Experiment 12B* - EQUIVALENT MASS OF AN UNKNOWN SOLID ACID
As the continuation of the acid-base titration experiment, you will determine the equivalent
mass of a solid organic acid using your standardized NaOH solution. If the acid molecule has
only one replaceable (acidic) hydrogen atom, such as HC2H3O2, then the number of
equivalents is the same as the number of moles of the acid and the equivalent mass is the
same as the molar mass. However, if the acid has two, or more replaceable hydrogens, such as
H2SO4 and H3PO4, then the titration data give only the equivalent mass, which is the molar
mass divided by the number of replaceable hydrogens. The key here is that with an unknown
acid the number of replaceable hydrogens is not known. The data from the experiment only
gives the mass of the sample divided by the number of moles of NaOH needed for titration.
This mass, then, is the equivalent mass and the molar mass cannot be determined without
some additional information. (The molar mass = the equivalent mass time an integer; the
integer being the number of replaceable hydrogen atoms in the acid molecule.
Procedure
Prepare a data table in your bound notebook. Refer to the suggested format. Record all data
using a pen (not a pencil) directly in your bound notebook at the time of observation.
Obtain an unknown sample from your instructor. Ask your instructor the size of sample to be
measured out. Write down the unknown number. Use weighing by difference on an analytical
balance to measure out three samples into Erlenmeyer flasks. Immediately clean up any spills
that occur in the balance or on the bench top next to the balance. Dissolve each sample in
about 50 ml of deionized water as you did with the oxalic acid in the course of standardization
of NaOH solution. If the samples do not dissolve after a few minutes, you may use a hot plate
to warm gently. If necessary, add another 25-50 ml of deionized water and warm again. When
the sample has dissolved, add phenolphthalein, and titrate.
Calculations and Results
Show the calculation of the equivalent mass (= sample mass/moles NaOH) for each run to the
proper number of significant figures in your bound notebook. Average the results and show
the calculation of the relative deviation and the average relative deviation in parts per
thousand. Prepare a summary table of results. Don't forget to record the unknown number on
in your lab notebook for the purposes of grading.
Experiment 12B* – “Determination of the Equivalent Mass of an Acid”
Molarity of NaOH solution determined in Exp. 12A = ____________ M
Data and Results Table:
Trial Number
2
3
4
N
O
TE
B
O
O
K
!!!
1
Mass of the unknown acid
Volume of NaOH(aq)
(2 digits after the decimal point!)
final buret reading
6
LA
B
− initial buret reading
5
TO
= volume of NaOH (aq) added
PY
C
O
Equivalent mass
of the acid
IN
Moles of NaOH
Runs used in averaging (x)
[Note: You must have at least three values that agree
K
!!!
Average Equivalent Mass: _________________
O
within 0.005. Use only values with this level of agreement
in calculating the average.]
O
Calculations:
TE
B
Run 1:
B
N
O
Run 2:
LA
Run 3:
IN
TO
Run 4:
C
Run 6:
O
PY
Run 5: