Passage V

In a chemistry class, the teacher placed 0.5 g of porous steel wool, composed mostly of iron (Fe), inside a small heat-resistant quartz tube. She then used silicone hoses to connect the quartz tube to 2 airtight glass syringes (see figure). Each syringe contained 8 mL of air, and the total volume of air in the closed apparatus was 20 mL.

A Bunsen burner was then used to heat the contents of the quartz tube for 2 min. During heating, the plungers were moved up and down to pass the air back and forth through the steel wool. The total volume of gas in the apparatus steadily declined over the 2 min. Once the apparatus and its contents returned to room temperature, the total volume of gas in the apparatus was 16 mL.

The teacher asked each of 4 students to explain what occurred during the demonstration.

Student 1

During heating, the Fe in the steel wool reacted with all the in the air to form solid iron nitride (FeN), which was deposited on the steel wool. Air contains about 20% by volume. As a result of the reaction, the total volume of gas in the apparatus decreased by about 20%, so almost all the gas remaining in the apparatus was .

Student 2

During heating, the Fe in the steel wool reacted with some of the in the air to form solid iron oxide (), which was deposited on the steel wool. Air contains about 80% by volume. As a result of the reaction, the total volume of gas in the apparatus decreased by about 20%. Almost all the gas remaining in the apparatus was a mixture of about 75% and 25% by volume.

Student 3

Student 2 is correct, except that (1) the Fe in the steel wool reacted with all the , in the air and (2)air contains about 20% by volume. After the reaction, almost all the gas remaining in the apparatus was .

Student 4

During heating, the Fe in the steel wool reacted with all the in the air to form solid iron carbonate (), which was deposited on the steel wool. Air contains about 20% by volume. As a result of the reaction, the total volume of gas in the apparatus decreased by about 20%, so almost all the gas remaining in the apparatus was .