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H-2
EXPERIMENTAL METHOD
The acids used as unknowns in this experiment are solids and have molar masses in the range 50-
250 g mol
-1
. To determine the moles of acid present, you must carry out at least three titrations.
The first titration will utilize a pH meter. The other two, (three, if time permits), will employ an
indicator (properly chosen using the information obtained from the pH titration curve). You will
evaluate:
1.
the number of moles of base required to neutralize a weighed sample of the unknown
acid
2.
the molar mass of the acid
3.
the acid dissociation constant of your unknown acid
You will weigh out a known mass of a solid acid into a container, dissolve it in water, and then
titrate it with a previously standardized NaOH solution.
Note: Some of the acids are slightly hygroscopic. Therefore you should handle your
unknown acid in such a way, as to minimize exposure to the atmosphere when
weighing out the samples.
The volume of water added to dissolve the acid does NOT affect the volume of NaOH solution
required to reach the equivalence point, because it is the moles of acid present in the weighed
sample that determines the moles of titrant required.
Since you do not know the pH at the equivalence point for your titration, you will need to use a pH
meter for the first titration. Once this titration is complete, you will need to plot the graph of pH
versus volume of NaOH added in order to determine (graphically) the pH of the equivalence point.
This will allow you to select a suitable indicator that will be used to do several more regular
titrations of your unknown acid.
NOTE:
You will be required to plot your data from the pH titration on graph paper as
you perform the titration. Therefore, come prepared to plot the graph during the
laboratory session by labeling, in advance, the axes on a piece of 1.0 mm grid
graph paper in the following manner: pH scale on the y-axis (short axis) from 0-
14 and the volume of NaOH on the x-axis (long axis) from 0 – 48 mL.
