Understanding Solutions: Mixtures, Types, and Colligative Properties in Chemistry

1. Solutions Dr. MuonHazlett Mandeville High School

2. 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

3. 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”

4. 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

5. 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

6. 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

7. 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

8. Colligative Properties of Solns • Properties of solns that depend on the number of solute particles in the solvent • Vapor Pressure – affecting evaporation rates. This is amount of solvent found in gas phase above the solution. Lowers vapor pressure. • Boiling Point – more solute, higher the Bp • Freezing Point – more solute, lower Fp • Osmotic Pressure – affects the passing through of semi-permeable membranes

9. 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)

10. Concentration Formula • General: • C = mass of solute mass of soln • % C = mass of solute x 100 mass of soln • C ppm = mass of solute x 106 mass of soln

11. 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

12. 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

13. 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

14. 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) = MdilutedsolnVdilutedsoln

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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)