AP CHEM: Unit 4 Questions

In the laboratory, the hydrate CaSO4 ⋅ 2 H2O(s) can be heated in a crucible to completely drive off the water of hydration to form the anhydrous salt, CaSO4(s). A 2.49 g sample of pure CaSO4 ⋅ 2 H2O(s) is heated several times until the mass is constant. Calculate the mass, in grams, of the solid that remains after the dehydration reaction is complete.

- molar mass of CaSO4 ⋅ 2 H2O(s) = 172.172 g mol - molar mass of CaSO4(s) = 136.14 g mol - 2.49 g CaSO4 ⋅ 2 H2O(s) x 136.14g / 172.172g = 1.97 g of CaSO4

W(g) + X(g) → Y(g) + Z(g) Gases W and X react in a closed, rigid vessel to form gases Y and Z according to the equation above. The initial pressure of W(g) is 1.20 atm and that of X(g) is 1.60 atm. No Y(g) or Z(g) is initially present. The experiment is carried out at constant temperature. What is the partial pressure of Z(g) when the partial pressure of W(g) has decreased to 1.0

0.20 atm

Ni(s) → Ni2+ + 2e− Ag+ + e− → Ag Which of the following is the balanced net ionic equation for an oxidation-reduction reaction between Ag+ and Ni(s) based on the half-reactions represented above?

2 Ag+(aq) + Ni(s) → 2 Ag(s) + Ni2+(aq)

2H2O(l)+4MnO-+3ClO-→4MnO2(s)+3ClO-+4OH- 2 (aq) According to the balanced equation above, how many moles of ClO - are needed to react completely with 20. mL of 0.20 M KMnO4 solution?

A) 0.0030 mol

3 Ag(s) + 4 HNO3 → 3 AgNO3 + NO(g) + 2 H2O The reaction of silver metal and dilute nitric acid proceeds according to the equation above. If 0.10 mole of powdered silver is added to 10. milliliters of 6.0-molar nitric acid, the number of moles of NO gas that can be formed is

A) 0.015 mol

If 0.40 mol of H2 and 0.15 mol of O2 were to react as completely as possible to produce H2O, what mass of reactant would remain?

A) 0.20 g of H2

What volume of O2(g) is required to react with excess CS2(l) to produce 4.0 L of CO2(g)? (Assume all gases are measured at 0°C and 1 atm.)

A) 12 L

Which of the following is an oxidation-reduction reaction that is also a synthesis reaction

A) 2 Mg(s) + O2(g) → 2 MgO(s).

NH4NO3(s) → N2O(g) + 2 H2O(g) A 0.03 mol sample of NH4NO3(s) is placed in a 1 L evacuated flask, which is then sealed and heated. The NH4NO3(s) decomposes completely according to the balanced equation above. The total pressure in the flask measured at 400 K is closest to which of the following? (The value of the gas constant, R, is 0.082 L atm mol-1 K-1.)

A) 3 atm

2 N2H4(g) + N2O4(g) → 3 N2(g) + 4 H2O(g) When 8.0 g of N2H4 (32 g mol-1) and 92 g of N2O4 (92 g mol-1) are mixed together and react according to the equation above, what is the maximum mass of H2O that can be produced?

A) 9.0 g

A student was studying physical and chemical changes. The student carried out some procedures in the laboratory and recorded observations. For one of the procedures, the student concluded that a physical change took place, but not a chemical change. - Which of the following could have been the results of the procedure?

A) A cube of metal was changed into a flat sheet of metal.

If equal masses of the following compounds undergo complete combustion, which will yield the greatest mass of CO2?

A) Benzene, C6H6

MnO4− + 5 Fe2+ + 8 H+ → Mn2+ + 5 Fe3+ + 4 H2O In the reaction represented above, the number of MnO4- ions that react must be equal to which of the following?

A) One-fifth the number of Fe2+ ions that are consumed

Equal volumes of 0.2M solutions of lead(II) nitrate and potassium bromide are combined to form lead(II) bromide as a yellow precipitate. Which of the following is the correct net ionic equation for the reaction?

A) Pb2+(aq) + 2Br−(aq) → PbBr2(s)

The reaction between C6H12O6 and O2 is represented by the balanced equation above. In an experiment, 0.30 mol of CO2 was produced from the reaction of 0.05 mol of C6H12O6 with excess O2. The reaction was repeated at the same temperature and in the same container, but this time 0.60 mol of CO2 was produced. Which of the following must be true?

A) The initial amount of C6H12O6 in the container must have been 0.10 mol

When 100 mL of 1.0 M Na3PO4 is mixed with 100 mL of 1.0 M AgNO3, a yellow precipitate forms and [Ag+] becomes negligibly small. Which of the following is a correct listing of the ions remaining in solution in order of increasing concentration?

A) [PO43-] < [NO3-] < [Na+]

Potassium hydrogen phthalate, KHP, is used as a primary standard for determining the concentration of a solution of NaOH by titration. If the KHP has not been dried before weighing, the calculated molarity of the NaOH would be

A) higher than the actual value, since water is included in the apparent mass of KHP

A mixture of CO(g) and O2(g) is placed in a container, as shown opposite. A reaction occurs, forming CO2(g). - Which of the following best represents the contents of the box after the reaction has proceeded as completely as possible?

B oOo oOo oOo oOo oo

1. What is the maximum number of moles of Al2O3 that can be produced by the reaction of 0.40 mol of Al with 0.40 mol of O2 ?

B) 0.20 mol

What mass of Au is produced when 0.0500 mol of Au2S3 is reduced completely with excess H2?

B) 19.7 g

Al(s) → Al3+ + 3e− Zn2+ + 2e −→ Zn The half-reactions for the oxidation-reduction reaction between Al(s) and Zn2+ are represented above. Based on the half-reactions, what is the coefficient for All(s) if the equation for the oxidation-reduction reaction is balanced with the smallest whole-number coefficients?

B) 2

A particle view of a sample of H2O2(aq) is shown opposite. The H2O2(aq) is titrated with KMnO4(aq), as represented by the equation below. 2 MnO + 5 H O + 6 H → 2 Mn + 5 O + 8 H O −+2+ Which of the following particle views best represents the mixture when the titration is halfway to the equivalence point? (H2O molecules and H+ ions are not shown.)

C

. . . C10H12O4S(s) + . . . O2(g) → . . . CO2(g) + . . . SO2(g) + . . . H2O(g) When the equation above is balanced and all coefficients are reduced to their lowest whole-number terms, the coefficient for O2(g) is

C) 12

Answer the following questions about the solubility of Ca(OH)2 a) Write a balanced chemical equation for the dissolution of Ca(OH)2(s) in pure water. b) In the box below, complete a particle representation diagram that includes four water molecules with proper orientation around the Ca2+ ion.

Ca(OH)2 ⥦ Ca2+ + 2 OH- The diagram should show the oxygen side of the water molecules oriented closer to the Ca2+ ion.

2MnO-+5CO2-+16H+→2Mn2++10CO+8HO Permanganate and oxalate ions react in an acidified solution according to the balanced equation above. How many moles of CO2(g) are produced when 20. mL of acidified 0.20 M KMnO4 solution is added to 50. mL of 0.10 M Na2C2O4 solution?

D) 0.010 mol

Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) When a student adds 30.0 mL of 1.00 M HCl to 0.56 g of powdered Fe, a reaction occurs according to the equation above.

D) 0.22 L of H2 has been produced.

C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(l) In the reaction represented above, what is the total number of moles of reactants consumed when 1.00 mole of CO2(g) is produced?

D) 2.00 mol

. . . Ca3(PO4)2(s) + . . . H3PO4(l) → . . . Ca(H2PO4)2(s)When the equation above is balanced and all coefficients are reduced to lowest whole-number terms, what is the coefficient for H3PO4(l) ?

D) 4

. . . C3H8(g) + . . . O2(g) → . . . H2O(g) + . . . CO2(g) When the equation for the reaction represented above is balanced and all coefficients are reduced to the lowest whole-number terms, the coefficient for O2(g) is

D) 5

The diagrams opposite represent solutes present in two different dilute aqueous solutions before they were mixed. Water molecules are not shown. When the solutions were combined, a precipitation reaction took place. Which of the diagrams below is the best particle representation of the mixture after the precipitation reaction occurred?

D) Ag+ and Cl- at bottom.

The particle diagram opposite represents an aqueous solution of a weak monoprotic acid. The white circles represent H atoms. Which of the following shows the species that act as a Brønsted-Lowry base and its conjugate acid, in that order, in the solution?

D) BASE + H = CA

Which of the following is the net ionic equation for the reaction between aqueous sodium fluoride and hydrochloric acid?

D) F−(aq) + H+(aq) → HF(aq)

Which of the following represents a process in which a species is reduced?

D) NO3-(aq) → NO(g)

1. When C2H4(g) reacts with H2(g), the compound C2H6(g) is produced, as represented by the equation opposite. The reaction is correctly classified as which of the following types?

D) Oxidation-reduction, because H2(g) is oxidized.

I2(aq)+C6H8O6(aq)→C6H6O6(aq)+2I−+2H+ The compound C6H8O6 reacts with I2 according to the reaction represented by the equation above. The reaction is correctly classified as which of the following types?

D) Oxidation-reduction, because I2 is reduced.

When a buret is rinsed before a titration, which of the techniques below is the best procedure?

D) Rinse the buret two times: each time with some of the titrant solution.

A 100mL sample of 0.1M MgCl2(aq) and a 100mL sample of 0.2MNaOH(aq) were combined, and Mg(OH)2(s) precipitated, as shown by the equation above. If the experiment is repeated using solutions of the same molarity, which of the following changes in volume will double the amount of Mg(OH)2(s) produced?

D) Using twice the volume of MgCl2(aq) and twice the volume of NaOH(aq)

2 H2O2(l) → 2 H2O(l) + O2(g) The exothermic process represented above is best classified as a

D) chemical change because covalent bonds are broken and new covalent bonds are formed.

Which of the following particle diagrams best represents the products when four molecules of H2O2(l) decompose into water and oxygen gas at room temperature?

D) oOo oOo oOo oOo OO OO

Which of the following compounds at 25°C and 1.0 atm contains an atom in a +1 oxidation state?

E) Cu2O

HSO4-+H2O⇌H3O++SO4 2- In the equilibrium represented above, the species that act as bases include which of the following? I. HSO4- II. H2O III. SO42-

E) II and III

In which of the following species does sulfur have the same oxidation number as it does in H2SO4?

E) SO2Cl2

The diagram opposite represents H2(g) and N2(g) in a closed container. Which of the following diagrams would represent the results if the reaction shown below were to proceed as far as possible? N2(g) + 3 H2(g) → 2 NH3(g)

E) o8o o8o OO OO

Which of the following is the conjugate acid of NH-?

B) NH3

The reaction between aqueous strontium chloride (SrCl2) and aqueous potassium sulfate (K2SO4) forms a precipitate of strontium sulfate (SrSO4) . Which of the following represents the net ionic equation for the reaction?

B) Sr2+(aq) + SO42−(aq) → SrSO4(s)

At 27°C, five identical rigid 2.0 L vessels are filled with N2(g) and sealed. Four of the five vessels also contain a 0.050 mol sample of NaHCO3(s), NaBr(s), Cu(s), or I2(s), as shown in the diagram above. The volume taken up by the solids is negligible, and the initial pressure of N2(g) in each vessel is 720 mm Hg. All four vessels are heated to 127°C and allowed to reach a constant pressure. At 127°C, the entire sample of I2 is observed to have vaporized. How does the mass of vessel 5 at 127°C compare to its mass at 27°C?

B) The mass is the same, since the number of each type of atom in the vessel is constant.

At 27°C, five identical rigid 2.0 L vessels are filled with N2(g) and sealed. Four of the five vessels also contain a 0.050 mol sample of NaHCO3(s), NaBr(s), Cu(s), or I2(s), as shown in the diagram above. - The volume taken up by the solids is negligible, and the initial pressure of N2(g) in each vessel is 720 mm Hg. All four vessels are heated to 127°C and allowed to reach a constant pressure. - At 127°C, the pressure in vessel 1 is found to be higher than that in vessel 2. - Which of the following reactions best accounts for the observation?

C) 2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g)

H2 + F2(g) → 2 HF(g) In the reaction represented above, what mass of HF is produced by the reaction of 3.0 x 1023 molecules of H2 with excess F2 ? (Assume the reaction goes to completion.)

D) 20 g

10 HI + 2 KMnO4 + 3 H2SO4 → 5 I2 + 2 MnSO4 + K2SO4 + 8 H2O According to the balanced equation above, how many moles of HI would be necessary to produce 2.5 mol of I2, starting with 4.0 mol of KMnO4 and 3.0 mol of H2SO4

D) 5.0

2SO2(g) + O2(g) → 2SO3(s) A mixture of gases containing 0.20 mol of SO2 and 0.20 mol of O2 in a 4.0 L flask reacts to form SO3. If the temperature is 25ºC, what is the pressure in the flask after reaction is complete?

B) 0.3(0.082)(298)/4 atm

. . . LiHCO3(aq) + . . . H2SO4(aq) → . . . Li2SO4(aq) + . . . H2O(l) + . . . CO2(g) When the equation above is balanced and the coefficients are reduced to lowest whole-number terms, what is the coefficient of H2O(l) ?

B) 2

2 KClO3(s) → 2 KCl(s) + 3 O2(g) What is the percentage yield of O2 if 12.3 g of KClO3 (molar mass 123 g) is decomposed to produce 3.2 g of O2 (molar mass 32 g) according to the equation above?

B) 67%

1. 2 Al(s) + 6 HCl(aq) → 2 AlCl3(aq) + 3 H2(g) According to the reaction represented above, about how many grams of aluminum (atomic mass 27 g) are necessary to produce 0.50 mol of hydrogen gas at 25°C and 1.00 atm?

B) 9.0 g

A sample of a compound that contains only the elements C, H, and N is completely burned in O2 to produce 44.0 g of CO2, 45.0 g of H2O, and some NO2 . A possible empirical formula of the compound is

B) CH5N

2F2(g) + 2NaOH(aq) → OF2(g) + 2NaF(aq) + H2O(l) A 2 mol sample of F2(g) reacts with excess NaOH(aq) according to the equation above. If the reaction is repeated with excess NaOH(aq) but with 1 mol of F2(g), which of the following is correct?

B) OB The amount of OF2(g) produced is halved.

A mixture of H2(g) and O2(g) is placed in a container as represented opposite. The H2(g) and O2(g) react to form H2O(g) . Which of the following best represents the container after the reaction has gone to completion?

B) OO oOo oOo oOo oOo OO

1. A 1.0 L sample of an aqueous solution contains 0.10 mol of NaCl and 0.10 mol of CaCl2. What is the minimum number of moles of AgNO3 that must be added to the solution in order to precipitate all of the Cl- as AgCl(s) ? (Assume that AgCl is insoluble.)

C) 0.30 mol

Equimolar samples of CH3OH(l) and C2H5OH(l) are placed in separate, previously evacuated, rigid 2.0 L vessels. Each vessel is attached to a pressure gauge, and the temperatures are kept at 300 K. In both vessels, liquid is observed to remain present at the bottom of the container at all times. The change in pressure inside the vessel containing CH3OH(l) is shown below. - Which of the following best describes the change that takes place immediately after the CH3OH(l) is introduced into the previously evacuated vessel?

D) A physical change takes place because intermolecular attractions are overcome.

A student mixes 20.0g of white KCl crystals with distilled water in a beaker. After the mixture was stirred, no crystals are visible and the solution is clear. After several days, all of the water evaporates and white crystals are found in the beaker. Which of following pieces of experimental evidence would best help the student to confirm that a new compound had not been made and that only a physical change occurred?

D) After the water has evaporated, the white crystals in the beaker have a mass of 20.0g

HCl(aq) + H2O(l) → H3O+(aq) + Cl−(aq)For the dissolution of HCl in water represented above, which of the following pairs includes the Brønsted-Lowry bases?

D) H2O(l) and Cl−(aq)

A student pipetted five 25.00-milliliter samples of hydrochloric acid and transferred each sample to an Erlenmeyer flask, diluted it with distilled water, and added a few drops of phenolphthalein to each. Each sample was then titrated with a sodium hydroxide solution to the appearance of the first permanent faint pink color. The following results were obtained. Volumes of NaOH Solution: 35.22 mL , 36.14 mL , 36.13 mL , 36.15 mL and 36.12 mL Which of the following is the most probable explanation for the variation in the student's results?

D) The pipette was not rinsed with the HCI solution.

2MnO-+10Br-+16H+→2Mn2++5Br+8HO How many electrons are transferred in the reaction represented by the balanced equation above?

E) 10

Zn(s) is used to reduce other compounds in chemical reactions. If a chemist needs a substance that is more effective in its reducing ability, which of the following species would be the best choice?

A) Na

2KClO3 -> 2KCl + 3O2 According to the equation above, how many moles of potassium chlorate, KClO3, must be decomposed to generate 1.0 L of O2 gas at standard temperature and pressure?

C) 2/3(1/22.4) mol

Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s) The reaction between solid copper and aqueous silver nitrate produces solid silver and a blue solution, as represented by the balanced equation shown above. Based on the balanced equation, which of the following identifies the oxidation and reduction half-reactions?

B) Cu= Ox Ag= Red

H2C2O4(aq) + H2O(l) ⇄ H3O+(aq) + HC2O4-(aq) HC2O4-(aq) + H2O(l) ⇄ H3O+(aq) + C2O42-(aq) H2O(l) + H2O(l) ⇄ H3O+(aq) + OH- (aq) All the reactions represented above occur in an aqueous solution of oxalic acid. Which of the following represent a Brønsted-Lowry conjugate acid-base pair?

B) HC2O4-(aq) and C2O42-(aq)

Which of the following describes the changes in forces of attraction that occur as H2O changes phase from a liquid to a vapor?

B) Hydrogen bonds between H2O molecules are broken.

M+ is an unknown metal cation with a +1 charge. A student dissolves the chloride of the unknown metal, MCl, in enough water to make 100.0 mL of solution. The student then mixes the solution with excess AgNO3 solution, causing AgCl to precipitate. The student collects the precipitate by filtration, dries it, and records the data shown below. (The molar mass of AgCl is 143 g/mol.) Mass of unknown chloride, MCl 0.74 g Mass of filter paper 0.80 g Mass of filter paper plus AgCl precipitate 2.23 g

B) KCl

2. NH3(aq)+HCl(aq)→NH4++Cl− The Brønsted-Lowry bases in the reaction represented above are

B) NH3(aq) and Cl−(aq)

NH4+(aq) + OH−(aq) → NH3(aq) + H2O(l) When 0.20 M NH4Cl(aq) and 0.20 M NaOH(aq) are mixed, the reaction represented by the equation above occurs and a strong smell of ammonia, NH3, is observed. Based on this information, which of the following statements is true?

B) NH4+(aq) is a stronger acid than H2O(l) is.

Which of the following is a precipitation reaction

B) Pb2+(aq) + CrO42-(aq) → PbCrO4(s)

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) A student had two dilute, colorless solutions, HCl(aq) and NaOH(aq), which were at the same temperature. The student combined the solutions, and the reaction represented above occurred. - Which of the following results would be evidence that a chemical reaction took place?

B) The temperature of the reaction mixture increases.

Reaction 1: CaC2(s) + 2 H2O(l) → C2H2(g) + Ca(OH)2(s) Reaction 2: NaOCl(aq) + 2 HCl(aq) → Cl2(g) + NaCl(aq) + H2O(l) Reaction 3: C2H2(g) + Cl2(g) → C2H2Cl2(g) Reaction 2 occurs when an excess of 6 M HCl(aq) solution is added to 100. mL of NaOCL(aq) of unknown concentration. If the reaction goes to completion and 0.010 mol of Cl2(g) is produced, then what was the molarity of the NaOCL(aq) solution?

C) 0.10 M

A 6.0 mol sample of C3H8(g) and a 20. mol sample of Cl2(g) are placed in a previously evacuated vessel, where they react according to the equation above. After one of the reactants has been totally consumed, how many moles of HCl(g) have been produced?

C) 20 mol

Which of the following particulate diagrams best shows the formation of water vapor from hydrogen gas and oxygen gas in a rigid container at 125°C?

C) 6O2 3H2 --> 6 H2O

5H2O2(aq)+2MnO-+6H+→2Mn2++8HO+5O In a titration experiment,H2O2(aq)reacts with aqueous MnO- as represented by the equation above. The dark purple KMnO4 solution is added from a buret to a colorless, acidified solution of H2O2(aq) in an Erlenmeyer flask. (Note: At the end point of the titration, the solution is a pale pink color.) Which of the following best describes what happens to the pH of the H2O2 solution as the titration proceeds?

C) As H+ ions are consumed, the solution becomes less acidic and the pH increases.

The table above summarizes data given to a student to evaluate the type of change that took place when substance X was mixed with water. The student claimed that the data did not provide enough evidence to determine whether a chemical or physical change took place and that additional tests were needed. Which of the following identifies the best way to gather evidence to support the type of change that occurred when water and X were mixed?

D) Measuring the electrical conductivities of X and the mixture of water and X .

H2Se(g) + 4 O2F2(g) → SeF6(g) + 2 HF(g) + 4 O2(g) Which of the following is true regarding the reaction represented above?

D) The oxidation number of Se changes from -2 to +6.

A student learns that ionic compounds have significant covalent character when a cation has a polarizing effect on a large anion. As a result, the student hypothesizes that salts composed of small cations and large anions should have relatively low melting points. a) Select two compounds from the table and explain how the data support the student's hypothesis. b) Identify a compound from the table that can be dissolved in water to produce a basic solution. Write the net ionic equation for the reaction that occurs to cause the solution to be basic.

a- LiI/KI, LiI/LiF, or LiI/NaF LiI and (KI, LiF, or NaF). LiI has a small cation and a large anion and KI has a large cation and the same large anion. The melting point of LiI (with its smaller cation) is lower than that of KI. b- Either LiF or NaF F- + H2O ⇄ HF + OH-

A student places a sample of a pure metal in a crucible and heats it strongly in air. Data from the experiment are given in the table above. The final mass was determined after the sample was cooled to room temperature. Which of the following statements related to the experiment is correct?

B) The mass of the sample increased, so a chemical change occurred when bonds formed between the metal and another substance.

When water is added to a mixture of Na2O2(s) and S (s) , a redox reaction occurs, as represented by the equation below. 2 Na2O2(s) + S(s) + 2 H2O(l) → 4 NaOH(aq) + SO2(aq) Atoms of which element are reduced in the reaction?

C) O in Na2O2 - each atom gains one electron

2 H2O2(aq) → 2 H2O(l) + O2(g) The decomposition of H2O2(aq) is represented by the equation above. Which of the following identifies the element(s) being oxidized and reduced in the reaction?

C) Oxygen is both oxidized and reduced.

A student was asked to formulate a hypothesis about what would happen if 100.mL of 0.1MNaOH(aq) at 25°C was combined with 100.mL of 0.1MMgCl2(aq) at 25°C . - Which of the following hypotheses indicates that the student thought a chemical change would occur?

C) The resulting solution would contain a precipitate.