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Determining pH and Titrations

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

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Determining pH and Titrations

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  1. Determining pH and Titrations Section 15.2

  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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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)

  9. 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

  10. 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] = ?

  11. Continued • Step 3: find the mol of the known volume Mol = [ 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

  12. Continued • Step 5 calculate [unknown] • 2.9988 x 10-4 mol HNO3 / 0.0101 L = • 3.0 x 10-2 M HNO3

  13. 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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