Solutions, Solubility Rules, and Molarity

Solutions Solutions are defined as homogeneous mixtures of two or more pure substances. Aqueous solution – solution in which water is the dissolving medium The solvent is present in greatest abundance. All other substances are solutes; they are dissolved in the solvent. Example: NaCl dissolved in water: NaCl = solute water = solvent Water can dissolve many ionic or molecular compounds – “universal solvent”

Dissociation When an ionic substance dissolves in water, the solvent separates the individual ions from the crystal. This process is called dissociation.

Dissociation This occurs because water is POLAR – it has poles: a partial positive (δ+) end and a partial negative (δ-) end δ- O H H δ+ δ+ O H H H H H Solvation – the surrounding of ions by H2O molecules to help stabilize the ions Keeps the ions from recombining NaCl(aq)  Na+(aq) + Cl-(aq) O H H H O Cl- O O Na+ O H H H H H H O O H H

Electrolytes An electrolyte is a substance that dissociates into ions when dissolved in water. May be strong or weak Strong electrolyte - completely dissociates into ions Solutions conduct electricity well Ex: HCl(aq) H+(aq) + Cl-(aq) Weak electrolyte – partially dissociates into ions, but some molecules remain intact Solutions conduct electricity poorly Ex: CH3COOH(aq) CH3COO-(aq) + H+(aq)

Electrolytes A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. Solutions do not conduct electricity

Identifying Strong & Weak Electrolytes & Nonelectrolytes Strong electrolytes Weak electrolytes Nonelectrolytes 1) Strong acids Weak acids Molecular compounds, except acids and bases 2) Strong bases Weak bases 3) Ionic compounds These are on the list below Any other acid or base NOT on the list below Memorize!

Solution Chemistry Pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution). If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction… the driving force! The driving force is what makes the reaction react (Ex: formation of a precipitate, gas, or liquid) Without a driving force, the reaction won’t occur 8

Precipitation Reactions When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed.

Solubility Rules Solubility of a substance is the amount that can be dissolved in a given quantity of solvent at a given temperature. Insoluble – the attraction between the ions in the solid is too great to separate the ions to any significant extent; substance doesn’t dissolve Memorize! Left off ***All compounds of alkali metals (Group 1A) and of NH4+ ion are soluble ***

Solubility Rules Classify the following as soluble or insoluble: c) CaCl2 d) SrSO4 a) CuCO3 e) (NH4)3PO4 b) Mg(OH)2 f) Fe2S3

Electrolytic Properties ≠ Solubility Do not confuse extent of solubility with strong or weak electrolyte Examples: Acetic acid (CH3COOH or HC2H3O2): very soluble in water but weak electrolyte because most remains in form of molecule, not ions, in solution BaCl2: not very soluble in water but strong electrolyte because what does dissolve dissociates completely

moles of solute liters of solution Molarity Concentration – tells how much solute is dissolved in a solvent Molarity is one way to measure the concentration of a solution. moles of solute volume of solution in liters Molarity (M) = Ex: 2.1 M MgCl2 = 2.1 mol MgCl2/L MgCl2 solution (read as:) “2.1 molar solution of MgCl2” Because the units of molarity are mol/L, we can use molarity as a conversion factor in stoichiometric calculations to interconvert: moles of solute liters of solution 2.1 mol MgCl2 1 L solution 1 L solution 2.1 mol MgCl2 or

Using Molarities in Stoichiometric Calculations 14

Molarity: Examples Calculate the molarity of 142 mL of an aqueous solution containing 13.31 g NaNO3. Calculate the molarity of a 5.623 g sample of NaHCO3 dissolved in enough water to make 250 ml of solution. How many grams of KOH are needed to make 500 ml of a 4.8M solution? 1.10 M

Dilution: Adding water If an original solution is diluted, the molarity of the new solution can be determined from the equation: M1  V1 = M2  V2 where M1 and M2 are the molarity of the initial (concentrated) and final (dilute) solutions, respectively, and V1 and V2 are the volumes of the two solutions. moles of solute before dilution = moles of solute after dilution (because all we are doing is adding solvent to the original solution)

Dilution: Example If a stock solution of 25.0 mL of 1.00 M HCl is diluted to a total of 100.0 mL, what is the final concentration of the HCl solution? M1  V1 = M2  V2 M2 = M1V1 V2 M2 = (1.00 M)(0.0250 L) 0.1000 L M2 = 0.250 M

Dilution: Example What is the molarity of a 500 ml HNO3 solution that was made from 35 ml of a 20. M stock solution?