AP Chemistry Unit 3 review: Properties of Substances and Mixtures

• Saying boiling breaks the covalent bonds within molecules. Ordinary boiling separates molecules by overcoming intermolecular attractions. CED printed p. 55 flags this distinction.
• Calling forces merely strong or weak. Name dispersion, dipole–dipole, hydrogen bonding or ion–dipole attractions and explain the comparison. CED p. 55.
• Using Celsius in a gas-law ratio. Absolute temperature is required; 20 °C is about 293 K. CED topic 3.4.
• Using solvent volume as final solution volume. Molarity is moles per liter of solution. CED topic 3.7.

1. Question 1 · 3.4

   A 0.200 mol ideal gas occupies 4.92 L at 300 K. With R = 0.0820 L atm mol⁻¹ K⁻¹, what is its pressure?

   • A0.500 atm
   • B2.00 atm
   • C4.00 atm
   • D1.00 atm
   Show the answer

   D. P = nRT/V = 0.200(0.0820)(300)/4.92 = 1.00 atm. The chosen R matches liters and atmospheres.

2. Question 2 · 3.7

   What volume of 0.800 M stock solution is needed to prepare 250.0 mL of 0.120 M solution?

   • A37.5 mL
   • B24.0 mL
   • C60.0 mL
   • D96.0 mL
   Show the answer

   A. V₁ = M₂V₂/M₁ = 0.120(250.0)/0.800 = 37.5 mL. Dilute this aliquot to 250.0 mL total volume, rather than adding 250.0 mL of water.

3. Question 3 · 3.13

   A calibration line through the origin is A = 3,200c, with c in mol L⁻¹. What concentration gives absorbance 0.480?

   • A6.67 × 10⁻⁴ M
   • B1.54 × 10³ M
   • C1.50 × 10⁻⁴ M
   • D6.67 × 10⁻⁵ M
   Show the answer

   C. c = A/3,200 = 0.480/3,200 = 1.50 × 10⁻⁴ M. This calculation assumes the sample is measured under the calibration conditions and lies in the linear range.

4. Question 4 · 3.12

   What is the energy of a 600 nm photon? Use h = 6.626 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.

   • A1.19 × 10⁻²⁵ J
   • B3.31 × 10⁻¹⁹ J
   • C3.31 × 10⁻²⁸ J
   • D1.99 × 10⁻¹⁹ J
   Show the answer

   B. 600 nm = 6.00 × 10⁻⁷ m. E = hc/λ = 1.9878 × 10⁻²⁵ / 6.00 × 10⁻⁷ = 3.31 × 10⁻¹⁹ J per photon.

5. Question 5 · 3.4

   A gas mixture contains 0.30 mol He and 0.70 mol Ne at a total pressure of 2.40 atm. What is the partial pressure of He?

   • A0.30 atm
   • B1.68 atm
   • C0.72 atm
   • D2.10 atm
   Show the answer

   C. The helium mole fraction is 0.30/(0.30 + 0.70) = 0.30. Its partial pressure is 0.30(2.40) = 0.72 atm. The other 1.68 atm comes from neon.

6. Question 6 · 3.1

   Ethanol and dimethyl ether have the same molecular formula, C₂H₆O. Why does ethanol have the higher boiling point?

   • AEthanol has more electrons per molecule
   • BEther forms an ionic lattice in the liquid
   • CEthanol breaks C–C bonds while boiling
   • DEthanol molecules can hydrogen-bond to each other
   Show the answer

   D. Ethanol has an O–H group that can donate hydrogen bonds to neighboring molecules. Dimethyl ether lacks an O–H donor, so pure ether does not form the same intermolecular hydrogen-bond network.

For each calculation, name the conserved amount: gas moles in a sealed container, solute moles during dilution or path length during an absorbance comparison. Make a rough prediction before entering numbers so a reversed ratio is easier to catch.

Draw particles before and after boiling, dissolving and diluting. Keep molecular identity and atom ratios consistent. For a physical-property comparison, identify the force, describe the particle feature that changes its strength and connect that force to the observed property.

Ashore's Chemistry course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.

• AP Chemistry: all 9 units, exam format and scores
• AP Chemistry Unit 1: Atomic Structure and Properties
• AP Chemistry Unit 2: Compound Structure and Properties
• AP Chemistry Unit 4: Chemical Reactions
• AP Chemistry Unit 5: Kinetics
• AP Chemistry Unit 6: Thermochemistry
• AP Chemistry Unit 7: Equilibrium
• AP Chemistry Unit 8: Acids and Bases
• AP Chemistry Unit 9: Thermodynamics and Electrochemistry