Name: ___________________
Date: _________ Per: ______
General Chemistry
Acids and Bases
Titration Lab
Introduction
A titration uses neutralization to determine the concentration (and identity) of an unknown acid
or base. If you know the concentration and volume needed to reach equivalence (molsacid =
molsbase), you can calculate molar mass and identify your unknown. You will know that you have
reached equivalence or neutralization when your indicator changes color.
The tricky part of this lab is that it is VERY EASY to overshoot and get wrong results so:
1. Watch for flashes of color. This means you are close to the equivalence point
2. At this point, add base DROP BY DROP until the color lingers for at least 30 seconds.
3. Do a trial run first to get a rough idea of how much base you will need for neutralization
Purpose to use titration to determine the identity of an unknown acid
The possible acids used as unknowns in this lab are:
Oxalic Acid • 2 H2O
Ascorbic Acid
Potassium Hydrogen
Phthalate
Potassium Hydrogen
Sulfate
HOOCCOOH • 2 H2O
HC6H7O6
KHC8H4O4
Molar mass = ____________
Molar mass = ____________
Molar mass = ____________
KHSO4
Molar mass = ____________
Materials buret, clamp, ring stand, 125 mL Erlenmeyer flask, unknown, standard NaOH, DI water
Procedure
PART I: Preparing the solid acid solution
1. Measure between 0.50 and 0.75 g of unknown into your clean flask and record in your
data table.
2. Add about 25 mL of ultrapure DI water to the flask to begin dissolving the solid acid.
3. Add about 3 drops of phenolphthalein to the flask and swirl (DON’T SPILL!) to dissolve
the acid as much as possible before titrating.
PART II: Titration Using Phenolphthalein
A. Trial Run
1. Record the initial buret reading in your data table. Round to the tenths place.
2. Place the acid solution beneath the buret tip, then slide a sheet of white paper under
the flask.
3. Open the buret valve to GENTLY release the base. Continuously swirl until the
solution goes pink.
4. Close the valve and record the final buret reading. This tells you roughly how much
base you need to neutralize your acid.
B. The Real Deal
5. Rinse out your flask, measure more unknown, and repeat steps 1 and 2 of the trial run.
6. Open the buret valve so that it releases base in stream at first, then a succession of
DROPS,as you get close to the equivalence point. Continuously swirl the flask.
7. When you see flashes of color, you know you are getting close.
8. Finish the titration by adding ONE DROP AT A TIME, with a lot of swirling, until the
color persists for at least 30 seconds. Record the final buret reading in the data table.
9. Repeat until you have completed three accurate and precise titrations
Data
Record the letter of your unknown here: _______
Trial
Concentration of
Standard Base
Sol’n (M)
Initial Buret
Reading (mL)
Final Buret
Reading (mL)
Volume of Base
Used (mL)
Mass of
Acid (g)
Trial
Run
1
2
3
Calculations
1. For each titration, (a) use the molarity equation to calculate the moles of base. Show
your work, then (b) calculate the molar mass of your unknown (HINT: it’ll help to know
what equivalence is as well as the units for molar mass). Show your work.
#1 (a)
(b)
#2 (a)
(b)
#3 (a)
(b)
2 Calculate the average molar mass, and identify your acid from the list on front:
3. Calculate the % Range and % Error using the values for molar mass in question 1 & 2. Show
your work.
% Range
%R = high value - low value x 100
average value
(this is a measure of lab precision)
% Error
%E = ave. value - “true value” x 100
“true value”
(this is a measure of lab accuracy)
Questions
1. From the error analysis above and the ratings to the
right, determine how good your accuracy and
precision were and explain the quality of your work.
Less than 1% error
Less than 5% error
Less than 10% error
10% or more error
=
=
=
=
excellent
good
fair
poor
2. In each of the following situations what change would have occurred to your value for the
molar mass of the unknown solid acid (be too large, too small, or have no effect)?
Explain. If you have trouble visualizing this, do some calculations like you did in question 2
above to help you out.
(a) you over-shot your equivalence point?
(b) not all your solid acid dissolved?
(c) you misread your final buret reading to be less than it actually was?
3. As the base was added to the acid, why does the color pink appear first in the center of
the flask (where the drop hits) and then disappear? (HINT: you learned this concept in
biology!!)
4. You may have noticed that nowhere in this lab did you record the amount of deionized
water that you used to dissolve the solid acid. Why will your titration results not be
affected by the amount of deionized water added to the acid? (HINT: What data about
the acid did you need to know to figure out its identity?)