Section 16.2 Determining the Acidity of a Solution 1.To understand and determine pH and pOH 2.To learn methods for measuring pH of a solution Objectives

Section 16.2 Determining the Acidity of a Solution Water as an Acid and a Base Amphoteric substance – can behave as either and acid or a base; water is the most common. Ionization of water – water molecules react with each other to form hydronium ions and hydroxide ions: H + acid base transfer hydronium hydroxide H 2 O(l) + H 2 O(l)  H 3 O + (aq) + OH - (aq) This forward reaction happens very little; in pure water, only a tiny amount of hydronium and hydroxide ions exists. At 25 o C, the actual concentrations are [H 3 O + ] = [OH - ] = 1.0 x M (a very small number…)

Section 16.2 Determining the Acidity of a Solution The Ion-Product Constant for Water (K w ) The mathematical product of the hydronium and hydroxide concentrations is always constant! To simplify, we write H 3 O + as H+; brackets mean concentration (molarity): K w = [H + ] [OH - ] = 1.0 x M The point: In any aqueous solution at 25 o C, no matter what it contains, this product remains constant: if [H + ] goes up, [OH - ] must go down.

Section 16.2 Determining the Acidity of a Solution Neutral, Acidic and Basic Solutions Neutral solution - a solution in which the concentrations of hydronium/hydrogen and hydroxide ions are equal: [H + ] = [OH - ] Acidic solution – a solution in which the concentration of hydrogen ions is greater that the concentration of hydroxide ions: [H + ] > [OH - ] Basic solution – a solution in which the concentration of hydroxide ions is greater than the concentration of hydrogen ions: [H + ] < [OH - ] In each case, K w = [H + ] [OH - ] = 1.0 x M (constant)

Section 16.2 Determining the Acidity of a Solution The pH scale The “p scale” is used to express small numbers. pH =  log [H + ] Because the pH scale is a log scale based on 10, the pH changes by 1 for every power of 10 change in the [H + ].

Section 16.2 Determining the Acidity of a Solution A. The pH Scale pOH scale pOH =  log [OH  ] pH + pOH = 14.00

Section 16.2 Determining the Acidity of a Solution B. Measuring pH Indicators – substances that exhibit different colors in acidic and basic solutions –In an acidic solution the indicator will be in the HIn form. –In a basic solution the indicator will be in the In  form. Other methods –Indicator paper –pH meter

Section 16.2 Determining the Acidity of a Solution Useful Ph Ranges for Several Common Indicators

Section 16.2 Determining the Acidity of a Solution Calculating the pH of Strong Acid Solutions (skip) A strong acid dissociates completely in water; therefore, the ions formed have the same concentration as the acid (unless the acid yields more than one proton per formula unit). What is the pH of 1.0 M HCl? 1.0 M HCl  1.0 M H + and 1.0 M Cl - pH = -log [H + ] = -log (1.0) = 0 (very acidic) What is the pH of 0.10 M HNO 3 ? 0.10 M HNO 3  0.10 M H + and 0.10 M NO 3 - pH = -log [H + ] = -log (0.10) = 1.00