Chapter 18: Solutions

Solubility

maximum amount of solute in a given amount of solvent at a specific temperature

• increases proportional to temperature (amount)

• rate that solute dissolves also increases proportional to temperature

Saturation

Maximum solute that can be dissolved (max solubility) in a solvent is dissolved (at a specific temperature)

Unsaturated

Less solute than can theoretically be dissolved in a solvent

Supersaturated

More solute is dissolved than the given solubility

Factors affecting solubility

1. Temperature

   1. solubility increases proportionally

   2. but gases dissolve faster in cold H₂O because a higher temperature means a higher Kinetic Energy, so particles escape faster

2. Agitation

   1. solubility increases proportionally

      1. Ex: stirring sugar in water

3. Particle size

   1. smaller particles dissolve faster

   2. inversely proportional to solubility

Henry’s Law

At a constant temperature, the solubility of a gas is directly proportional to the pressure to gas above the liquid

• S₁/P₁=S₂/P₂

Dilute

more solvent than solute

Concentrated

more solute than solvent

Units of concentration

• Molarity (M)

• %(m/v) aka %(mass/volume)

• %(v/v) aka %(volume/volume)

• mole fraction

• molality (m)

• normality

Molarity (M)

(moles solute)/(liters solution)

%(m/v)

(grams solute)/(mL solution) x 100

%(v/v)

(volume solute)/(volume solution) x 100

• units of volume for solute and solution must be the same

mole fraction

(moles solute)/(moles of solution)

molality (m)

(moles solute)/(kg solvent)

Making dilutions

M₁V₁ = M₂V₂

Stock solution

higher M solution in the dilution formula

Colligative properties

properties of solutions that depends on the number of solute particles

1. Boiling point elevation

   1. not only break IMFs between solvent molecules, but also between solute and solvent molecules

2. Freezing point depression

   1. same as BP elevation but must stabilize extra IMFs

3. Vapor pressure lowering

   1. because BP elevation, less vapor is produced so VP decreases

Van’t Hoff Factor (i)

number of particles that a solute dissociates into

• for biomolecules/organic molecules, i always = 1

K_b

molal BP elevation constant

K_f

molal FP depression constant

K_b and K_f for H₂O

• K_b: 0.512 degrees C/m

• K_f: 1.86 degrees C/m