Titration of a Solid Unknown Acid

 

Introduction:

            In this laboratory you will titrate an unknown acid and determine its molecular weight (the acid will be monoprotic).  At the equivalence point of the titration, the moles of acid equal the moles of base. The moles of base is given by the volume of base used, in liters, times the molar concentration of the base.

 

            moles of base =

 

As for any compound, the moles of acid is given by the grams of acid used divided by the formula weight of the acid.

 

            moles of acid =

 

Since moles of acid equals mole of base at the equivalence point:

 

            M base x V base  =  grams of acid/MW acid

 

Or:      Molecular Weight of acid =

 

You will be determining the molecular  weight of the acid. The first calculation you will do is to determine  the moles of NaOH used, which will be equal to the moles of acid present  in your sample as the unknown acid will be a monoprotic acid. The moles of acid are equal to the weight of acid used divided by its formula weight.  Therefore, the formula weight of the acid is equal to the grams of acid  titrated divided by the moles of NaOH used.  The moles of NaOH used are the product of the molarity of the NaOH times the volume of NaOH solution used in the titration, in liters.

            The equivalence point of the reaction is determined by the use of a visual  indicator. For this titration the indicator is phenolphthalein, which changes from colorless at pH = 8 to red at pH = 10. It is easier to  follow the color change from colorless to red, and so the acid is placed  in a container (generally an Erlenmeyer flask) and slurried in about 25.0 mL distilled water (it needn't all dissolve at first).  Then the NaOH solution is added using a buret so that the addition can be stopped, and the volume of base measured, when the first permanent pinkish color appears.

            The acid used is carefully weighed to the nearest 0.001 gram. The volume of base added is measured (to the nearest 0.02 mL) by use of a buret.  And the exact molarity of the base is written on the reagent bottle.  Be sure to record the exact molarity for the bottle you use - not all NaOH solutions have the same concentrations.

                                   

Equipment

50.00 mL buret

3 ·125 or 250 mL Erlenmeyer flasks

short stem funnel  

stir bar (optional)  

 

Chemicals    

Students

standardized (approx. 0.10 M) Sodium Hydroxide (NaOH)

solid unknown acid

phenolphthalein indicator

 

Spill/Disposal   

NaOH Spill/Disposal: B1                          

All else  Spill/Disposal: A

 

Procedure

1.  Obtain a 250 mL bottle of the standardized NaOH solution.  Record the exact concentration of the base. Rinse out a 50.00 mL buret with two 5mL portions of the NaOH solution.  Be sure the stopcock is closed.  Then, using a funnel, carefully fill the buret  to between the 0.00 mL and 1.00 mL mark, and record the volume. 

 

2.  Accurately weigh out (to 0.001 g) two samples of your unknown acid (~ 0.2 +/- 0.01 gram). Transfer the samples  to Erlenmeyer flasks. Add 25 to 30 mL of distilled water. Add 2 drops of the indicator solution.  If you forget the indicator, the color will never change.  ("Professor?  I've drained two burets and it still isn't pink.")

 

3.  Titrate each of the acid samples with the NaOH solution until the first permanent pink color appears (the color remains a light pink for at least 15 seconds after swirling). Record the volume of base added, to the nearest 0.02 mL.

 

4  . Discard the solutions after the titrations are completed. Rinse out  the flasks with small quantities (about 5 mL) of distilled water. You  do not have to dry the inside of the flask.  Determine the formula weight of the unknown acid. The unknown acid used in this experiment will be a monoprotic acid.

 

Disposal

            All solutions from this experiment may be disposed of into the sink.       

 

 

 

 

 

 

 

 

 

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