determining ph and titrations
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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
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