Chemistry 223 Identification of Weak Acid
Identification of an Unknown Organic Acid
Understanding weak acids and weak bases is a central part of quantitative analysis.
involved are important in all aqueous chemistry (thus, biochemistry), and the methods to study them
help develop your critical thinking skills.
Furthermore, to get good results, you have to carefully deal
with measurement precision, data reduction, and chemical handling. You also must multi
task. If you
stare at boiled water while it cools, rather than working on other tasks, you will not finish this lab.
In Week One of this lab, you will:
Make concentrated NaOH
Make carbonate-free water
Dilute NaOH to leave it carbonate free
Dry potassium hydrogen phthalate (KHP) to use as a primary standard
Weigh 4 aliquots of KHP
Titrate KHP with NaOH to standardize the NaOH
Obtain an unknown weak acid
Dry the unknown
In Week Two, you will:
Weigh 4 aliquots of unknown weak acid
Titrate the weak acid with standard NaOH, monitoring each titration with a pH meter
From the shape of the titration curve, determine the equivalent weight of the acid
From the shape of the titration curve, determine the pK
s) of the acid
By comparison to published data, determine the identity of your unknown
In light of what you have determined, specify pH's where your unknown could be used to make a
buffer, and answer questions about what would be different if the unknowns were weak bases.
How many times on CSI have you seen detectives encounter "an unknown white powder?" The powder
could be sugar, artificial sweetener, an illegal substance, or one of the unknowns from this lab. While
you can be sure we will only give you a weak acid unknown (there goes the suspense … ), you can
experience the rush of solving a problem and knowing you've "nailed it."
Instructions for Week One
A. Make Concentrated NaOH
Prepare some 50% by weight NaOH. Procedure: weigh out approximately 25 g of NaOH (this can not be
done accurately; NaOH is hygroscopic. Getting it close on a top
loading or triple beam balance is Good
Enough). Add as many mL of distilled water as the mass of NaOH (so if you weigh 24 g of NaOH, use 24
mL water). Store in a tightly sealed 50 mL: (or 100 mL) plastic bottle. Swirl well to mix and dissolve the
NaOH, then let settle so the Na
settles out. Once the settling starts, DO NOT MOVE THE BOTTLE.