Determination of Molar Mass by Freezing Point Depression

A series of vocabulary flashcards covering key concepts from the lab on determining molar mass through freezing point depression.

Colligative Property

Properties that depend on the number of solute particles in a solution, not their identity.

Freezing Point Depression

The decrease in the freezing point of a solvent when a nonvolatile solute is added.

Boiling Point Elevation

The increase in the boiling point of a solvent when a nonvolatile solute is added.

Kf

The freezing point depression constant, which is specific to each solvent.

Molality (m)

The number of moles of solute per kilogram of solvent.

Solution

A homogeneous mixture composed of two or more substances.

Tertiary Butyl Alcohol

A solvent with the formula C₄H₁₀O, noted for its use in freezing point depression experiments.

Supercooling

A phenomenon where a liquid is cooled below its freezing point without becoming solid.Then goes back up

Freezing Point of Pure Solvent

The temperature at which the pure solvent transitions from liquid to solid.

Graphical Analysis

A method of evaluating data by plotting temperature versus time to observe freezing and boiling behaviors.

Unknown Molar Mass

The molar mass of a solute that is determined through experiments involving freezing point depression.

Why freezing point drops

Because solute lowers vapor pressure → harder to freeze

What is ΔTf

(pure – solution)

What is molality

m=moles solute​/kg solvent

What is the purpose of this experiment?

To determine the molar mass of an unknown using freezing point depression

What happens to freezing point when solute is added?

It decreases

What type of property is freezing point depression?

Colligative property

What does colligative mean?

Depends on number of particles, not identity

What is Kf for t-butyl alcohol?

9.1 °C·kg/mol

What is i in this lab?

1 (non-dissociating solute)

Why do we ignore supercooling?

It gives incorrect freezing point

Why do you stir continuously?

To keep temperature uniform

Why record temperature every 30 sec?

To make a cooling curve

What indicates freezing point for pure solvent?

Flat/constant temperature

What indicates freezing point for solution?

Intersection of two lines

Why must solute fully dissolve?

Undissolved solid = wrong results

How do you calculate ΔTf?

Tf∘​−Tf​

How do you find molality?

m=​ΔTf​​/Kf

How do you find moles of solute?

moles=m×kg solvent

How do you find molar mass?

moles/grams​

If you add MORE solute, what happens to ΔTf?

It increases

If you accidentally add 5g instead of 2g, freezing point is:

LOWER

If your molar mass is too small, what might be wrong?

Solute dissociated (i ≠ 1)