CR - Solutions Stoichiometry (Lesson)

Titrations

A titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution. It is a quantitative analysis for acid base and redox reactions. 

Important Titration Terms:

• The titrant is the solution with known concentration.  The analyte is the solution of unknown concentration.
• An auret is  a precise, graduated delivery device that allows very small quantities of the solution to be delivered to the reaction container.
• An indicator is a material added to the analyte that will change colors very near the stoichiometric balance between the materials reacting. The actual stoichiometric balance is called the equivalence point, and the point where the colors change is called the experimental end point. And generally the end point = the equivalence point. 

Titration Calculations

Let's walk through an example calculation using titration techniques together: 

A student prepares a solution of nitrous acid that is approximately 0.10 M and wishes to determine its precise concentration. This is the analyte. A 25.00 mL portion of the acid is transferred to a flask and indicator added. The analyte is in the flask. The analyte sample is titrated with the titrant, a 0.0775 M KOH solution. The titration requires exactly 37.46 mL of the standard KOH titrant to reach the end point. What is the molarity of the HNO₂?

Redox Titrations

Not all titrations are acid-base titrations. An oxidation-reduction reaction (redox) can be used to determine the concentration of an unknown solution.

Some redox titrations may not require an indicator that changes color to show the end point but will change color naturally near the equivalence point.  The permanganate ion in solution has a dark purple color but as it is reduced, it becomes colorless. So the equivalence point of the titration can be detected when the solution becomes a faint pink color.

Follow along with this example:

A student was given the task of determining the molarity of an unknown concentration of hydrogen peroxide solution. She analyzed a 10.0 mL sample of H₂O₂ (aq) by titrating it with 0.0330 M KMnO₄, which has a dark purple color. The balanced equation is:

2 MnO₄^- (aq) + 5 H₂O₂ (aq) + 6 H⁺ (aq) $\longrightarrow$ 2 Mn²⁺ (aq) + 5 O₂ (g) + 8 H₂O (l)

A total of 10.69 mL of 0.0330 M KMnO₄ was required to reach the equivalence point. 

Part A: Calculate the number of moles of MnO₄ ^- that reacted.

Part B: Calculate the [H₂O₂ ] in the solution. 

Using the answer we just found for moles 3.53 x 10^-4 moles of permanganate solution, we can find  the concentration of H₂O₂ solution. 

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