Passage VI

When hydrogen and oxygen are mixed under certain conditions, H2O will form. Students did the following experiments to study how H2O forms.

Experiment 1

A thick-walled gas syringe fitted with a sparking device was filled with 20 mL of hydrogen gas (H2) and 10 mL of oxygen gas (O2)，as shown in Figure 1.

The syringe plunger was then locked in place and the gases were sparked. A reaction took place, forming droplets of water. The plunger was then released. The gas in the syringe was allowed to adjust to room temperature and pressure, and the final volume of gas was recorded. The remaining gas, if any, was then analyzed to determine its composition. The procedure was repeated with different volumes of gases, and the results were recorded in Table 1.

Since it is known that equal numbers of molecules of different gases occupy equal volumes at the same pressure and temperature, the following equation was suggested:

2 H2 + O2 → H2O

Experiment 2

In the apparatus shown in Figure 2, a stream of H2 was passed over hot copper oxide (CuO), producing H2O vapor. The vapor was then absorbed by calcium chloride (CaCl2).

The changes in the masses of the tube sections containing the CuO and CaCl2 were used to calculate the mass of CuO reacted and the mass of H2O formed. It was determined that 1 molecule of H2O was produced for every CuO reacted:

CuO + H2 → Cu + H2O

31. In Trial 1 of Experiment 1, after the reaction had taken place, but before the syringe plunger was unlocked, one would predict that, compared to the pressure in the syringe before the spark, the pressure in the syringe after the spark was:

Answer and Explanation

Your Answer is

Correct Answer is A

Explanation

When H2 and O2 have just reacted and the plug has not moved, due to H2 and O2< /sub>is consumed to generate water molecules, so the gas molecules become less and the air pressure decreases