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 Strong acid and strong base No EQ, because it is 100% ionized. pH=7 Strong acid and weak base The base is neutralized, need K a for conjugate acid EQ Weak acid and strong base The acid is neutralized, need K b for conjugate base EQ Weak acid and weak base Depends on the strength of both. Could use any of the above 3 ways to find EQ.

What is K a and K b ? K a for the example reaction HA (aq) + H 2 O (l)  H 3 O + (aq) + A - (aq) [H 3 O + ][A - ] K a = [HA] K b for the example reaction B (aq) + H 2 O (l)  HB + (aq) + OH - (aq) [HB + ][OH - ] K b = [HB]

How to navigate through titration problems

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 K a the acid dissociation constant, for an acid is 9 x 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, K a, for a weak monoprotic acid if a 0.30 molar solution has a pH of 4.0? A) 9.7 x B) 4.7 x C) 1.7 x D) 3.0 x 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 (K sp ) 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 K sp ? K sp = [products] [reactants] (except when reactants are solids, you do not include them in your K sp equation)

The K sp of Pb(OH) 2 (MM=241 g/mol) is 1.2 x 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 (C 6 H 5 NH 2 ) solution. (K b for aniline is 4.0 x ) A) 2.0 x M B) 4.0 x M C) 3.0 x M D) 5.0 x M E) 1.0 x 10 0 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 K sp. Many factors can effect solubility of solutions.

Now some example AP questions…