Question

Use the 100 mL graduated cylinder to measure and pour 50 mL of distilled water into Beaker 1.

Use the 100 mL graduated cylinder to measure and pour 48 mL of distilled water into Beaker 2.

Use the 10 mL gradated cylinder to measure and pour 1 mL of saturated sodium bicarbonate solution into Beaker 1. Then, add 2 drops of phenol red to Beaker 1. Gently mix the solution.

Use the straw to blow bubbles into Beaker 1. Start the stopwatch as soon as you start blowing into the straw. Observe the color of the solution as you blow into the straw, and record how long it takes for the pH indicator (phenol red) to change from pink/red (basic) to yellow (acidic).

Use the 10 mL graduated cylinder to measure and pour 1 mL of saturated sodium bicarbonate solution into Beaker 2.

Use a pipette to add five drops of the sodium bicarbonate buffer solution to Beaker 2. Then, add 2 drops of phenol red to Beaker 2. Gently swirl the solutions to create a homogeneous mixture.

Use the straw to blow bubbles into Beaker 2. Start the stopwatch as soon as you start blowing into the straw.

Observe the color of the solution as you blow into the straw, and record how long it takes for the pH indicator (phenol red) to change from pink/red (basic) to yellow (acidic).

Was the time required to change the solution different for the two beakers? Why or why not?

Answer 1

There are two beakers. Beaker 1 contains sodium bicarbonate, water and phenol red. Beaker 2 has sodium bicarbonate, sodium bicarbonate buffer, water and phenol red.

In beaker 1, Sodium bicarbonate reacts with water to form carbonic acid and NaOH. This carbon dioxide breaks down to carbon dioxide and water, which gives bubbles. There is reduction of pH of the solution due to CO2, that will change the color of phenol red from red to yellow. Phenol red is yellow under acidic conditions and red under alkaline conditions.

Sodium bicarbonate buffer stabilizes the pH of the solution. In solution, it dissolves into sodium and bicarbonate ions. As the bicarbonate ions in the second beaker increases when the buffer is added to sodium bicarbonate solution. As the bicarbonate concentration increases, the pH will increase. Thus, the buffer will resist the change in pH. Hence, the color change of phenol red in beaker 2 will be much slower than in beaker 1.