Passage V

Students were given unknown aqueous acid solutions UAX, UAY, and UAZ. Each solution had an acid concentration of 0.100 mole/L but contained a different acid. The students were also given Table 1, which gives the , of each of 5 acids (the lower the the stronger the acid).

In a titration, the titrant (a solution of known identity and concentration) is slowly added to the analyte (a solution of unknown identity or concentration). Students used titration to attempt to identify the acid present in UAX,UAY, and UAZ.Experiment 1

A 0.100 mole/L aqueous sodium hydroxide (NaOH, a base) solution was added to a buret (a graduated tube with a valve at the bottom that can be opened to dispense precise volumes of liquid). The initial volume of solution in the buret was recorded. A beaker containing 20.00 mL of UAX was placed under the buret. A pH probe was placed in the beaker. Then 25.00 mL of the NaOH solution was added in small increments to the acid solution while the acid solution was continuously stirred. After each addition the pH of the solution in the beaker was recorded (see Figure 1).

In Figure 1, the equivalence point indicates the volume of NaOH solution needed to react with all the acid in the solution in the beaker. At the halfway point, the volume of NaOH solution added is half of the volume needed to reach the equivalence point, and the pH of the solution in the beaker equals theof the acid in the solution.

Experiment 2

The procedure from Experiment 1 was repeated with UAY instead of UAX (see Figure 2).

Experiment 3

The procedure from Experiment 1 was repeated with UAZ instead of UAX (see Figure 3).