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Lab Score: ______ Unknown Score: ______
STANDARDIZATION of NaOH
EVERY DAY: When you are finished with the buret for the day, rinse the buret four times with DI water,
and fill the buret to the very top with DI water. Release the pressure from the tip by delivering a little DI
water while inserting the cork. There should be no air space left in the buret. Ask your instructor to check
the buret(s) and sign below. Points will be deducted for failure to obtain initials.
Date Initials Date Initials Date Initials
______ _________ ______ _________ ______ _________
______ _________ ______ _________ ______ _________
This experiment involves the preparation of a standard oxalic acid solution and a base solution, aqueous
sodium hydroxide. The sodium hydroxide solution will be standardized using solid potassium hydrogen
phthalate, a primary standard acid. These solutions will be used in later experiments for the titration of
an unknown acid solution and possibly an acetic acid solution. The accuracy of the calculated molarity
for your unknown acid depends on the accuracy of the molarities (or normalities) of the standard solutions
prepared in this experiment.
Read the Analytical Techniques in the GCN. You will use several of these methods during this
experiment. Consider the calculated concentrations as intermediate values. Carry two extra
nonsignificant digits in the molarity (or normality) values. Calculations should be set up using the unit
equation (dimensional analysis) approach.
Part I: Preparation of a Standard Oxalic Acid Solution Anhydrous oxalic acid is not readily available, but oxalic acid dihydrate, H2C2O4.2H2O, can be
purchased. Oxalic acid dihydrate does not meet the requirements of a primary standard and should not be
used as a titrant without first being standardized. However, because only small volumes of low
concentration oxalic acid will be used, it is an acceptable standard for our purpose.
Write the balanced molecular, complete ionic and net ionic equations for the reaction of sodium
hydroxide solution with oxalic acid solution.
Calculate the mass (g) of oxalic acid dihydrate required to prepare 250.0 mL of 0.0300 N H2C2O4.
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From your instructor, obtain a test tube containing the approximate amount of oxalic acid dihydrate
needed to prepare the solution. Obtain a 250.0 mL volumetric flask and powder funnel from the cabinet
in the front of the lab. Use all of the sample that will come out of the test tube with tapping.
Using the Analytical Techniques described in the GCN, carefully prepare 250.0 mL of standard oxalic
acid solution. ______ water will be used to prepare this solution. Record the masses that will be used to
calculate the mass of oxalic acid dihydrate in the table below.
DATA TABLE Part I
initial mass of test tube containing oxalic acid dihydrate ______________
final mass of test tube ______________
mass of oxalic acid dihydrate dispensed ______________
Use the actual mass of oxalic acid dihydrate that you placed in the 250.0 mL volumetric flask to
calculate the normality of your standard oxalic acid solution.
Instructor’s Initials _______________
Transfer the standard oxalic acid solution to a properly prepared 250 mL plastic bottle for storage and
later use. Return the test tube to your instructor. Keep the lid on the storage bottle as much as possible.
Part II: Preparation of the Sodium Hydroxide Solution
Calculate the volume (mL) of 6.0 M NaOH solution required to prepare 1.0 L of 0.09000 N NaOH
solution.
Instructor’s Initials _______________
Rinse your 1 L plastic bottle four times with 5 mL DI water. Use your 50 mL graduated cylinder to
add the calculated volume of 6 M NaOH to the plastic bottle. Add DI water to the bottle until the level is
at the shoulder of the bottle. (You may fill with DI water from the DI tap if you are careful to keep the
plastic tubing on the spout above the top of the bottle.) Cap the bottle, invert and mix 25 times. This
solution should be kept tightly covered: Carbon dioxide in the atmosphere reacts with the hydroxide ion
in the solution changing the concentration of the sodium hydroxide in the solution.
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Part III: Preparation of Potassium Hydrogen Phthalate Samples Potassium hydrogen phthalate, KHC8H4O4, is often abbreviated as KHP. Note that KHP is not the
formula! KHP is a monoprotic acid that can be obtained with a high level of purity. Its high purity,
relatively large molar mass, and stability make KHP an appropriate substance to use as a primary standard.
Potassium hydrogen phthalate is commonly used to standardize sodium hydroxide solutions.
KHP molar mass: _____________________ KHP eq mol–1: ___________________
The method of “Weighing by Difference” will be used to weigh out four samples of KHP with
approximate masses between 0.6 g and 0.8 g. The precise mass of the samples should be obtained by
difference using measurements from an analytical balance (a.u. = 0.0001 g). The mass of the samples
should differ from each other by at least 0.01 g. “Weighing by Difference” (in the GCN) is a two-balance
method you will use to analytically transfer your samples of KHP to Erlenmeyer flasks. Answer the
following questions.
What object is placed on the top loader’s pan:
What mass is measured on the top loader balance (Why?):
What object is placed on the pan of the analytical balance:
What mass is determined on the analytical balance (Why):
Why is it important not to weigh the top loader object on the analytical balance?
Obtain a set of two burets on a buret stand from the shelf in the middle of the lab bench. Choose the
buret set that has the same number as the last two digits of your lab station. Using proper conditioning
techniques, prepare a buret for use in delivering your sodium hydroxide solution. Be sure there are no
bubbles in the tubing or the buret tip.
The first titration will be performed without any back-titration. The first titration will be primarily to
determine the approximate volume of NaOH solution needed to get to the end point, and to gain some
experience in titration. During these titrations, when will you reach the endpoint if you forget to add the
indicator? _____________ Would you reach the equivalence point? ______________
Instructor’s Initials _______________
From your instructor, obtain a test tube containing enough KHP for several titrations. Prepare four
clean, but not necessarily dry, 250 mL glass Erlenmeyer flasks (lab locker). Dry the flasks on the outside,
then number them in pencil 1, 2, 3, and 4. Understand the technique before weighing out any sample.
Record the data for the first two samples of KHP in the data table on the next page, but keep the test
tube with the remaining KHP in your drawer for additional runs later.
If any mass is lost from the test tube and does not go into the flask during the transfer, the mass of KHP
will be incorrect and that sample should be discarded. Your hands should be dry when handling the test
tube and flasks.
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Part VI: Standardization of NaOH Solution with Solid KHP Including Back-
Titration with a Standard Solution of Oxalic Acid
For these titrations, your standard oxalic acid solution will be used to back-titrate the reaction mixture.
Refill the NaOH soln buret as usual. Prepare the second buret by rinsing it four times with 5 mL portions
of your standard oxalic acid solution. Place between 10 and 15 mL of your standard oxalic acid solution
in the second buret. Remember to record both initial and final volumes in the data table below. You may
want to label the buret stand with A for acid and B for base. Keep the storage containers adjacent to the
appropriate buret to prevent refilling a buret with the wrong solution.
Prepare Flask #3 as before. Titrate the KHP with your NaOH solution. Intentionally titrate past the
endpoint, back-titrate with the oxalic acid solution, then re-approach the endpoint by adding NaOH
solution. Use a minimum of 1.00 mL of back-titrant. For the final approach to the endpoint, titrate very
carefully. Repeat as often as necessary.
Refill the NaOH soln buret with your solution and repeat the procedure with Flasks #4, #5 and #6.
The goal is to have the normalities (from Runs #3 through #6) of three, preferably consecutive,
titrations with an extreme relative deviation of 1 ppt or less. This will most likely take more practice in
Part VII. The extreme relative deviation is just the absolute deviation between the lowest and highest
values, divided by the highest value (multiplied by 1000 to get ppt). This is a measure of precision (or
consistency), which has a direct correlation with the confidence we have in your technique.
DATA TABLE for Part VI
sample number 3 4 5 6
initial mass of tt + KHP _________ _________ _________ _________
mass of tt – sample _________ _________ _________ _________
mass of KHP in flask _________ _________ _________ _________
sodium hydroxide solution
Buret # and letter: _________ _________ _________ _________
final buret reading _________ _________ _________ _________
initial buret reading _________ _________ _________ _________
volume NaOH delivered _________ _________ _________ _________
oxalic acid solution
Buret # and letter: _________ _________ _________ _________
final buret reading _________ _________ _________ _________
initial buret reading _________ _________ _________ _________
volume oxalic acid delivered_________ _________ _________ _________
normality of NaOH solution _________ _________ _________ _________
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Calculate the normalities of your NaOH solutions for Flasks #3 through #6.
RUN #3:
RUN #4:
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RUN #5:
RUN #6:
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Part VIII: Result Analysis A.1. Enter five calculated normalities (exclude #1) for your sodium hydroxide solution below. Include
two non-significant digits and the dashed line.
____________ ____________ ____________ ____________ _____________
Order the normalities from the smallest to the largest.
____________ < ____________ < ____________ < ____________ < _____________
Circle the most extreme value (the outlier). You may be able to discard this value, but you can only
discard values by showing that the value is statistically different from the other values. Refer to the
GCN, and see if the 4-d rule can be applied to eliminate this value. The steps are below:
2. Calculate the mean for the four results that agree best with each other. If you have three values
with an extreme relative deviation of 1 ppt or less, you may use those three values instead of four.
3. Calculate the average absolute deviation from the mean for the four (or three) values from A.2.
4. Multiply the above average absolute deviation by 4.
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B. Calculate the absolute deviation of the next closest result, or the “outlier” (the one circled in A.1.) from
the mean value of the best results (from A.2.)
C. Which is the larger A.4. or B.? (Circle one.)
If B is larger, you may statistically eliminate the worst result. Put a single line through the column
of data in your data sheet and note that it was eliminated using the 4-d rule. If A.4 is larger, you must
use all of the values and will need to recalculate the average normality, the average deviation and the
rad in ppt. Attach a separate sheet showing the calculations.
Record the average and the average deviation you will use for your results of this experiment.
avg normality ________________ avg deviation: ________________
D. Determine the relative average deviation (in ppt) of any remaining values.
Instructor’s Initials: __________
If your rad is larger than 2 ppt (0.8 ppt for A work) you should titrate more samples of KHP. After
completing the additional titrations, repeat the process of analyzing your results.
If your RAD is still larger than 2 ppt, take your bottle of NaOH solution and your clean, dry 250 mL
storage bottle to your instructor and exchange it for a standardized NaOH solution (10 pt deduction!).
When you are finished with the standardization lab, place the KHP test tube and stopper in the labeled
container. Save the standardized NaOH solution (yours or ours) for the next experiment.
Average Normality of NaOH for the next experiment: ___________________
- Chem 180 Spring 20 Title page #1
- Standardization of NaOH Oct 2008 Matt2020
- Unknown Acid Titration Matt S2020
