Determining pH and Titrations

Section 15.2

Acid-Base Indicators

• acid-base indicators: compounds whose colors are sensitive to pH

• Indicators change colors because they are either weak acids or weak bases

Indicators

• Indicators come in many different colors• There are specific pH ranges over which

an indicator changes color• Transition interval: the pH range over

which an indicator changes color

pH Meter and Paper

• pH meter: determines the pH of a solution by measuring the voltage between the two electrodes that are placed in the solution

• pH paper is used by comparing the color the paper turns when it is in contact with the solution

Titration

• Titration: 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

Equivalence Point

• Equivalence point: the point at which the two solutions used in a titration are present in chemically equivalent amounts

• Indicators and/or pH meters can be used to determine the equivalence point

More about Titrations

• End point (of the indicator): the point in a titration at which an indicator changes color

• Indicators that undergo transition at about pH 7 are used to determine the equivalence point of strong acid/strong base titrations

Standard Solution

• Standard solution: (AKA known solution) the solution that contains the precisely known concentration of a solute

• See pages 518-519 in the book at school for the procedure for carrying out an acid-base titration (pages 500-501 in your book at home)

Sample Problem for a Titration

• Suppose that 10.1 mL HNO3 is neutralized by 71.4 mL of a 4.2 x 10-3 M solution of KOH in a titration. Calculate the concentration of the HNO3 solution.

• Step 1: write and balance the equation.HNO3 + KOH H2O + KNO3

Problem Continued

• Step 2: define what you are given in the units required

• V of KOH = 71.4 mL = 0.0714 L• [KOH] = 4.2 x 10-3 M• V of HNO3 = 10.1 mL = 0.0101 L

• [HNO3] = ?

Continued

• Step 3: find the mol of the known volumeMol = [ KOH] (volume in liters)Mol = 4.2 x 10-3 mol/L KOH 0.0714 L Mol = 2.9988 x 10-4 mol KOH

• Step 4: find the mol of the unknown (HNO3)

2.9988 x 10-4 mol KOH 1 mol HNO3 / 1 mol KOH

2.9988 x 10-4 mol HNO3

Continued

• Step 5 calculate [unknown]• 2.9988 x 10-4 mol HNO3 / 0.0101 L =

• 3.0 x 10-2 M HNO3

Calculations

• See pages 520-521 in the book at school and pages 502-503 in the book at home for sample calculations