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Unformatted text preview: ss determined. The
sample of vinegar is then titrated with a NaOH solution of known concentration (Mb as e), and the volume of
NaOH solution required to reach the endpoint (Vb as e) is determined. Vb as e, Mb as e, and Vacid are all known, so
the concentration of the acid (Macid ) can be determined as described above. In addition, the mass of acetic acid
in the sample can be determined from the number of moles present and the molar mass of acetic acid (gacid =
MWacid × molesacid ). Finally, the mass percent of acetic acid in the vinegar can be determined from the mass of
the acetic acid in the sample and mass of the vinegar solution that was titrated.
Mass % =
mass of acetic acid in sample
mass of vinegar solution titrated
In the titration of acetic acid with aqueous NaOH, phenolphthalein is used as the indicator. Phenolphthalein is
nearly colorless in acidic solution, but turns pink at a pH of about 8. This indicates that the base has neutralized
all the acid. As you titrate the vinegar, you will observe that the pink color is more persistent as you add more
base. This is a signal to slow the addition of base, and control it carefully. The endpoint has been reached when a
faint pink color persists for at least 30 seconds. It is easy to overshoot the endpoint. If this happens, you will
have a dark purple- pink solution, and you will have to repeat the titration, so be careful. Note the volume you
have used; stop short of this volume in subsequent titrations, and add the last milliliter or so dropwise. Your
instructor will show you how to control the stopcock of the buret to facilitate this.
Note that the volume
measurements in titrations are usually reported to four s ignificant figure s , so the concentrations are usually
reported to four significant figures as well. Watch this in your work; when you calculate molar masses, make sure
you have four significant figures. Equipment
1 10.0 mL graduated cylinder
1 30 mL beaker
1 100 mL beaker
1 25 mL buret
1 ring stand and buret clamp
2 125 mL Erlenmeyer flasks
1 deionized water squirt bottle Re...
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- Spring '14