Solutions and Solubility

Solutions

• 1.

Solution – a homogenous mixture of a solute dissolved in a solvent. The solubility (ability to dissolve) of a solute in a solvent is dependent on the Temperature For solid solutes: as temperature increases, solubility increases. For gas solutes: as temperature increases, solubility decreases.

Solutions con

2. Pressure For solid solutes: as pressure increases, solubility remains the same.

For gas solutes: as pressure increases, solubility increases 3. Nature of Solute/Solvent “Like dissolves in like.”

Solute/Solvents cont.

Solubility Summary Solute Type Nonpolar solvent Soluble (soap) Nonpolar (Fat Grease) Polar Ionic (salt) Insoluble insoluble Polar Solvent Insoluble (water) Soluble (water) Soluble (water) High solubility-soluble Low solubility-insoluble

Solubility Curves

• • • • Shows the number of grams of solute that can be dissolved in 100.g of water at temperatures between 0 degrees C and 100 degrees C.

Each line represents the maximum amount of that substance that can be dissolved at a given temperature.

Lines that show an increase in solubility as temperatures increase represent solids being dissolved in water.

Lines that show a decrease in solubility as temperatures increase represent gases being dissolved in water. These are NH3, SO2, and HCl

There are three types of solutions

• • • 1. An unsaturated solution is a solution in which more solute can be dissolved at a given temperature.

2. a saturated solution is a solution containing the maximum amount of solute that will dissolve at a given temperature.

3. a supersaturated solution is a solution that contains more solute than would dissolve in a saturated solution at a given temperature.

Solubility Rules

• • Not all ionic compounds are water soluble There are some general rules for compounds that are water soluble: – Group 1 ionic cmpds and ammonium (NH 4 + ) are always water soluble – Group 17 ionic cmpds are water soluble except when paired with Ag, Pb, and Hg ions – See Table F for full rules and exceptions

Examples

• • • • • • AgNO3 = water soluble AgCl = insoluble Na 2 S = soluble NaCl = soluble CaCO AlPO 4 3 = insoluble = insoluble

Precipitation reactions

• Recall that double-replacement reactions have the general formula: AB(aq) + CD(aq) AD(aq) +CB(s) A precipitation reaction will take place if one or both of the products is listed as an insoluble solid.

2KI(aq) + Pb(NO 3 ) 2 (aq) -> 2KNO 3 (aq) + PbI(s) • • • • • KI = soluble Pb(NO 2KNO 3 3 ) 2 = soluble (aq) = soluble PbI(s) = insoluble solid In a precipitation reaction, two clear aqueous solutions are combined to form a cloudy, solid precipitate that can be collected by filtration.

Concentrations of Solutions

• Because solutions are homogeneous mixtures, their compositions can vary. Sometimes it is adequate to refer to a solution as dilute or concentrated. These are qualitative descriptions of concentration. It is more precise to describe the concentration of solutions in quantitative measures.

Molarity

• Molarity (M)- number of moles of solute in 1L of solution. Table T Molarity= moles of solute liters of solution

Calculating molarity

Highly concentrated HCl(aq) has a molarity of 12M This means there are 12 moles of HCl dissolved in 1 Liter of water 12M = 12 moles 1 Liter

Sample Problem

• How many grams of NaCl must be added to 1 Liter of water to make a 3M solution?

• 3M = 3 moles NaCl I Liter 1 mole NaCl = 58g x 3moles = 174g NaCl

Parts Per Million

• Parts per million is another way of measuring the concentration of a solution • The general formula is: Parts per million = grams of solute x 1,000,000 grams of solution

Parts per million example

• 5 grams of NaCl is dissolved in 2.5L of water. What is the concentration of NaCl in parts per million (ppm)?

• Remember 1mL water = 1 g ppm = 5g NaCl x 1,000,000 = 2000 ppm 2500 g H 2 O

Colligative Properties

Freezing and boiling points of water change when salts (nonvolatile solutes) are added. Colligative properties depend on the number of particles in a substance 1. Freezing Point Depression: when any salt is added to water, the freezing point of the water decreases.

Freezing point of pure water= 0 degrees C Freezing point of salt water (NaCl solution)= -21 °C (-6°F) under controlled lab conditions In the real world, on a real sidewalk, sodium chloride can melt ice only down to about -9 °C (15°F)

Molecular vs. Ionic

• • • When one mole of sucrose is dissolved in water, one mole of particle is produced in solution: C 12 H 22 O 11 (s) C 12 H 22 O 11 (aq) When one mole of an ionic substance is dissolved in water, the results are different. The ionic substance dissociates into individual ions: NaCl(s) Na(aq) + Cl (aq) The greater number of ions, the lower the freezing point.

Boiling Point Elevation

• • • When any salt is added to water, the boiling point of the water increases.

Boiling point of pure water= 100 ° C Boiling point of salt water solution increases