Phase Change Basics
Vapor Pressure & IMFs
Clausius-Clapeyron & Heat
Concentration Units
Calculation Lab
100
Name the three exothermic phase changes.
Freezing, Condensation, and Deposition.
100
Q: Define "normal boiling point."
A: The temperature at which vapor pressure equals exactly 1 atm (760 torr).
100
Q: State the two primary variables related by the Clausius-Clapeyron equation.
A: Vapor Pressure (P) and Temperature (T).
100
Q: Define Molarity (M) and provide its standard units.
A: Moles of solute per Liter of solution (mol/L).
100
Q: A solution contains 5.0g of salt in 45.0g of water. What is the mass %?
A: 10% (5g salt / 50g total solution × 100).
200
Q: Define the Triple Point on a phase diagram.
A: The specific T and P where solid, liquid, and gas coexist in equilibrium.
200
Q: Which has a higher vapor pressure at 25°C: Water or Diethyl Ether (C4H10O)? Why?
A: Diethyl Ether, because it has weaker IMFs (no H-bonding).
200
How much heat (q) is needed to melt 2.0 moles of ice? (ΔHfus=6.02 kJ/mol).
A: 12.04 kJ (2.0mol×6.02kJ/mol).
200
Q: Which concentration unit is independent of temperature and why?
A: Molality (m), because mass does not change with temperature.
200
Q: Word Prob: You have 25.0 g of a 10.0% (w/w) sugar solution. If you add 75.0 g of pure water to this solution, what is the new mass percent of the sugar?
A: 2.50%.
Step 1: Calculate initial solute (0.10×25.0 g=2.5 g sugar).
Step 2: Divide by new total mass (2.5 g sugar/[25.0 g+75.0 g]×100).
300
Q: What unique state of matter is formed at the Critical Point?
A: A Supercritical Fluid (where liquid/gas densities are equal).
300
Q: As the strength of Intermolecular Forces increases, what happens to the ΔHvap?
A: It increases (more energy is needed to pull molecules apart).
300
Q: Why does a liquid boil at a lower temperature at the top of a mountain than at sea level? Answer in terms of Vapor Pressure and the Clausius-Clapeyron relationship.
A: At high altitudes, atmospheric pressure is lower. Boiling occurs when the liquid's vapor pressure equals the external pressure. According to the Clausius-Clapeyron relationship, the liquid reaches this lower required vapor pressure at a significantly lower temperature.
300
Q: Define Mole Fraction and state what the sum of all fractions in a solution must equal.
A: nsolute/ntotal; The sum must always equal 1.
300
Word Prob: Find the molality of 6.44g biphenyl in 91.6g benzene.
A: 0.457 m (0.0418mol/0.0916kg).
400
Q: Word Prob: In a heating curve, name the two specific regions where Kinetic Energy remains constant.
(Hint: Where does temperature remain constant?)
A: The plateaus (Melting and Boiling points).
400
Q: Ethanol (CH3CH2OH) has a ΔHvap of 38.6 kJ/mol, while Dimethyl Ether (CH3OCH3) has a ΔHvap of 21.5 kJ/mol. Even though they have the same molecular formula, explain this 17 kJ difference in terms of specific Intermolecular Forces and how this affects their Vapor Pressure.
A: Ethanol contains an -OH group and can form strong Hydrogen Bonds, whereas Dimethyl Ether only has weaker Dipole-Dipole forces. Because Ethanol's "sticks" are stronger, it requires more energy to vaporize, resulting in a significantly lower vapor pressure at the same temperature.
400
Q: Word Prob: Setup the 2-point equation to find P2 if T1=35°C, P1=100 mmHg, and T2=55°C
A: ln(100/P2)=RΔHvap(3281−3081).
(various forms possible)
400
Q: A chemist prepares a 1.00 M solution of NaCl and a 1.00 m solution of NaCl at 20°C. If she then heats both solutions to 80°C, which concentration value will change (m or M), and will it increase or decrease? Explain why.
A: The Molarity (1.00 M) will decrease. As the temperature rises, the volume of the solution expands (the denominator increases), making the molarity lower. The molality (1.00 m) remains constant because mass does not change with temperature.
400
Q: Word Prob: A 0.258 m sucrose solution has a density of 1.0173 g/mL. What is the Molarity?
A: 0.243 M (Requires calculating total mass and volume).
500
Q: At 1 atm, which substance can undergo sublimation: H2O or CO2?
A: CO2 (its triple point is above 1 atm).
500
Q: Why does vapor pressure increase exponentially with temperature?
A: The fraction of molecules with enough KE to escape grows exponentially (Boltzman dist.).
500
Q: In the Clausius-Clapeyron equation, what is the exact value and units of the constant R?
A: 8.314 J/(mol⋅K).
500
A solution is 30% H2O2 by mass. Calculate the mole fraction of H2O2.
A: 0.185 (0.88 mol H2O2 per 4.77 total mol).
500
Q: Word Prob: You have 30% H2O2 (density 1.11 g/mL). Calculate the Molarity (M).
A: 9.79 M (0.882 mol / 0.0901 L).