AP Chemistry - Midterm Review

1s²2s²2p⁶3s²3p⁶ 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.

How many carbon atoms are contained in 2.8 g of C2H4 ? A) 1.2 x 10²³ B) 3.0 x 10²³ C) 6.0 x 10²³ D) 1.2 x 10²⁴ E) 6.0 x 10²⁴

A) 1.2 x 10²³

Which of the following represents the ground state electron configuration for the Mn³⁺ ion? (Atomic number Mn =25) A) 1s²2s²2p⁶3s²3p⁶3d⁴ B) 1s²2s²2p⁶3s²3p⁶3d⁵4s² C) 1s²2s²2p⁶3s²3p⁶3d²4s² D) 1s²2s²2p⁶3s²3p⁶3d⁸4s² E) 1s²2s²2p⁶3s²3p⁶3d³4s¹

A) 1s²2s²2p⁶3s²3p⁶3d⁴

Consider atoms of the following elements. Assume that the atoms are in the ground state. The atom that contains exactly two unpaired electrons A) S B) Ca C) Ga D) Sb E) Br

A) S

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 mass percent of carbon in pure glucose, C₆H₁₂O₆, is 40.0 percent. A chemist analyzes an impure sample of glucose and determines that the mass percent of carbon is 38.2 percent. Which of the following impurities could account for the low mass percent of carbon in the sample? A) Water, H₂O B) Ribose, C₅H₁₀O₅ C) Fructose, C₆H₁₂O₆, an isomer of glucose D) Sucrose, C₁₂H₂₂O₁₁

A) Water, H₂O -An impurity that contains no carbon will decrease the mass percent of carbon relative to the pure substance. The mass percent of carbon in the sample is lower than that of pure glucose. So this option is consistent with the observation.

A student has two samples of NaCl, each one from a different source. Assume that the only potential contaminant in each sample is KCl. The student runs an experiment to determine the percent by mass of chlorine in each sample. From the results of this experiment alone, which of the following questions is most likely to be answered? A) Which sample has the higher purity? B) Which sample has the higher density? C) What is the source of the contaminants present in each of the samples? D) Which sample came from a salt mine, and which sample came from the ocean?

A) Which sample has the higher purity? -For each sample, the percent of chlorine by mass can be compared to the percent of chlorine by mass of pure NaCl. The sample with the greater percent of chlorine by mass has the higher purity.

Which of the following represents an electron configuration that corresponds to the valence electrons of an element for which there is an especially large jump between the second and third ionization energies? (Note: n represents a principal quantum number equal to or greater than 2.) A) ns² B) ns²np¹ C) ns²np² D) ns²np³

A) ns² -The third ionization of this element would involve removing an electron that is closer to the nucleus (in the adjacent lower shell) than are the first two electrons. This would require much more energy than the removal of the first two electrons.

4 ions that are always soluble

1. Na⁺ 2. K⁺ 3. NH₄⁺ 4. NO₃⁻

Conversion Factor

A ratio (or fraction) which represents the relationship between the same quantity in 2 different units

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.

Calcium: -Atomic Radius: 194pm -First Ionization Energy: 590kJ/mol Based on periodic trends and the data in the table above, which of the following are the most probable values of the atomic radius and the first ionization energy for potassium , respectively? A) 242 pm, 633 kJ/mol B) 242 pm, 419 kJ/mol C) 120 pm, 633 kJ/mol D) 120 pm, 419 kJ/mol

B) 242 pm, 419 kJ/mol -Potassium is larger than calcium because it has a smaller effective nuclear charge. Because potassium is larger, less energy is required to remove an electron from it than is required to remove an electron from calcium.

Which of the following numerical expressions gives the number of moles in 5.0g of CaO? A) 5.0g × 56g/mol B) 5.0g/56g/mol C) 56g/mol/5.0g D) 1/5.0g × 1/56g/mol

B) 5.0g/56g/mol -Dividing the mass of the sample by the molar mass of CaO gives the number of moles of CaO in the sample.

Atoms of Mg combine with atoms of F to form a compound. Atoms of which of the following elements combine with atoms of F in the same ratio? A) Li B) Ba C) Al D) Cl E) Ne

B) Ba

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.

A 23.0g sample of a compound contains 12.0g of C, 3.0g of H, and 8.0g of O. Which of the following is the empirical formula of the compound? A) CH₃O B) C₂H₆O C) C₃H₉O₂ D) C₄H₁₂O₂

B) C₂H₆O

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 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.

Mass: 27.3g Mass % of Na: 35.2% Density: 2.05 g cm⁻³ Color: 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.

Compound : Molar Mass -Na₂O: 62.0g -MgO: 40.3g -K₂O: 94.2g -CaO: 56.1g According to the information above, a 1.00 g sample of which of the following contains the greatest mass of oxygen? A) Na₂O B) MgO C) K₂O D) CaO

B) MgO

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 2s2s 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.

Which of the following is the correct electron configuration for a ground-state atom of magnesium (atomic number 12) ? A) 1s²2s²2p⁸ B) 1s²2s²3s²3p⁶ C) 1s²2s²2p⁶3s² D) 1s²2s²3s⁴3p⁴

C) 1s²2s²2p⁶3s² -This electron configuration has the proper sequence of orbitals filled with the correct number of electrons.

Complete combustion of a sample of a hydrocarbon in excess oxygen produces equimolar quantities of carbon dioxide and water. Which of the following could be the molecular formula of the compound? A) C2H2 B) C2H6 C) C4H8 D) C6H6

C) C4H8 -The balanced chemical equation is C4H8 + 6 O2 ➝ 4 CO2 + 4 H2O, so combustion produces CO2 and H2O with a mole ratio of 1 to 1, in agreement with the observed equimolar ratio.

A sample of carbonate rock is a mixture of CaCO3 and MgCO3. The rock is analyzed in a laboratory, and the results are recorded in the table above. Which columns in the table provide all the information necessary to determine the mole ratio of Ca to Mg in the rock? A) total mass of the sample (g), Mass of C in sample (g), Molar mass of C (g/mol) B) Mass of C in sample (g), Molar mass of C (g/mol), Molar mass of Mg (g/mol) C) Mass of Mg in sample (g), Mass of Ca in sample (g), Molar mass of Mg (g/mol), Molar mass of Ca (g/mol) D) Mass of C in sample (g), Mass of Mg in sample (g), Mass of Ca in sample (g), Molar mass of C (g/mol)

C) Mass of Mg in sample (g), Mass of Ca in sample (g), Molar mass of Mg (g/mol), Molar mass of Ca (g/mol) -From the masses of Ca and Mg and their respective molar masses (i.e., columns 3, 4, 6, 7), the number of moles of Ca and Mg can be calculated. The ratio of these two numbers is the desired answer.

A student obtains a sample of a pure solid compound. In addition to Avogadro's number, which of the following must the student know in order to determine how many molecules are in the sample? A) Mass of the sample, volume of the sample B) Mass of the sample, density of the sample C) Molar mass of the compound, mass of the sample D) Molar mass of the compound, density of the sample

C) Molar mass of the compound, mass of the sample -Dividing the mass of the sample by the molar mass of the compound yields the number of moles of the compound, which, when multiplied by Avogadro's number, yields the number of molecules of the compound in the sample.

Element : Atomic Radius N: 65pm P: 100pm As: 115pm Sb: 145pm Bi: 160pm 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. -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.

All the chlorides of the alkaline earth metals have similar empirical formulas, as shown in the table above. Which of the following best helps to explain this observation? A) Cl₂(g) reacts with metal atoms to form strong, covalent double bonds. B) Cl has a much greater electronegativity than any of the alkaline earth metals. C) The two valence electrons of alkaline earth metal atoms are relatively easy to remove. D) The radii of atoms of alkaline earth metals increase moving down the group from Be to Ra.

C) The two valence electrons of alkaline earth metal atoms are relatively easy to remove. -By easily losing their two valence electrons, the alkaline earth metal atoms react with chlorine atoms to form M²⁺ cations that combine in a 1-to-2 ratio with Cl⁻ anions.

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.

The percentage of silver in a solid sample is determined gravimetrically by converting the silver to Ag+(aq) and precipitating it as silver chloride. Failure to do which of the following could cause errors in the analysis? I. Account for the mass of the weighing paper when determining the mass of the sample II. Measure the temperature during the precipitation reaction III. Wash the precipitate IV. Heat the AgCl precipitate to constant mass A) I only B) I and II C) I and IV D) II and III E) I, III, and IV

E) I, III, and IV

When a 3.22 g sample of an unknown hydrate of sodium sulfate, Na2SO4 ⋅ H2O(s), is heated, H2O (molar mass 18 g) is driven off. The mass of the anhydrous Na2SO4(s) (molar mass 142 g) that remains is 1.42 g. The value of x in the hydrate is... A) 0.013 B) 1.8 C) 6.0 D) 10. E) 20.

D) 10.

Which of the following ground-state electron configurations represents the atom that has the lowest first-ionization energy? A) 1s²2s¹ B) 1s²2s²2p² C) 1s²2s²2p⁶ D) 1s²2s²2p⁶3s¹

D) 1s²2s²2p⁶3s¹ -An electron in a 3s sublevel is farthest from the nucleus. Based on Coulomb's law, this weakens the attractive force between the electron and the nucleus, making it easier to remove.

A sample of CaCO3 (molar mass 100. g) was reported as being 30. percent Ca. Assuming no calcium was present in any impurities, the percent of CaCO3 in the sample is... A) 30% B) 40% C) 70% D) 75% E) 100%

D) 75%

Which of the following correctly identifies which has the higher first-ionization energy, Cl or Ar, and supplies the best justification? A) Cl, because of its higher electronegativity B) Cl, because of its higher electron affinity C) Ar, because of its completely filled valence shell D) Ar, because of its higher effective nuclear charge

D) Ar, because of its higher effective nuclear charge

The photoelectron spectra of the 1s electrons of two isoelectronic species, Ca2+ and Ar, are shown above. Which of the following correctly identifies the species associated with peak X and provides a valid justification? A) Ar, because it has completely filled energy levels B) Ar, because its radius is smaller than the radius of Ca²⁺ C) Ca²⁺, because its nuclear mass is greater than that of Ar D) Ca²⁺, because its nucleus has two more protons than the nucleus of Ar has

D) Ca²⁺, because its nucleus has two more protons than the nucleus of Ar has

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 student has a 1g sample of each of the following compounds: NaCl, KBr, and KCl. Which of the following lists the samples in order of increasing number of moles in the sample? A) NaCl < KCl < KBr B) NaCl < KBr < KCl C) KCl < NaCl <KBr D) KBr < KCl < NaCl

D) KBr < KCl < NaCl -The number of moles is represented by the ratio mass/molar mass. Since all samples have the same mass, the greater the molar mass, the smaller the number of moles.

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 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.

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.

The photoelectron spectrum for the element nitrogen is represented above. Which of the following best explains how the spectrum is consistent with the electron shell model of the atom? A) The leftmost peak represents the valence electrons B) The two peaks at the right represent a total of three electrons. C) The electrons in the 1s1s sublevel have the smallest binding energy. D) The electrons in the 2p2p sublevel have the smallest binding energy.

D) The electrons in the 2p2p sublevel have the smallest binding energy. -The electrons in the 2p sublevel have the smallest binding energy because these electrons have the weakest attraction to the nucleus and are more easily removed from the atom.

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.

Which of the following is the electron configuration of an excited atom that is likely to emit a quantum of energy? A) 1s²2s²2p⁶3s²3p¹ B) 1s²2s²2p⁶3s²3p⁵ C) 1s²2s²2p⁶3s² D) 1s²2s²2p⁶3s¹ E) 1s²2s²2p⁶3s¹3p¹

E) 1s²2s²2p⁶3s¹3p¹

1s²2s²2p⁶3s²3p³ Atoms of an element, X, have the electronic configuration shown above. The compound most likely formed with magnesium, Mg, is A) MgX B) Mg₂X C) MgX₂ D) MgX₃ E) Mg₃X₂

E) Mg₃X₂

The compound K₂O₂ also exists. A chemist can determine the mass of K in a sample of known mass that consists of either pure K₂O or pure K₂O₂. From this information, can the chemist answer the question of which compound is in the sample? Indicate yes or no, and explain.

From this information, a chemist can answer the question of which compound is in the sample by weighing the sample and then, using conversion, can find how much K and O ae in the sample. If the sample has a higher value than 1g of K2O, the sample is probably K2O. However, if the sample is about the weight of K2O, it is probably K2O.

In terms of atomic structure, explain why the first-ionization energy K is less than that of Ca.

K has a lower first-ionization energy than Ca because it has a larger atomic radius. This is due to the fact that K has 19 protons while Ca has 20. Those 20 protons in Ca are going to create a smaller atomic radius because of an increase in coulombic attraction. In turn, this will create a higher first-ionization energy than K.

K forms the compound K₂O, which is an ionic compound that is brittle. Identify another element, M, that is likely to form a brittle, ionic compound with the formula M₂O. Justify your answer in terms of periodic trends.

K has an ionic charge of 1+. Oxygen has an ionic charge of 2-. This means that you need 2 K and 1 O in order to make the charge neutral. Another element that is likely to have the same occurrence as K₂O is Na₂O. This is because K and Na are in the same family, which means that they will share similar chemical properties.

Empirical Formula

Lowest whole number ratio of atoms of the elements in a compound

Mass Spectroscopy

Technique used to identify the isotopes of an element and their relative abundance in nature

In terms of atomic structure, explain why the atomic radius of K is larger than that of Na.

The atomic radius of K is larger than Na because K has an extra valence shell.

Ba(OH)₂

barium hydroxide

Ca(OH)₂

calcium hydroxide

Sr(OH)₂

calcium hydroxide

HBr

hydrobromic acid

HCl

hydrochloric acid

HI

hydroiodic acid

LiOH

lithium hydroxide

HNO₃

nitric acid

HClO₄

perchloric acid

NaOH

sodium hydroxide

H₂SO₄

sulfuric acid