest molar mass. Since each sample contains the same number of molecules (1.0mol of molecules), the sample of N2O5 has the largest mass.

The complete photoelectron spectrum of an element is given above. Which labeled peak corresponds to the 1s electrons and why?

Peak XX, because 1s1s electrons have the strongest attractions to the nucleus WHY? Greater binding energies correlate to stronger attractions to the nucleus.

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?

The attractive force between the valence electrons and the nuclei of the atoms decreases. WHY? 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 best helps to explain why the atomic radius of K is greater than that of Br?

The effective nuclear charge experienced by valence electrons is smaller for KK than for BrBr. WHY? Because the effective nuclear charge of KK is smaller than that of BrBr, the attraction between its valence electron and the nucleus is less than the attraction between the valence electrons in BrBr and the nucleus.

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.

The molar mass of Cu WHY? 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 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?

The molar masses of X and Z WHY? 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 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?

The spectrum shows a single electron in the 2p2p subshell. WHY? The spectrum is consistent with the electron configuration for boron: 1s22s22p11s22s22p1. The leftmost peak represents the two electrons in the filled 1s1s subshell. The two peaks on the right represent the two electrons in the filled 2s2s subshell and the single electron in the 2p2p subshell.

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?

The spectrum shows equal numbers of electrons in the three occupied electron subshells. WHY? The peak on the left represents two 1s1s electrons, and the two peaks on the right represent two 2s2s electrons and two 2p2p electrons, respectively.

Which of the following best helps explain why the first ionization energy of K is less than that of Ca?

The valence electron of KK experiences a lower effective nuclear charge than the valence electrons of CaCa. WHY? The amount of energy required to remove the electron (IE1IE1) will depend on the strength of the attractive force between the electron and the nucleus, as predicted by Coulomb's law.

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?

What is the empirical formula of the compound? WHY? 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.

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?

What is the molar mass of M? WHY? 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.