Objectives Describe how an acid-base indicator functions. Explain how to carry out an acid-base titration. Calculate the molarity of a solution from titration data. Chapter 15 Section 2 Determining pH and Titrations

Indicators and pH Meters Acid-base indicators are compounds whose colors are sensitive to pH. Indicators change colors because they are either weak acids or weak bases. Chapter 15 Section 2 Determining pH and Titrations H In and In  are different colors. In acidic solutions, most of the indicator is H In In basic solutions, most of the indicator is In –

Indicators and pH Meters The pH range over which an indicator changes color is called its transition interval. Indicators that change color at pH lower than 7 are stronger acids than the other types of indicators. They tend to ionize more than the others. Indicators that undergo transition in the higher pH range are weaker acids. Chapter 15 Section 2 Determining pH and Titrations

Indicators and pH Meters A pH meter determines the pH of a solution by measuring the voltage between the two electrodes that are placed in the solution. The voltage changes as the hydronium ion concentration in the solution changes. Measures pH more precisely than indicators Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Titration Neutralization occurs when hydronium ions and hydroxide ions are supplied in equal numbers by reactants. H 3 O + ( aq ) + OH  ( aq ) 2H 2 O( l ) Chapter 15 Section 2 Determining pH and Titrations Titration 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.

Titration, continued Equivalence Point The point at which the two solutions used in a titration are present in chemically equivalent amounts is the equivalence point. The point in a titration at which an indicator changes color is called the end point of the indicator. Chapter 15 Section 2 Determining pH and Titrations

Titration, continued Equivalence Point, continued Indicators that undergo transition at about pH 7 are used to determine the equivalence point of strong- acid/strong base titrations. The neutralization of strong acids with strong bases produces a salt solution with a pH of 7. Chapter 15 Section 2 Determining pH and Titrations

Titration, continued Equivalence Point, continued Indicators that change color at pH lower than 7 are used to determine the equivalence point of strong- acid/weak-base titrations. The equivalence point of a strong-acid/weak-base titration is acidic. Chapter 15 Section 2 Determining pH and Titrations

Titration, continued Equivalence Point, continued Indicators that change color at pH higher than 7 are used to determine the equivalence point of weak- acid/strong-base titrations. The equivalence point of a weak-acid/strong-base titration is basic. Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Molarity and Titration The solution that contains the precisely known concentration of a solute is known as a standard solution. A primary standard is a highly purified solid compound used to check the concentration of the known solution in a titration The standard solution can be used to determine the molarity of another solution by titration. Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Chapter 15 Section 2 Determining pH and Titrations

Molarity and Titration, continued To determine the molarity of an acidic solution, 10 mL HCl, by titration 1.Titrate acid with a standard base solution mL of 5.0  10  3 M NaOH was titrated 2.Write the balanced neutralization reaction equation. HCl( aq ) + NaOH( aq ) NaCl( aq ) + H 2 O( l ) Chapter 15 Section 2 Determining pH and Titrations 1 mol 1 mol 3.Determine the chemically equivalent amounts of HCl and NaOH.

Molarity and Titration, continued 4.Calculate the number of moles of NaOH used in the titration mL of 5.0  10  3 M NaOH is needed to reach the end point Chapter 15 Section 2 Determining pH and Titrations 5.amount of HCl = mol NaOH = 1.0  10  4 mol 6.Calculate the molarity of the HCl solution

Molarity and Titration, continued 1.Start with the balanced equation for the neutralization reaction, and determine the chemically equivalent amounts of the acid and base. 2. Determine the moles of acid (or base) from the known solution used during the titration. 3.Determine the moles of solute of the unknown solution used during the titration. 4. Determine the molarity of the unknown solution. Chapter 15 Section 2 Determining pH and Titrations

Molarity and Titration, continued Sample Problem F 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 until the equivalence point is reached. What is the molarity of the acid solution? Chapter 15 Section 2 Determining pH and Titrations

Molarity and Titration, continued Ba(OH) 2 + 2HCl BaCl 2 + 2H 2 O 1 mol 2 mol Chapter 15 Section 2 Determining pH and Titrations Sample Problem F Solution Given: volume and concentration of known solution = 27.4 mL of M Ba(OH) 2 Unknown: molarity of acid solution Solution: 1.balanced neutralization equation chemically equivalent amounts

Molarity and Titration, continued Sample Problem F Solution, continued 2. volume of known basic solution used (mL) amount of base used (mol) Chapter 15 Section 2 Determining pH and Titrations 3. mole ratio, moles of base used moles of acid used from unknown solution

Molarity and Titration, continued Sample Problem F Solution, continued 4. volume of unknown, moles of solute in unknown molarity of unknown Chapter 15 Section 2 Determining pH and Titrations

Molarity and Titration, continued Sample Problem F Solution, continued 1.1 mol Ba(OH) 2 for every 2 mol HCl. Chapter 15 Section 2 Determining pH and Titrations 2. 3.

Molarity and Titration, continued Sample Problem F Solution, continued Chapter 15 Section 2 Determining pH and Titrations 4.