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Titrations and Solubility . By Courtney Page and Jake Estes. The Basics of Titrations. A titration is when a solution of accurately known concentration is gradually added to another solution of unknown concentration until the chemical reaction between the two is complete.
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Titrations and Solubility By Courtney Page and Jake Estes
The Basics of Titrations • A titration is when a solution of accurately known concentration is gradually added to another solution of unknown concentration until the chemical reaction between the two is complete. • Equivalence point is the point when the reaction is complete. (In an acid-base titration the EQ point is when the moles of H+ from the acid equals the moles OH- from the base.) • Indicator is the substance that changes color at the equivalence point.
How it works • Slowly add base to the unknown acid until the indicator changes color. Before After
When do we use titrations? • We use titrations to find different aspects of the experiment. You could be asked to find the concentration of a solution before the EQ point, at EQ point, or after EQ point. You could also be asked to find the pH at a given point. • To do this, we use a BCA or ICE table. • But what’s the difference??
BCA Tables • Use a BCA table when titrating a strong acid by a strong base.
ICE Tables • Use an ICE table when titrating a weak acid and weak base.
Finding EQ point through calculations • Strongacid and strongbase No EQ, because it is 100% ionized. pH=7 • Strongacid and weakbase The base is neutralized, need Ka for conjugate acid EQ • Weakacid and strongbase The acid is neutralized, need Kb for conjugate base EQ • Weakacid and weakbase Depends on the strength of both. Could use any of the above 3 ways to find EQ.
What is Ka and Kb? • Ka for the example reaction HA(aq) + H2O(l) H3O+(aq) + A-(aq) [H3O+][A-] Ka= [HA] • Kb for the example reaction B(aq) + H2O(l) HB+(aq) + OH-(aq) [HB+][OH-] Kb= [HB]
A 25.0 mL sample of 0.20 M HCl is titrated with 0.10 M NaOH • What is the pH when 0 mL of titrant has been added? • What is the pH when 60 mL of titrant has been added?
Sample Multiple Choice #1 • Ka the acid dissociation constant, for an acid is 9 x 10-4 at room temperature. At this temperature, what is the approximate percent dissociation of the acid in a 1.0 M solution? • A) 0.03 % • B) 0.09 % • C) 3 % • D) 5 % • E) 9 %
Sample Multiple Choice #2 • What is the ionization constant, Ka, for a weak monoprotic acid if a 0.30 molar solution has a pH of 4.0? • A) 9.7 x 10-10 • B) 4.7 x 10-2 • C) 1.7 x 10-6 • D) 3.0 x 10-4 • E) 3.3 x 10-8
The Basics of Solubility • Solubility is the amount of stuff that can dissolve in a given amount of solvent at a given temperature. (Units are usually mols/L or g/L) • Solubility product is the equilibrium constant (Ksp) for a given dissolution at a given temperature. • The common-ion effect is the shift in equilibrium caused by an addition of a compound having an ion in common with the dissolved substance.
What’s Ksp? • Ksp = [products] [reactants] • (except when reactants are solids, you do not include them in your Kspequation)
The Ksp of Pb(OH)2 (MM=241 g/mol) is 1.2 x 10-15. • What is the solubility in units of mols/L and g/L?
Sample Multiple Choice #3 • Determine the OH- (aq) concentration in 1.0 M aniline (C6H5NH2) solution. (Kb for aniline is 4.0 x 10-10) • A) 2.0 x 10-5 M • B) 4.0 x 10-10 M • C) 3.0 x 10-6 M • D) 5.0 x 10-7 M • E) 1.0 x 100 M
Factors that affect solubility • Solubility depends on what else is in the solution. • The common-ion effect. • Acids and bases.
Review of Titrations and Solubility • Titrations are a laboratory technique to determine the concentration of an acid or base solution. • An indicator is used in a titration to indicate the equivalence point, or end point. • The solubility product constant is Ksp. • Many factors can effect solubility of solutions.
