Passage IV

Acid-base titration is a technique in which precise volumes of a titrant (an acid or base solution) are added incrementally to a known volume of a sample solution (a base or acid solution, respectively). This process can be monitored by adding an acid-base indicator (a substance that changes color over a certain pH range) to the sample solution or by measuring the sample solution's conductivity. Conductivity (measured in kilosiemens per centimeter, kS/cm) is a measure of a substance's ability to conduct electricity.

Two titration experiments were done at 25°C using a 0.10 M sodium hydroxide (NaOH) solution and either a 0.0010 M hydrochloric acid (HCl) solution or a 0.0010 M acetic acid solution (where M is moles of acid or base per liter of solution). All solutions were aqueous. An acid-base indicator solution of nitrazine yellow was also used. Nitrazine yellow is yellow if the pH is less than 6.0 or blue if the pH is greater than 7.0.

Experiment 1

A drop of nitrazine yellow solution was added to a flask containing 100.0 mL of the HCl solution. A probe that measures conductivity was placed in the solution. The NaOH solution was slowly added to the HCl solution in small increments. After each addition, the HCl solution was stirred and then the solution's color and conductivity were recorded (see Figure 1).

Experiment 2

Experiment 1 was repeated, except that the acetic acid solution was used instead of the HCl solution (see Figure 2).

21. In Experiment 2, which solution was the titrant and which solution was the sample solution?

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

Look at the first line of the text below Experiment 2, it has been clearly stated that the sample solution uses acetic acid, and D is excluded;

Look at figure 2 again, the sample solution gradually changes from yellow to blue, indicating the original sample The solution is acidic, and then it becomes alkaline, so the titrant titrant should be alkaline, so A & B is excluded