Lecture 5 CHEM 1212 F25 SIEMER – Colligative Properties and Colloids

Flashcards covering key concepts from Lecture 5: colligative properties, Raoult’s Law, molarity/molality, osmotic pressure, semipermeable membranes, electrolytes, and colloids.

What is a colligative property?

A property that depends on the number of solute particles in a solution, not the identity of the solute.

Name the four colligative properties discussed in this lecture.

Vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

What does the van’t Hoff factor i represent?

The ratio of the number of solute particles in solution to the number of formula units dissolved; for electrolytes it reflects the degree of dissociation.

What is the predicted van’t Hoff factor for NaCl (sodium chloride)?

2 (NaCl dissociates into two ions: Na+ and Cl−).

What is the predicted van’t Hoff factor for C6H12O6 (glucose)?

1 (glucose does not dissociate in solution).

What is the predicted van’t Hoff factor for CaCl2 (calcium chloride)?

3 (CaCl2 dissociates into Ca2+ and 2 Cl−, giving 3 particles).

What is the predicted van’t Hoff factor for Na3PO4 (sodium phosphate)?

4 (Na3PO4 dissociates into 3 Na+ and PO4^3−, totaling 4 particles).

Why are measured van’t Hoff factors often lower than predicted?

Due to incomplete dissociation and ion pairing, which lowers the effective concentration (activity) of particles.

State Raoult’s Law for an ideal solution with a nonvolatile solute.

Psolution = Xsolvent × P°solvent, where Xsolvent is the mole fraction of the solvent.

What effect does adding a nonvolatile solute have on the solvent’s vapor pressure?

It lowers the solvent’s vapor pressure (vapor pressure lowering).

What is the boiling point elevation equation?

ΔTb = i × Kb × m (molality m, Boiling-point elevation constant Kb).

What is the freezing point depression equation?

ΔTf = i × Kf × m (molality m, Freezing-point depression constant Kf).

What does molality (m) measure?

Moles of solute per kilogram of solvent; it is independent of temperature and pressure.

What does molarity (M) measure?

Moles of solute per liter of solution; it can depend on temperature due to volume changes.

Define mass percent (m/m%), mass/volume percent (m/v%), and volume percent (v/v%).

m/m% = (mass solute / mass solution) × 100%; m/v% = (mass solute / volume solution) × 100%; v/v% = (volume solute / volume solution) × 100%.

Why are volume-based concentration units limited?

Because concentrations that relate solute amount to solvent volume can change with external conditions like pressure and temperature.

What is mole fraction X_A and what is a key property about it?

XA = nA / (nA + nB + …); mole fractions sum to 1 for all components.

Which concentration measure is independent of temperature and pressure?

Molality (m).

What is osmosis?

Diffusive net transfer of solvent through a semipermeable membrane from a region of higher solvent concentration to lower solvent concentration.

What is osmotic pressure?

The pressure required to prevent osmosis; for solutions, π = i × M × R × T (using molarity M).

What is a semipermeable membrane?

A membrane that allows passage of some molecules (usually solvent) but restricts others (solutes) based on size, polarity, charge, etc.

How do red blood cells respond to hypotonic, isotonic, and hypertonic solutions?

Hypotonic: swell and may lyse; isotonic: normal volume and shape; hypertonic: shrink and may die.

How can osmosis be reversed?

By applying external pressure greater than the osmotic pressure (reverse osmosis).

Which colligative properties are commonly used to determine molar mass?

Freezing point depression (ΔTf) and osmotic pressure (Π).

Outline a typical procedure to determine molar mass from colligative properties.

1) Add known mass of solute to known mass of solvent or known volume of solvent. 2) Measure ΔTf or Π. 3) Calculate concentration (m or M). 4) Determine moles of solute. 5) Compute molar mass from mass and moles.

What is the Tyndall effect?

Visible scattering of light by colloidal-sized particles, making a beam visible in the dispersion.

Differentiate among solution, colloid, and suspension.

Solution: homogeneous; colloid: dispersed particles that do not settle; suspension: heterogeneous with particles that settle.

Provide examples of colloids and identify dispersed phase and medium.

Examples: aerosol (gas in solid or liquid), sol (solid in liquid, e.g., starch in water), emulsion (liquid in liquid, e.g., milk), gel (solid in liquid, e.g., jelly), foam (gas in liquid, e.g., whipped cream; also foam gas in solid like pumice).

What is the effect of solute concentration on vapor pressure?

Adding solute lowers the vapor pressure of the solvent (Raoult’s Law).

Which of the following is not a strong electrolyte?

HF (hydrofluoric acid) is a weak electrolyte, while HCl, NH4NO3, and NaOH are strong electrolytes.