Quick Lab: Molarity of Vinegar
Objective: Collect data to help you calculate the molarity of acetic acid (HC2H3O2) in vinegar. WARNING: Do not
mix up pipettes. Use the vinegar pipette for vinegar and the sodium hydroxide pipette for sodium hydroxide!
Procedure:
1.
2.
3.
4.
5.
Lay your 24-well reaction plate on a piece of white paper.
Add exactly 10 drops of vinegar to each of three adjacent clean wells.
Add exactly 1 drop of phenolphthalein to each of the three wells. This is an acid-base indicator. When
there is an excess of base present, the phenolphthalein will turn pale pink.
Use the toothpick to mix the contents of each well.
Add 0.2 M NaOH, dropwise, to each well and mix after the addition of every drop of base. STOP when the
resulting solution stays a very faint pale pink. Keep count of the number of drops of base required, and
record this in the chart.
Data:
Trial 1 (first well)
Trial 2 (second well)
Trial 3 (third well)
# drops of NaOH needed
Average # drops of
NaOH needed
Questions:
1.
Write a balanced neutralization equation for the reaction of acetic acid with sodium hydroxide.
2.
Calculate the molarity of acetic acid in vinegar:
a) Calculate volume of vinegar added to one
well, in L. (1 drop = 0.05 mL)
b) Calculate average volume of the NaOH
drops, in L.
c)
Calculate average # moles of NaOH. (#
moles = molarity of NaOH x volume in L)
d) According to the balanced equation, how
many moles of acid reacted with base?
(Use stoichiometry)
e)
Divide # moles acid reacted (answer in d))
by the volume of the vinegar in L. This is
the molarity of the vinegar.
3.
Calculate the molarity of acetic acid in vinegar
another way. (I will show you this.)