lab quiz - colligative properties

What are colligative properties?

Colligative properties are physical changes in a solvent that depend on the number of solute particles in solution, not their identity. These include freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.

What are the key formulas used in colligative properties?

Freezing Point Depression: ΔTf = i × Kf × m; Boiling Point Elevation: ΔTb = i × Kb × m

What does each variable in the colligative property formulas represent?

ΔTf / ΔTb = change in freezing or boiling point; i = van’t Hoff factor; Kf / Kb = freezing or boiling point constant; m = molality

Why is molality used instead of molarity in these calculations?

Molality uses mass (kg), which doesn’t change with temperature. Molarity uses volume, which does.

How do you calculate molality?

m = moles of solute / kg of solvent

How do you calculate moles of solute?

mol = molality × kg of solvent

How do you calculate molar mass from lab data?

Molar mass = mass of solute (g) / moles of solute

What were the solutes and solvent used in your experiment?

Solvent: Water (H₂O); Solutes: Glucose (i = 1), KNO₃ (i = 2)

What boiling point elevation data was used for glucose?

ΔTb = 1.0°C; Kb = 0.512°C·kg/mol; i = 1; m = 1.953 mol/kg

How was glucose’s molar mass calculated using boiling point data?

m = 1.953 mol/kg; mol = 1.953 × 0.050 = 0.09765 mol; Molar mass = 10 g / 0.09765 mol = 102.4 g/mol

What is the theoretical molar mass of glucose?

180.16 g/mol

What was the % error in the experimental molar mass of glucose?

43.2%

What freezing point depression data was used for KNO₃?

ΔTf = 9.0°C; Kf = 1.86°C·kg/mol; i = 2; m = 2.42 mol/kg

How was KNO₃’s molar mass calculated?

m = 2.42 mol/kg; mol = 2.42 × 0.050 = 0.121 mol; Molar mass = 10 g / 0.121 mol = 82.64 g/mol

What is the theoretical molar mass of KNO₃?

101.11 g/mol

What was the % error in the experimental molar mass of KNO₃?

18.2%

Why was boiling point used for glucose and freezing point for KNO₃?

Boiling was easier to measure for glucose; freezing point gave a larger and clearer ΔTf for ionic KNO₃.

What were key sources of error in the lab?

Thermometer touching glass, poor mixing, incomplete dissolution, supercooling or no crystallization

Why does CaCl₂ melt ice better than NaCl?

CaCl₂ makes 3 ions (Ca²⁺ + 2Cl⁻); NaCl makes 2. More ions = stronger freezing point depression.

What is the van’t Hoff factor (i) for each solute?

Glucose: i = 1; KNO₃: i = 2; CaCl₂: i = 3