Unit 4: Topic 6 - Introduction to Titration Flashcards
Explain what occurs in a titration experiment.
Titration experiments are used to determine the unknown concentration of a reagent called the analyte by slowly reacting it with a reagent of known concentration and volume called the titrant.
To determine when the reaction has completed, we add an indicator into the container. Indicators are substances that change color based on a change in acidity of the solution and are helpful for determining when a reaction has completed. For example, phenolphthalein is a commonly used indicator that is clear under acidic conditions but turns pink when the solution becomes neutral or basic and is therefore used in many acid-base titrations.
Indicators can be used to determine the equivalence point, the point in the reaction when the amount of titrant added is exactly enough to fully neutralize all of the analyte. At this stage, there are equimolar amounts of titrant and analyte. The observable change is the endpoint of the titration. Since the volume and concentration of the titrant used is known, we can calculate the concentration of the analyte through stoichiometry.
Usually, the titrant is added using a buret with stop cocks for accuracy, and the analyte/titrant mixture is swirled to more accurately determine the endpoint.
When acetic acid (weak acid) is titrated against sodium hydroxide (strong base), explain why phenolphthalein is a good indicator for this titration.
Phenolphthalein is a compound that does not affect an acid-base reaction. Phenolphthalein is clear in acidic solution and turns bright pink at a basic pH, around 8.3. This is perfect for the titration of acetic acid against sodium hydroxide, because at the completion point the strong base will slightly dominate the acid, and the pH will be slightly basic, around 8.3. Therefore, once the reaction reaches completion, phenolphthalein will turn from clear to bright pink.
10mL of acetic acid is placed in an Erlenmeyer flask with phenolphthalein. 1mL 0.10M sodium hydroxide is added to the flask every 10 seconds. After 20mL has been added, the solution turns a bright pink. If the balanced equation of acetic acid with sodium hydroxide is CH3COOH + NaOH –> H2O + NaCH3COO, what is the concentration of the acetic acid?
From knowledge of titration, if the solution turns a bright pink after 20mL of sodium hydroxide has been added, then the titration has reached the endpoint and the reaction has proceeded to completion. Therefore, we can perform stoichiometry to determine the concentration of acetic acid. 20mL of 0.10M sodium hydroxide is equivalent to 0.002 moles of sodium hydroxide. Since the mole ratio of acetic acid and sodium hydroxide is 1:1, there must also be 0.002 moles of acetic acid in the 10 mL of acetic acid. Therefore, the concentration of acetic acid is 0.20M.
