Illustrated Guide to Home Chemistry Experiments

Chapter 11 Laboratory: Acid-Base Chemistry 207

dISpoSAL: Retain the stock sodium carbonate solution

and the standardized hydrochloric acid solution for later

use. The other solutions can be flushed down the drain

with plenty of water.

PRTIII: A ANRST dA dIzE A BENCH SoLUTIoN
of ~1 m HydRoCHLoRIC ACId

1. 
   

   If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.

   
   


2. 
   

   Use the 10 mL pipette to transfer about 10 mL of the
   ~1.0 M hydrochloric acid bench solution to a 150 mL
   beaker. Record the volume to 0.01 mL on line E of
   Table 11-4.

   
   


3. 
   

   Add about 25 mL of water to the beaker and swirl the
   beaker to mix the solution.

   
   


4. 
   

   Add a drop or two of phenolphthalein indicator to the
   beaker and swirl to mix the solution.

   
   


5. 
   

   Calculate the approximate amount of titrant you
   expect to need to neutralize the 10 mL of ~1.0 M
   hydrochloric acid solution, and enter that value on line
   F of Table 11-4.

   
   


6. 
   

   Rinse the burette thoroughly, first with water and then
   with a few mL of the titrant solution.

   
   


7. 
   

   Install the burette in the burette clamp and fill it to or
   above the top (0.00 mL) line with titrant solution.

   
   


8. 
   

   Run a few mL of titrant through the burette, making
   sure that no air bubbles remain and that the level of
   titrant is at or below the top index line at 0.00 mL.
   (It’s not important that the initial reading be exactly
   0.00 mL, but it is important to know the initial reading
   as closely as possible.) Record the initial reading as
   accurately as possible on line G of Table 11-4.

   
   


9. 
   

   While swirling the beaker, use the burette to dispense into
   the beaker a few mL less of the titrant than you estimated
   in step 5 will be required to neutralize the hydrochloric acid.
   Begin adding the titrant dropwise but quickly and with
   continuous swirling. As you approach the equivalence
   point, you’ll see the solution turn pink where the titrant
   is being added, but the pink color will disappear with
   swirling. This indicates that you are rapidly approaching
   the equivalence point. Using a sheet of white paper or
   other white background under or behind the beaker
   makes it easier to detect the first hint of a color change.
   Continue adding titrant slowly dropwise, with swirling,
   until the solution in the beaker shows an overall very
   slightly pink color that does not disappear when the
   solution is swirled. A permanent slight pink coloration
   indicates that you’ve reached the equivalence point. All
   of the hydrochloric acid is neutralized, and there is a tiny
   excess of sodium carbonate. A dark pink color (or any
   color other than pale pink) indicates that the equivalence
   point has been met and exceeded, which means you need
   to redo the titration. If you use a different indicator, such
   as universal indicator, the indicative color change may be
   slightly different.

   
   

Record the final burette reading on line H of Table 11-4.

Subtract line G from line H and record the difference on

line I as the volume of titrant required to neutralize the

aliquot of hydrochloric acid.

Calculate the number of moles of sodium carbonate

contained in the volume of titrant you used, and record

that value on line J of Table 11-4.

Calculate the number of moles of hydrochloric acid

present in the aliquot (remember that two moles

of hydrochloric acid react with one mole of sodium

carbonate) and enter that value on line K of Table 11-4.

Calculate the molarity of the HCl bench solution, and

enter that value on line L of Table 11-4.

12.

13.

14.

15.

oALIpToN ACTIvITIES

If you have time and the required materials, consider

performing these optional activities:

• Repeat the titration once or twice and compare these
  results with your initial results. If the results are not
  in close agreement, run additional titrations until
  you have an accurate value for the molarity of the
  hydrochloric acid bench solution.


• Use your standard sodium carbonate solution
  and your newly standardized hydrochloric acid
  solution to determine accurate concentrations for
  various household acids and bases. For example,
  you might use the sodium carbonate solution (after
  serial dilution) to titrate aliquots of distilled vinegar
  (acetic acid) and lemon juice (citric acid), and the
  standardized HCl solution to titrate aliquots of
  household ammonia and liquid drain cleaner
  (sodium hydroxide).

10.

11.