Acids- Base Titration and pH

Aqueous Solutions and the Concept of pH

 When two molecules of water produce a hydronium and hydroxide ion by the transfer of a proton.  H 2 O + H 2 O   H 3 O + + OH -

 At 25  C the [H 3 O + ] = [OH - ]= M  So, the products of [H 3 O + ] and [OH - ] can be represented by:  [H 3 O + ] x [OH - ]  M x M = M 2  Where M 2 Is the Kw ( ionization constant of water) So, [H 3 O + ] x [OH - ]= Kw at 25  C

 Neutral solutions: [H 3 O + ] = [OH - ]= M  Acidic solutions: [H 3 O + ] > [OH - ]  Basic solutions: [H 3 O + ] < [OH - ]

 We can find the [H 3 O + ] and[OH - ] by using the formula:  [H 3 O + ] x [OH - ]= Kw= M 2 Practice p: 484

 It is the negative logarithm of the hydronium ion concentration pH= - log [H 3 O + ]

 Likewise, pOH It is the negative logarithm of the hydroxide ion concentration  pOH= - log [OH - ]  So, pH + p OH= 14 at 25  C

 [H 3 O + ]=10 -pH  Practice p: 487

 1- [H 3 O + ] x [OH - ]= Kw  2- pH= - log [H 3 O + ]  3- pOH= - log [OH - ]  4- pH + p OH= 14 at 25  C  5- [H 3 O + ]=10 -pH

Determining pH and Titrations

 1- Acid- Base indicators: are compounds whose colors are sensitive to pH. Indicators come in many different colors. The exact pH range over which an indicator changes color also varies.

 2-Universal indicators: the pH of solution can be determined by comparing the color it turns with the scale of paper.

3- pH meter: determines the pH of a solution by measuring the voltage between the electrodes that are placed in the solution. (It’s the most accurate way to determine the pH)

 Is the controlled addition 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.  The equation of titration is:  H 3 O + + OH -  2H 2 O

 1- strong acid- strong base  2- strong acid- weak base  3- weak acid- strong base

 Indicators are specific, each type of titration needs a specific indicator  For example:  Strong acid- strong base titration: We use bromothymol blue ( )  For strong acid- weak base titration: We use bromophenol blue ( 3-4.6)  For weak acid- strong base titration We use phenolphtalein(8-10)

 The point at which the two solutions used in a titration are present in chemically equivalent amounts is called the equivalence point.

 The endpoint in titration marks the point at which the color of indicator changes.

 The solution that contains precisely known concentration of solute is known as the standard solution.  To find the molarity or the volume of an unknown solution we use the equation:  No of moles of acid= No of moles of base  CxV (acid)= CxV (base)  Practice p: 503 (1 and 2)