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” 2

Dissociation When an ionic substance dissolves in water, the solvent separates the individual ions from the crystal. This process is called dissociation. 3

Dissociation This occurs because water is POLAR – it has poles: a partial positive (δ + ) end and a partial negative (δ - ) end O HH δ+δ+ δ+δ+ δ-δ- O HH O H H HH O O H H Na + O HH O H H HH O O H H Cl - Solvation – the surrounding of ions by H 2 O molecules to help stabilize the ions -Keeps the ions from recombining NaCl(aq)  Na + (aq) + Cl - (aq) 4

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 Electrolytes 5 Ex: CH 3 COOH(aq) CH 3 COO - (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 6

Identifying Strong & Weak Electrolytes & Nonelectrolytes Strong electrolytes 1) Strong acids 2) Strong bases 3) Ionic compounds Weak electrolytes 1)Weak acids 2)Weak bases Nonelectrolytes Molecular compounds, except acids and bases 7 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. 9

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 ***All compounds of alkali metals (Group 1A) and of NH 4 + ion are soluble *** 10 Memorize!

Solubility Rules Classify the following as soluble or insoluble: a) CuCO 3 b) Mg(OH) 2 c) CaCl 2 d) SrSO 4 e) (NH 4 ) 3 PO 4 f) Fe 2 S 3 11

Electrolytic Properties ≠ Solubility Do not confuse extent of solubility with strong or weak electrolyte Examples: Acetic acid (CH 3 COOH or HC 2 H 3 O 2 ): very soluble in water but weak electrolyte because most remains in form of molecule, not ions, in solution BaCl 2 : not very soluble in water but strong electrolyte because what does dissolve dissociates completely 12

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) = 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 Ex: 2.1 M MgCl 2 = 2.1 mol MgCl 2 /L MgCl 2 solution (read as:) “2.1 molar solution of MgCl 2 ” mol MgCl 2 1 L solution 2.1 mol MgCl 2 or

Using Molarities in Stoichiometric Calculations 14

Molarity: Examples 1.Calculate the molarity of 142 mL of an aqueous solution containing g NaNO 3. 2.Calculate the molarity of a g sample of NaHCO 3 dissolved in enough water to make 250 ml of solution. 3.How many grams of KOH are needed to make 500 ml of a 4.8M solution? M

Dilution: Adding water If an original solution is diluted, the molarity of the new solution can be determined from the equation: M 1  V 1 = M 2  V 2 where M 1 and M 2 are the molarity of the initial (concentrated) and final (dilute) solutions, respectively, and V 1 and V 2 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) 16

Dilution: Example If a stock solution of 25.0 mL of 1.00 M HCl is diluted to a total of mL, what is the final concentration of the HCl solution? M 1  V 1 = M 2  V 2 M 2 = M 1 V 1 V 2 M 2 = (1.00 M)( L) L M 2 = M 17

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