Passage V

When vinegar (a solution of acetic acid, CH3COOH, in H2O) is mixed with baking soda (sodium bicarbonate, NaHCO3), the following reaction occurs:

CH3COOH + NaHCO3 → CH3COONa + H2O + CO2

A teacher constructed the apparatus shown in Figure 1 for a demonstration. First, 30 mL of vinegar was poured into a bottle. Then, 5 g of baking soda was placed in a bal­loon. The balloon was affixed to the top of the bottle with wire to form an airtight seal.

The apparatus was placed on a balance, and the mass reading was 68.7 g. The balloon was then lifted up so that all the baking soda fell into the bottle. As bubbles formed in the bottle, the balloon inflated and the mass reading on the balance steadily decreased. When the reaction was fin­ished, the mass reading was 68.1 g (see Figure 2).

The teacher asked 3 students to explain what occurred in the demonstration. The students agreed that the balloon inflated because bubbles of CO2 formed and then exited the reaction mixture. They disagreed about why the mass read­ing on the balance decreased.

Student 1

No gas exited the apparatus because it was airtight. Gases, however, have no mass, so as CO2 was produced, the mass of the apparatus decreased.

Student 2

Gases have mass, so the mass of the apparatus should not have changed. A balloon, however, will deflate over time because there are tiny pores in its surface. Due to the rapid pressure increase in the apparatus, a measurable mass of CO2 exited through the pores in the quickly expanding balloon, which caused the mass of the apparatus to decrease.

Student 3

Gases have mass, and although there are tiny pores in a balloon's surface, there was not enough time for a mea­surable mass of gases to have exited the balloon. The air exerted an upward buoyant force on the apparatus that was equal to the weight of the volume of air that was displaced by the apparatus. As the balloon inflated, the volume of the apparatus increased, causing it to displace a greater volume of air. Thus, the buoyant force increased, which caused the mass reading on the balance to decrease. The actual mass of the apparatus did not measurably change.

28. Suppose that the apparatus had been fitted with a rigid, airtight cap instead of the balloon, and that the baking soda had been contained in a chamber below the cap. If the baking soda in the chamber had been released by remote control into the vinegar, which of the students would have been likely to predict that the mass reading on the balance would decrease ?

Answer and Explanation

Your Answer is

Correct Answer is F

Explanation

If the balloon is replaced with a cover, student 1 will continue to think that the total mass has decreased, because he believes that the gas has no mass;

student 2 believes that the small hole on the balloon is the cause of the mass decrease, So when the balloon is replaced with a cover, there is no small hole, the gas will not escape, and the student will think that the mass remains the same;

student 3 believes that the reason for the buoyancy is that the filled balloon occupies more space , so after changing to the cover, the occupied space remains the same, so he will also think that the quality of the display remains the same