Passage VI

At constant temperature and pressure, the ratio in which the volumes of gases react and form products is indicated by the balanced chemical equation for the reaction. For example, consider the balanced chemical equation for hypothetical Gases A, B, and C:

A + 2B → C

At constant temperature and pressure, 1 L of Gas A will react with 2 L of Gas B to form 1 L of Gas C.

Students did experiments at constant room temperature and pressure to determine the ratio in which nitric oxide (NO) and O₂ (either pure, or from air) react to form nitrogen dioxide (NO₂).

Experiment 1

Two glass syringes (Syringes I and II) were lubricated

so their plungers would move freely. Then, 20 mL of NO was introduced into empty Syringe I through its tip and the tip was closed. Next, 20 mL of O₂ was introduced into empty Syringe II through its tip and the tip was closed. The syringes were then attached to each other and the tips were opened. The plunger of Syringe II was pushed to the bottom (emptying its content into Syringe I) and its tip was closed. After the reaction was complete, the final volume of gas in Syringe I was recorded and its contents were analyzed (see Figure 1).

Figure 1 adapted from Donald D. DuPre, "A Simple Demonstration of the Law of Combining Volumes." @1993 byDivision of Chemical Education, Inc., American Chemical Society.

The same procedure was used with various initial volumes of NO and O₂ (see Table 1).

Experiment 2

The procedure from Experiment 1 was used to test 20 mL of air (instead of O₂) mixed with various amounts of NO (see Table 2).