Name____________________________
Prelab
Standardization of NaOH
1) What is a primary standard?
2) Why is it necessary to standardize the NaOH?
3) Find the structural formula of KHP in a reference. Write it. Which hydrogen is acidic? ( Which
one ionizes?)
4) What is the net ionic equation for the acid- base reaction in this lab?
5) What is an endpoint? How will you know when it is reached?
6) Calculate what mass of NaOH you will need to make the solution of NaOH (see proc., and write
it in lab notebook for future reference.)
7) A student weighs 5.678 g of KHP and dissolves it in water and places it in a
250 mL Erlenmeyer flask to which 5 drops of the acid-base indicator
phenolphthalein is added. This solution is titrated with a sodium hydroxide
solution of unknown molarity. 26.64mL of sodium hydroxide solution is required
to reach the endpoint. Calculate the molarity of the sodium hydroxide solution.
Chemlab- Standardization of NaOH
Purpose - to determine the concentration of a NaOH solution to be used in another lab.
IntroductionThe controlled addition and measurement of solution of known concentration that is required to react
completely with a measured amount of solution of unknown concentration is called a titration.
When using NaOH for an acid-base titration, it is necessary to “standardize” the solution. NaOH is both
water absorbing and CO2 absorbing, thus, we virtually never have a pure sample of NaOH. Since we can’t
know the exact amount of NaOH in a sample, we can’t calculate the molarity of a solution made with NaOH
by weighing NaOH and then dissolving it.
We can, however, use a extremely pure acid to react with it. By knowing how much NaOH (aq) is required to
react completely with a known amount of this pure acid, we can calculate the concentration of the
NaOH(aq).
Once we know the concentration of NaOH(aq), it can be used in other
titrations to determine the acid content of other solutions.
Compounds which are used because of their high purity and ease of use in a titration are called primary
standards. The primary standard we will use is
potassium hydrogen phthalate (KHP). It is monoprotic.
The reaction is :
NaOH(aq) + KHC8H4O4(aq)  NaKC8H4O4(aq) + H2O(l)
In this titration, we will use phenolphthalein as an indicator to show when the reaction contains a slight
excess of OH-. Ideally, we don’t want any excess OH- , so we will look for the slightest color change. This
point in the titration is called the endpoint.
ProcedureAdd enough NaOH(s) to a plastic container so that with addition of 1 L of
deionized water, you will have ~ .1 M NaOH(aq). Add the water.
Weigh ~ .65 g (record actual mass using the analytical scale) of KHP into two 125 mL Erlenmeyer flasks.
Dissolve in ~ 50 mL
of water, warming if necessary.
Add 2 drops of indicator and titrate using a buret till first faint pink color appears. Add titrant rapidly at first,
then more slowly as endpoint nears. This will be indicated by a longer persistence in color change as you
stir.
Titrate other flask likewise. Calculate the molarity of NaOH (aq) from the two trials.
If they differ by more than .02 M, repeat a third trial.
NaOH reacts with glass slightly and should always be rinsed out of the buret
between days in lab.
Do not discard the solution after this lab. It will be used in the next lab.
Data- A buret will be used to keep track of how much NaOH is required to react with the measured amount of acid.
You will need to record the initial reading and final reading on the buret for each trial. You will also need to record the
mass of KHP for each trial.Organize a data table in your lab notebook for the procedure. Recopy for the lab
report.
Calculations- Label and show one example of how you calculated the mol KHP, mol NaOH and [NaOH].
Partner’s Name_________________________
Name________________________
PurposeStandardization of NaOH Lab Report
Balanced net ionic equation-
Data
Trial
1
Trial
2
Trial 3
(if needed)
Mass of KHP (g)
Final buret reading (mL)
Initial buret reading (mL)
Volume of NaOH added (mL)
CalculationsMols of KHPMols of NaOHMolarity of NaOH
1
Mols of KHP
Mol of NaOH
[NaOH]
Conclusion - [NaOH] =
2
3 (if needed)
average
NA
NA