Solutions

Dr. Muon HazlettMandeville High School

Solutions (solns)

• Definition:– It is a homogeneous mixture– It is made up of a solute (what is being dissolved)

and a solvent (what the solute is dissolved in)• Solute + Solvent = Solution

– It can be a gas, liquid, or a solid or even a combination of these phases

– It does not separate into its parts on its own

Solubility• If a solute dissolves in a solvent, it is soluble– Solubility affected by Temperature, Pressure,

Agitation, etc.• The process in which the solvent particles

surround the solute and “cause” it to dissolve is called solvation– If this occurs in water – called hydration– Rule of Solvation – “Like Dissolves Like”

• Rule of Solvation– “Like Dissolves Like”• Refers to polar solutes dissolving in polar solvent, but

not in nonpolar solvents• This is due to the charges found on the molecules and

opposite charges attracting each other• If it does mix – said to be miscible, and if doesn’t –

immiscible– Immiscible mixtures are not solutions since they will filter or

separate on their own – Like oil and water

• If the solute is an ionic compound and is dissolving completely, it is breaking down into its cations and anions– This is called ionization– Dependent upon the dissociation energy (Do), the

energy needed to break apart the ionic compound– This is a part of the solvation process

Saturation

• Saturation refers to the level of solute in the solvent

• If the solvent can not dissolve any more solute, then the two are in equilibrium and the solution is saturated

• If the solute “falls” out of the solvent – oversaturation

• If not enough solute - undersaturated

Solution Types• Suspension - the solute will filter out of the

solvent on its own (dirt in water)• Thixotrope - the solute and solvent adt as a

solid until agitated, then it acts as a liquid• Emulsion - a liquid with a solid solute

dissolved in it • Aerosol - a colloid using a gas as a propellant • Colloid - a heterogeneous mixture, the solute

does not settle from solvent

Colligative Properties of Solns

• Properties of solns that depend on the number of solute particles in the solvent1. Vapor Pressure – affecting evaporation rates.

This is amount of solvent found in gas phase above the solution. Lowers vapor pressure.

2. Boiling Point – more solute, higher the Bp

3. Freezing Point – more solute, lower Fp

4. Osmotic Pressure – affects the passing through of semi-permeable membranes

Concentration [C]

• Concentration symbolized by brackets [ ]• Refers to amount of solute in the solvent• Common measures are ppm (parts per

million) or ppb (parts per billion)

Concentration Formula• General:

1. C = mass of solute mass of soln

2. % C = mass of solute x 100 mass of soln

3. C ppm = mass of solute x 106

mass of soln

4. mass soln = mass of solute C

5. mass solute = C x mass solvent (1 - C)

6. Soln = solute + solvent

7. Volume % = vol. solute x 100 vol. soln

Molarity (M)

• Gives the number of moles of solute per liter of soln

•M = mols of solute = (n) liters of soln V

*Remember n = mass/molar mass

Molality (m)• Gives the number of mols of solute per kg of

solvent

• m = mols solute = n kg solvent kg

*Remember – 1 L = 1 kg 1 ml = 1 g = 1 cm3

Dilution• Mixing one substance into another to lower

the concentration levels• M1V1 = M2V2

• Molarity x Volume initial = Molarity x Volume final• Rearrange the formula to determine individual

variables• For multiple solns:–(M1V1) + (M2V2) = Mdiluted solnVdiluted soln

Mole Fraction• Ratio of mols of solute or solvent to the total

number of mols in soln– XA = solvent mol fraction = nA__

nA + nB

XB = solute mol fraction = ___nB__

nA + nB

Calculating Colligative Properties

• Vapor Pressure Lowering:

• Δ PV = XB PA◦

• Where Δ PV is change in vapor pressure; XB is solute mol fraction; and PA

◦ is vapor pressure of pure solvent found in a reference source like the CRC

• Boiling Point Elevation:

Δ BP = i KB m Where i is the van’t Hoff constant;

KB is the boiling constant from a reference source (H2O is 0.513); and m is the molality of the solution The van’t Hoff constant is the degree of

dissociation for a substance (the number of parts it breaks into) and we can use 2 if an ionic compound, 1 if it is covalent

• Freezing Point Depression:

Δ FP = i Kf m

Where i is van’t Hoff constant; FP is freezing point constant of pure solvent (with no solute

in it), (H20 is 1.86); and m is molality of solution

• Osmotic Pressure (Π)

Π = i M R T

Where i is van’t Hoff constant; M is the molar concentration; R is the gas constant (0.0821 L atm/mol K); and T is temperature in Kelvin

Henry’s Law• For a gas dissolved in a solution• At a constant T, amt. of gas dissolved in a liquid is

directly proportional to the partial pressure of that gas in equilibrium with that liquid

S1 = S2

P1 P2

Where S is initial and final solubility in g per L;and P is initial and final pressure usually in

atmospheres (atm)