Online Chemistry 11 Lab #18

 

Acid Base Titration

Safety:  Acids and bases are dangerous to your eyes, skin and clothing.  Wear safety goggles at all times during this lab.  Wash any spills on your skin immediately with plenty of running water.  Let me know of any spills on the lab benches or floor.

 

Theory The ideas behind a titration are these:

1. A titration is used to measure the concentration of a sample acid (or base) of unknown concentration by reacting it with a standard base (or acid) of known concentration.
2. A few drops of an indicator, such as phenolphthalein, are added to the unknown and the standard is added to this mixture until a color change is noted.  This should occur at the point where the standard has neutralized the unknown.
3. The volume of acid and base must be noted.
4. When neutralization occurs the number of moles of unknown acid (or base) are equal to the number of moles of known base (or acid).
5. The concentration of the unknown can then be calculated using the information from points 3 and 4 and given the known molarity of the standard:

            

Number of moles = Volume x concentration

so

(Volume unknown)x(Concentration unknown) = (Volume known)x(Concentration known)

and then

(Concentration unknown) = (Volume known)x(Concentration known)/ (Volume unknown)

 
Finding the concentration of an unknown sodium hydroxide solution

1. Put on your safety goggles.
2. Obtain about 60 mL each of standard 0.01M HCl and unknown NaOH solutions in clearly labeled beakers from the front of the lab.  Also obtain an Erlenmeyer flask with a couple of drops of phenolphthalein solution added to it, a funnel and a small beaker.
3. Label your burets A and B for acid and base.
4. Place the small beaker under the buret nozzle and make sure the nozzle is closed.  Add about 5 mL of base to the base buret and then run it through into the beaker.  Repeat with another 5 mL of base.  This cleans the buret.  Close the nozzle and now fill the buret with base until the bottom of the meniscus is between 0.00 and 1.00 mL.  Record this initial reading.  Dump the waste base used to clean the buret in a functioning sink and wash down with tap water.
5. Repeat step 4 for the acid buret USING THE ACID.
6. Add about 10 mL of acid to the Erlenmeyer flask  (What color do you note? What does this tell you?)  followed by an equal amount of base.  Note the color and figure out what it means in terms of which is higher concentration – the acid or base.
7. Add more of the lower concentration solution until between 5 and 10 ml of it remain in the buret.
8. Add the higher concentration solution until the color changes (ie neutralization occurs).
9. If you add too much solution in step 8 add a few more drops from the lower concentration buret until the color changes back and then proceed with step 8 MORE SLOWLY.  Add a drop at a time and swirl the flask to see when the color change endures.
10.  Note the final readings of each buret.
11.  Calculate the total volume added of acid and base, and use the formula above to find the unknown concentration.