AP Chemistry Unit 3 practice questions

Question 1

The Henry's law constant for oxygen gas ( $O_2$ ) in water at $25^\circ C$ is $1.3 \times 10^{-3}$ mol/(L·atm). Calculate the concentration of dissolved $O_2$ in a solution at equilibrium with air where the partial pressure of $O_2$ is 0.21 atm.

$2.7 \times 10^{-4}$ M

$6.2 \times 10^{-3}$ M

$1.6 \times 10^{2}$ M

$2.7 \times 10^{-3}$ M

Accepted Answer

$2.7 	imes 10^{-4}$ M

Answer

$6.2 	imes 10^{-3}$ M

Answer

$1.6 	imes 10^{2}$ M

Answer

$2.7 	imes 10^{-3}$ M

Question 2

Potassium nitrate ($KNO_3$) has a solubility of 85.0 g per 100.0 g of water at $50^\circ C$ and 32.0 g per 100.0 g of water at $20^\circ C$. A student prepares a saturated solution using 50.0 g of water at $50^\circ C$ and then cools it to $20^\circ C$. What mass of $KNO_3$ will precipitate?

Accepted Answer

26.5 g

Answer

16.0 g

Answer

53.0 g

Answer

58.5 g

Question 3

At $20^\circ C$, the solubility of sucrose ($C_{12}H_{22}O_{11}$, molar mass 342.3 g/mol) is 204 g per 100. mL of solution. What is the molarity of a saturated sucrose solution at this temperature?

Accepted Answer

5.96 M

Answer

0.596 M

Answer

1.68 M

Answer

16.8 M

Question 4

The solubility of ammonium chloride ($NH_4Cl$) at $20^\circ C$ is 37.0 g per 100.0 g of water. If a student dissolves 12.0 g of $NH_4Cl$ in 50.0 g of water at $20^\circ C$, how much additional $NH_4Cl$ is required to saturate the solution?

Accepted Answer

6.5 g

Answer

18.5 g

Answer

25.0 g

Answer

30.5 g

Question 5

A structural model of liquid water (H₂O) and liquid hydrogen sulfide (H₂S) shows both molecules with bent geometries and similar dipole moments. However, the model does not depict lone pair electrons. How well does this model explain why H₂O has a significantly higher boiling point than H₂S?

Accepted Answer

Poorly, because it omits the lone pairs needed to explain strong hydrogen bonding in water.

Answer

Fully, because the bent geometry creates the net dipole responsible for the attraction.

Answer

Fully, because the similar dipole moments imply similar strengths of intermolecular forces.

Answer

Poorly, because it fails to show the stronger London dispersion forces in hydrogen sulfide.

Question 6

A student uses ball-and-stick models to represent n-pentane (a straight chain) and neopentane (a compact sphere). Both models contain the same number of carbon and hydrogen atoms. The student claims the models predict identical boiling points. Which evaluation of this claim is most accurate?

Accepted Answer

Incorrect, because the straight chain allows more surface contact for stronger dispersion forces.

Answer

Correct, because isomers with identical molecular formulas have identical intermolecular forces.

Answer

Correct, because both molecules are nonpolar and rely solely on London dispersion forces.

Answer

Incorrect, because the compact sphere creates a stronger permanent dipole moment in the isomer.

Question 7

A representation of oxygen gas (O₂) dissolving in water depicts water molecules as permanent dipoles and O₂ molecules as neutral spheres with fixed electron distributions. To what extent does this representation explain the solubility of O₂ in water?

Accepted Answer

Incompletely, because it fails to show the induction of a temporary dipole in the O₂ molecule.

Answer

Completely, because nonpolar molecules dissolve via simple mixing with polar solvents.

Answer

Completely, because the fixed electron distribution prevents repulsion from water dipoles.

Answer

Incompletely, because it fails to show the formation of hydrogen bonds between O₂ and water.

Question 8

A kinetic model of a liquid in a closed container shows particles as spheres with a single uniform velocity vector at 298 K. The model is used to explain why vapor pressure increases at 350 K. Which limitation of the model most hinders its ability to explain this property?

Accepted Answer

It fails to show the distribution of kinetic energies needed to overcome attractive forces.

Answer

It fails to show that individual particles have different speeds at the same temperature.

Answer

It fails to show that some particles have sufficient kinetic energy to overcome intermolecular attractions.

Answer

It fails to show that the average kinetic energy of particles increases with temperature change.

Question 9

A student presents a model comparing hydrogen fluoride (HF) and hydrogen chloride (HCl). The model accurately depicts bond lengths but shows the intermolecular attraction between molecules as identical dashed lines. How well does this model account for the difference in boiling points between HF (293 K) and HCl (188 K)?

Accepted Answer

Poorly, because it does not distinguish hydrogen bonding in HF from dipole forces in HCl.

Answer

Well, because both molecules are linear polar acids with similar dipole strengths.

Answer

Well, because the bond length difference compensates for the difference in forces.

Answer

Poorly, because it does not show the stronger London dispersion forces present in HF.

Question 10

According to Coulomb's law ($$F \propto \frac{q_1q_2}{r^2}$$), if the charge of a dissolved metal cation doubles while its distance from a water molecule remains constant, how does the attractive force change?

Accepted Answer

It increases by a factor of 2; force is directly proportional to charge magnitude

Answer

It increases by a factor of 4; force is directly proportional to charge squared

Answer

It decreases by a factor of 2; force is inversely proportional to charge magnitude

Answer

It decreases by a factor of 4; force is inversely proportional to charge squared

AP Chemistry Unit 3 Practice Test

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