Wednesday, May 18, 2011

Final Exam Review Questions #s 29 and 31

29)  5.00mL of an unknown acid is titrated using 0.100M NaOH and phenolphthalein.

a. What role does the phenolphthalein play?
Phenolphthalein is a colorless crystalline solid, also known as C2OH14O4. In an alkaline solution,
phenolphthalein turns a solid pink color. In this equation, the phenolphthalein is used as an acid-base indicator. 

b. How would you know when the titration was complete? Why is this an estimation of the equivalence point and not an exact measurement of the equivalence point? 
The endpoint is the point at which the titration is determined complete by an indicator. The endpoint is ideally the same volume as the equivalence point. The equivalence point is the volume of added titrant, when the number of moles of titrant is equal to the number of moles of analyte or some other polyprotic acids. In the classic strong acid-strong base titration, the endpoint of a titration is the point when the pH of the reactant is just about equal to seven. At this point, the solution becomes a persisting solid color as in the pink of phenolphthalein indicator. Equivalence points can only be estimated, it is not possible to find the exact measurement of the equivalence point. The equivalence point of the reaction, the point at which equivalent amounts of the reactants have reacted, will have a pH dependent on the relative strengths of the acid and base used. The equivalence point can be estimated using the following rules: 
   -A strong acid will react with a strong base to form a neutral (pH=7) solution.
   -A strong acid will react with a weak base to form an acidic (pH<7) solution.
   -A weak acid will react with a strong base to form a basic (pH>7) solution.

c. If 12.15mL of NaOH are used to complete the titration, what is the concentration of the unknown acid? 
.01215 L NaOH (1.00) = mol = .01215 mol NaOH
.01215 mol NaOH/.005 L Unknown = M 
=2.43M of the unknown acid 

31) Did we cover it all? Think of a topic or question from this past trimester that        
you think should have been covered more by this review, and respond to it. You are welcome to recycle or reformulate good questions you have seen this past trimester. If you pick a really good one, I might use it on the exam.

Question: Explain how a decrease in the vapor pressure of a solution results in an increase in its boiling point. 
The more vapor pressure forced on a solution, the easier it is for the solution to boil (lower boiling point). The less vapor pressure there is, the harder it is for the solution to boil (higher boiling point). The pressure pushes the particles of H20 to the top of the solution because they are trying to escape. When there is less pressure, the particles have so big of a desire to escape and therefore they do not boil so quickly. 


  1. 29 b) The equivalence point is also when the number of moles of H+ ions equals the number of moles of OH- ions.

    31) A decrease in the vapor pressure of a solution results from an increase of the intermolecular forces among the molecules in solution. As a result, more energy is required to disrupt these intermolecular forces, leading to an increase in the boiling point.

  2. Great job, I agree with your answers.

  3. 29 c- your answer is a factor of 10 too large. The answer is O.243 M

    0.100M(0.01215) = 0.001215 mol

    Concentration of Acid = 0.001215mol/0.005L = 0.243 M

    You have the concentration of NaOH incorrect (you have 1.00 M, but it is 0.100M)