Exp' 9 Objectives

1
Exp. 9 Objectives

• To prepare and standardize a sodium hydroxide
  (NaOH) solution.
• To determine the molar concentration of a strong
  acid.

2
Exp. 9 Introduction

• Primary Standard a substance that has a known
  high degree of purity, a relatively large molar
  mass, is nonhygroscopic, and reacts in a
  predictable way.
• Standard Solution a solution having a known
  concentration of a solute.
• Hygroscopic readily able to absorb water from
  air

3
Exp. 9 Introduction

• Acid-Base Indicator a substance having an
  acidic structure with a different color than its
  basic structure.
• Stoichiometric amounts amounts corresponding to
  the mole ratio of the balanced equation.

4
Exp. 9 Introduction

• Polyprotic Acid an acid containing more than
  one ionizable hydrogen atom. H2SO4 (diprotic),
  H3PO4 (triprotic).
• The ionization of the acid occurs in steps
  depending on the number of hydrogen atoms. One
  mole of base is utilized in each step
• H3PO4 OH- H2O H2PO4-
• H2PO4- OH- H2O HPO42-
• HPO42- OH- H2O PO43-

5
Exp. 9 Introduction

• Because NaOH is very hygroscopic, it cannot be
  used as a primary standard. Therefore, it has to
  be standardized against a primary standard
  potassium hydrogen phthalate (KHP, KHC8H4O4)

6
Exp. 9 Procedural Notes

• Part A
• Omit steps 1,2, 3
• Accurately weigh KHP and react (titrate) with
  NaOH solution of approximately known
  concentration.

7
Exp. 9 Procedural Notes

• Calculate the molar concentration of NaOH needed
  to react stoichiometrically with KHP
• Mass (g) KHP
• Moles KHP
• Balanced equation (mole/mole)
• Moles NaOH
• Volume (mL) NaOH

8
Exp. 9 Procedural Notes

• Part B
• Calculate the molar concentration of an unknown
  acid
• Known volume (mL) HA, H2A, H3A
• Known molar concentration of NaOH (mol/L)
• Volume (mL) NaOH

9
Exp. 9 Report Sheet

• Questions 2 and 7