Titration lab

Posted: April 1, 2013

Titration of an Acid and a Base

Objectives:  -To understand the procedures necessary to
standardize the concentration of a basic solution.

-To gain experience in calculating the average molarity of
an unknown solution (NaOH)

Procedure:

[All lab group members must wear goggles, and
gloves throughout this experiment.  If
there is a spill or glassware is broken immediately inform the
instructor.]

1)      Set up titration apparatus including ring stand, double buret
clamps, two (labeled) 250 ml beakers and one unlabeled 250ml beaker, 1
Erlenmeyer flask 200ml, two (labeled) 50ml burets, wash bottle.  Be extremely careful with all glassware,
particularly the burets.

2)     Rinse out Erlenmeyer flask with distilled water.

3)      Turn stopcock on buret to closed position. Pour about 5ml of NaOH (unknown concentration) from
labeled beaker into the appropriately labeled buret (be careful not to let base
flow down the outside of buret).  Rinse
the walls of the buret by holding the open end of the buret over the sink and
tilting the buret back and forth at a nearly horizontal position.  Turn the buret to allow the solution to come
in contact with all inner walls, then discard solution in the sink. This will
coat the inside of the buret with the solution that you will be putting inside
the buret for the titration.  Repeat
this rinsing procedure two more times with 5ml of NaOH and the same buret.  Then perform three rinses of the HCl buret,
using 5ml of 0.5 M HCl each time, in
the same manner.

4)      Position the burets in the double buret clamp so that there is
about 2 cm or so distance from the buret opening and the Erlenmeyer flask
opening.  Fill each labeled buret with
the appropriate solution of HCl and NaOH to about the 3/4 point of the buret.

5)      With the unlabeled 250ml beaker underneath, open the stopcock
on the burets very briefly to let out any gas bubbles.  You are ready to begin the titration.

6)      In your Data Table, record the initial volumes from each of
the burets, estimating to the nearest 0.01ml.
Do not forget to do this step before every trial, otherwise, you
will have to start the trial over.

7)      Put 5ml of NaOH solution into the Erlenmeyer flask and then
add 2-3 drops of phenolphthalein indicator.
The solution should turn pink. Swirl and wash inside walls of Erlenmeyer
flask with a small amount of distilled water from wash bottle.  This water wash will be performed often
(after every few additions of acid to the Erlenmeyer flask) in order to be sure
that not even the tiniest amount of acid or base solution stays on the inside
walls of flask.

8)      At first adding larger quantities (1ml or so) of acid to the
Erlenmeyer will not greatly affect the pink color of the NaOH solution.  Be careful though, because as you approach
the equivalence point, you must considerably decrease the amount of acid you
add (to the point of adding a few drops at a time between swirl/dH2O wash, then
2 drops at a time between swirl/dH2O wash, then 1 drop at a time and so
on).  Turn the stopcock 360 degrees in
order to achieve a single drop (“dropwise” addition). The way you will know
that you have begun approaching equivalence point is when the pink color of the
solution begins to disappear at the place where you add the acid.  When the last drop you add of acid causes
the pink color to disappear from the whole solution, stop adding acid.  Now add base dropwise just to return the
pink color to the solution.  You have
reached equivalence point—prove it to yourself by going back and forth past the
equivalence point by adding acid dropwise to make the solution clear, and then
adding base to get a faint pink colored solution.  Swirl/dH2O wash frequently.

9)     Read the final volumes from the burets and record volumes in
Data Table.

10)  Discard liquid in Erlenmeyer flask in the sink.

11)  Repeat titration up to three times (as requested by
instructor).  Recording initial and
final volumes of acid and base each time.

12)  Clean and rinse all glassware with dH2O, except two
labeled beakers with HCl and NaOH.

 

DATA TABLE

TRIAL

BURET READINGS (mL)

HCl

NaOH

Initial

Final

Initial

Final

1

2

3

<!–[

Molarity of
HCl_________M

Calculations:

1)      Calculate the volumes of acid used in the three trials.  Record in Calculations Table.

2)      Calculate the volumes of base used in the three trials.  Record in Calculations Table.

3)      Use Molarity formula (M=moles solute/liters of solution) to
determine moles of acid used in the three trials.  Record in Calculations Table.

4)      What is the balanced reaction between HCl and NaOH?  Use the mole ratio and results from
Calculation #3 to determine moles of base used to neutralize the acid used in
the three trials.  Record in
Calculations Table.

5)      Use the Molarity formula and results from Calculations 2 and 4
to calculate molarity of the base used in the three trials.  Record in Calculations Table.

6)      Calculate the average molarity of the base used in the three
trials.  Record in Calculations Table.

CALCULATIONS TABLE

HCl

NaOH

Volume (ml)

Moles

Volume (ml)

Molarity (M)

1
2
3

Average Molarity of
NaOH=________M

Post-lab questions:

1)  Why does addition
of dH2O not affect titration results?

2)  What is meant by
the term “equivalence point”?

3)  How soluble is
phenolphthalein in water? What do we do to change the solubility of
phenolphthalein?

4) Compare the effectiveness at detecting the equivalence
point in this titration procedure as compared to the procedure you performed in
the Antacid lab?

5) What was one major difference in the way in which the
equivalence point was approached in this lab as compared to the Antacid lab?

6) What do you think the pH would be for the final solution
in the Erlenmeyer flask? Would all neutralization reactions have this pH at
their equivalence points?  Give an
example to support your answer.

 


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