AP Chemistry Unit 3 AP Exam Prep Quiz

The electron cloud of HF is smaller than that of F₂, however, HF has a much higher boiling point than F₂ has. Which of the following explains how the dispersion-force model of intermolecular attraction does not account for the unusually high boiling point of HF?

 

Table: Substances and their Normal Boiling Points

Substance	Normal boiling point
HF	293 K
F2	85 K

Liquid F2 has weak dispersion force attractions between its molecules, whereas liquid HF has both weak dispersion force attractions and hydrogen bonding interactions between its molecules.

Solid ethyl alcohol, C₂H₅OH

 

A molecular solid with hydrogen bonding

In solid methane, the forces between neighboring CH₄ molecules are best characterized as

 

London (dispersion) forces

The graph shows the heating curve for methane, CH₄.

 

The enthalpy of vaporization of water is 40.7 kJ/mol. Which of the following best explains why the enthalpy of vaporization of methane is less than that of water?

Methane does not exhibit hydrogen bonding, but water does.

PCl₅(g) decomposes into PCl₃(g) and Cl₂(g) according to the following equation:

PCl₅(g) ⇆ PCl₃(g) + Cl₂(g)

A pure sample of PCl₅(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl₅(g) is 1.00 atm. The temperature is held constant until the PCl₅(g) reaches equilibrium with its decomposition products. The figures show the initial and equilibrium conditions of the system.

 

 

Which of the following is the most likely cause for the increase in pressure observed in the container as the reaction reaches equilibrium?

An increase in the number of molecules, which increases the frequency of collisions with the walls of the container

The question refers to the 1 L flask shown in the figure.

Which flask contains the smallest number of moles of gas?

B

The table contains information about samples of four different gases at 273 K. The samples are in four identical rigid containers numbered 1 through 4.

 

Container	Gas	Pressure (atm)	Mass of Sample (g)
1	He	2.00	?
2	Ne	2.00	?
3	?	2.00	16.0
4	SO2	1.96	64.1

 

On the basis of the data provided in the table, the gas in Container 3 could be

CH4

A 0.5 mol sample of He(g) and a 0.5 mol sample of Ne(g) are placed separately in two 10.0 L rigid containers at 25°C. Each container has a pinhole opening. Which of the gases, He(g) or Ne(g), will escape faster through the pinhole and why?

He(g) will escape faster because the He(g) atoms are moving at a higher average speed than the Ne(g) atoms.

The Maxwell-Boltzmann distributions of molecular speeds in samples of two different gases at the same temperature are shown in the figure.

Which gas has the greater molar mass?

Gas A

The table below contains information about samples of four different gases at 273 K. The samples are in four identical rigid containers numbered 1 through 4.

 

Container	Gas	Pressure (atm)	Mass of Sample (g)
1	He	2.00	?
2	Ne	2.00	?
3	?	2.00	16.0
4	SO2	1.96	64.1

 

The best explanation for the lower pressure in container 4 is that SO₂ molecules

have stronger intermolecular attractions than the other three gases

When a sample of oxygen gas in a closed container of constant volume is heated until its absolute temperature is doubled, which of the following is also doubled?

The pressure of the gas

A 1.0 L sample of a pure gas is found to have a lower pressure than that predicted by the ideal gas law. The best explanation for the observation is that the molecule of the gas

are attracted to each other and do not exert as much force on the container walls as they would if they had no mutual attractions

If 200. mL of 0.60 M MgCl₂(aq) is added to 400. mL of distilled water, what is the concentration of Mg²⁺(aq) in the resulting solution? (Assume volumes are additive).

 

0.20 M

For an experiment, a student needs 100.0 mL of 0.4220 M NaCl. If the student starts with NaCl(s) and distilled water, which of the following pieces of laboratory glassware should the student use to prepare the solution with the greatest accuracy?

100 mL volumetric flask

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

 

Which of the following diagrams best represents the AgNO3 solution before the reaction occurs?

 

Note: water molecules are represented by the symbol

 

A mixture containing equal numbers of moles of ethyl acetate and butyl acetate was separated using distillation. Based on the diagrams shown below, which of the following identifies the substance that would be initially present in higher concentration in the distillate and correctly explains why that occurs?

 

Ethyl acetate, because it has a shorter carbon chain and weaker London dispersion forces

To gravimetrically analyze the silver content of a piece of jewelry made from an alloy of Ag and Cu, a student dissolves a small preweighed sample in HNO₃(aq). Ag⁺(aq) and Cu²⁺(aq) ions form in the solution. Which of the following should be the next step in the analytical process?

Adding a solution containing an anion that forms an insoluble salt with only one of the metal ions

 student placed a sample of a food coloring that contains a mixture of a blue dye and a red dye at the top of a chromatography column filled with a nonpolar stationary phase. When water is poured through the column, two bands of colors are seen in the column, as shown in the diagram.

 

Which of the following techniques would be best to investigate molecular vibrations in molecules of the red dye?

 

Infrared spectroscopy

Which statement correctly compares what occurs when molecules absorb photons in the microwave region with what occurs when molecules absorb photons in the infrared region?

Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause the molecules to increase their vibrational energy states.

Using the wavelength information provided in the table, what is the color of the light with photon energy of 3.3 ✕ 10^-19 J?

 

Color	Wavelength
Red	647 - 760 nm
Orange	585 - 647 nm
Yellow	575 - 585 nm
Green	491 -575 nm
Blue	424 - 491 nm
Violet	300 - 424 nm

Orange

The diagrams below show the ultraviolet absorption spectra for two compounds. Diagram 1 is the absorption spectrum of pure acetone, a solvent used when preparing solutions for an experiment. Diagram 2 is the absorption spectrum of the solute for which the absorbance needs to be measured to determine its concentration.

When the student reads the absorbance of the solution at 280 nm, the result is too high. Which of the following is most likely responsible for the error in the measured absorbance?

The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone

An experiment was performed to investigate the reaction between Zn metal and Ni²⁺ (aq)at different concentrations. Because the Ni²⁺ (aq)ion is green, the extent of the reaction was determined using spectrophotometric analysis. Four 20.0 mL standard solutions of Ni²⁺ (aq) were prepared by dissolving NiCl₂ ⦁ 6H₂0 (molar mass 240 g/mol)) in water. The absorbance of each solution was measured; the results are shown both in the table below and in the following plot of the absorbance data.

 

Solution	[Ni2+]	Absorbance
1	0.020	0.12
2	0.040	0.25
3	0.060	0.39
4	0.080	0.51

What is the expected absorbance of a standard solution made by dissolving 0.0070 mol of NiCl₂ ⦁ 6H₂0 in water to make 100. mL of solution?

  

0.45