SUSB-012 - A common calculation error
The primary purpose of the
phenolphthalein titration of the unknown acid (trial run) it to be able to compute
the weight of the acid to be used in the pH titration.
It incidentally produces a value of the molar mass of the acid which might be
helpful to confirm the molar mass subsequently found from the pH titration.
The section of the data sheet for the phenolphthalein titration serves two linked purposes.
1. To establish the weight of unknown acid needed in the pH titration to consume (25 +/- 3 mL) of NaOH at the equivalence point in Part 2 of the exercise
2. To estimate of the molar mass of the unknown acid
The table below seeks to elaborate the computations of the two with sample data. The TITER is used only to simplify the computation of the desired weight of acid for the pH titration.
Data Sheet - Run 1 (Phenolphthalein)
Item |
Origin |
Computation |
Value |
| Concentration of NaOH solution | From
Label of Carboy |
----- |
0.05263
M |
| Mass of acid sample | From
Analytical Balance |
----- |
217.1
mg |
| Net Volume of NaOH sol'n. | From
Buret Readings |
----- |
18.36
mL |
| mmol of NaOH | From
concentration and volume |
0.05263
* 18.36 |
0.9663
mmol |
| mmol of unknown acid | Equal
to mmol of NaOH |
----- |
0.9663
mmol |
| Titer | From
mass of acid and volume of NaOH |
217.1
/ 18.36 |
11.82
mg/mL |
| Weight of acid for 25 mL of NaOH for pH titration | From
titer and desired volume (25 +/- 3 mL) |
11.82
* (25.0 +/- 3) |
296
+/- 35 mg |
| Molar mass of unknown acid | From
mass of acid and mmol of acid |
217.1
/ 0.9663 |
224.7
g/mol |
The mass used in the
titration (217.1
mg) must be used to calculate the
molar mass of the unknown acid
not the mass calculated for Part 2 (296
+/- 35 mg)!
Similarly,
the amount weighed out for the pH titration should be in
the mass (range) calculated for Part 2 (296
+/- 35 mg)
Last
Update: RFS
2006-03-26
Maintained by: rschneider at notes.cc.sunysb.edu