Acid-Base Titration & pH

16-1 Objectives 1.Describe the self-ionization of water 2.Define pH and give the pH of a neutral solution at 25 o C 3.Explain and use the pH scale 4.Given concentrations of H+ & OH-, calculate pH 5.Given pH, calculate concentrations of H+ & OH-

Self-Ionization of Water Two water molecules produce a hydronium ion and a hydroxide ion by transfer of a proton Conductivity shows concentrations of H 3 O + and are 1.0 x mol/L at 25 o C K w = [H 3 O + ][OH - ] = 1.0 x M 2

Self-Ionization of Water

Neutral, Acidic, and Basic Solutions Neutral solutions: have equal [H+] and [OH-] Acidic Solutions: have greater [H+] than [OH-] Basic Solutions (alkaline): have greater [OH-] than [H+]

Calculating [H+] and [OH-] K w = [H+] [OH-] If given the concentration of one ion, the concentration of the other can be calculated

Example A 1.0 x M solution of HNO 3 has been prepared for a lab experiment. Calculate [H 3 O + ] and [OH - ] HNO 3 is a strong acid so assume 100% dissociation HNO 3 (l) + H 2 O(l)  H 3 O + (aq) + OH - (aq)

Solution Given: K w = [H + ] [OH - ] 1.0 x 10-4 M HNO 3 Find concentration of H+:

Solution Solve algebraically: Substitue:

pH Scale pH – pouvior hydrogène or “hydrogen power” pH is defined as the negative logarithm of the hydronium ion concentration [H 3 O + ] To calculate pH pH = -log[H 3 O + ] or pH = -log[H + ]

pH Scale To calculate pOH pOH = -log[OH - ] At 25 o C, [H + ] = [OH - ] = 1.0 x = pH =7 If [H + ] > [OH - ] the solution is acidic If [OH - ] > [H + ] the solution is basic

Examples: 1.What is the pH of 1.0 x M NaOH solution? 2.What is the pH of a solution if the [H 3 O + ] is 3.4 x M? 3.Determine the [H 3 O + ] of an aqueous solution that has a pH of The pH of a solution is found to be a)Find [H 3 O + ] b)Find [OH-] c)Is the solution acidic or basic?

16-2 Objectives Describe how a pH indicator works Explain an acid-base titration Calculate molarity of a solution from titration data

Indicators and pH Meters Acid-Base Indicator – compounds whose colors are sensitive to pH In acidic solutions, In - ions act as B-L bases In basic solutions, OH - react with H+ from indicator

Indicators Transition Interval – the pH range over which an indicator changes color Universal Indicators – combination of different indicators pH meters – determine pH by measuring the voltage difference between two electrodes

Titrations The controlled addition of and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentration Equivalence Point – the point in a titration where [H + ] and [OH - ] are present in chemically equivalent amounts End Point – the point of the titration at which the indicator changes color

Molarity and Titration Standard Solution – the solution that contains the precisely known concentration of a solute Primary Standard – a highly purified solid compound used to check the concentration of the known solution in a titration

Calculating Molarity 1.Write balanced chemical equation and determine chemical equivalents 2.Determine moles of acid or base from the known solution used during the titration 3.Determine moles of solute of the unknown used during the titration 4.Determine molarity

Example: in a titration, 27.4 mL of M Ba(OH)2 is added to a 20.0 mL sample of HCl solution of unknown concentration…find the molarity of the acid solution

Solution