EXP 5: Stoichiometry of the Neutralization of an Organic Acid
Introduction:
In this experiment you are going to experimentally determine the number of ionizable H+ in an unknown solid acid and by doing so determine the mole ratios of reactants and products.
All of the hydrogens are acidic in many of the common acids you have used thus far in laboratory work. This is true for acids such as sulfuric acid, nitric acid, and hydrochloric acid. This laboratory activity should show you that only some hydrogens within selected acid molecules may be capable of forming H1+.
The basic definition of an acid is that it is a substance which produces hydrogen ions in water. When hydroxide ion is added to an acid, (this is the technique of titration) the two ions come together to form water, one hydroxide ion for each hydrogen ion. This is a type of ionic reaction, and is specifically called a neutralization reaction, because if all the hydrogen ion is reacted with an equivalent number of hydroxide ions, the solution will be neutral. The exact point as which every hydrogen ion has been neutralized with an hydroxide ion is called the equivalence point. By knowing the concentration of the hydroxide ion and the amount of base used to titrate a known mass of acid, one can calculate the concentration of acid, and thus the amount of hydrogen ion. The net ionic reaction for this is
Procedure:
Select one of the un-named acid samples and record its molecular formula.
Obtain a buret, and if not suitably clean, wash with fresh, warm, soapy water and rinse well. Then rinse with distilled water, followed with a few mL of the 1.000 M sodium hydroxide sol'n to be used in the experiment. Make sure the stopcock does not leak, and then fill the buret with the 1.000 M NaOH. Be sure to record this and all buret readings to the nearest .01 mL.
Helpful tips for using a buret
Add between 2.00 and 3.20 g of the acid to a 250 mL Erlenmeyer flask and dissolve in 50.0 mL of distilled water. Add 4 drops of phenolphthalein.
Slowly add the sodium hydroxide to the acid solution, swirling the flask constantly until a faint pink color persists for at least 30 seconds. (Be sure there is no undissolved acid) (A sheet of white notebook paper will aid in seeing the pink color) Do not overshoot the equivalence point. If you add too much base, mass out a little more of the solid acid and add to the flask, dissolve, and continue the titration.
Repeat the titration with two more samples of the un-named solid acid.
Data Analysis:
First, for each titration, calculate:
Second, determine the average ratio of moles of hydrogen ions to moles of acid. This value will give the number of ionizable hydrogens in each acid molecule. Rewrite the molecular formula from the vial to reflect the number of ionizable hydrogens.
Write an equation to reflect the number of ionizable hydrogens in the total ionic reaction and then write the net ionic equation, balancing each equation.
Look up your acid in the Merek Index and find out its name, uses, any hazards and how it is obtained or prepared. Copy the structural formula into your lab book and use it to answer the following question: Do the ionizable hydrogens come from the -OH, -CH2, or -COOH groups in the acid? If the structural formula isn't given, then use the Lange's and/or CRC Handbooks and look up the acid and and see if the structure is given there. Only if you can't find the structure in either book do you ask Mrs. Sibert for help in finding it.
Applications:
Answer the following questions using the structural formulas in the Structures.pdf file. Refer to the notes on organic chemistry for help in deciding what constitutes an organic acid.