Passage III
When a solid metal (M) such as iron (Fe), nickel (Ni) or zinc (Zn) is placed in an aqueous hydrochloric acid (HCI) solution, a reaction that produces H2 gas occurs:
M + 2HCl → MCl2 +H2
Two experiments were conducted to study the production of H2, in this reaction. The apparatus shown in the diagram below was used to collect the H2 gas produced in each trial.
As H2 was produced in the stoppered flask, it exited the flask through the outlet tube and displaced the water that had been trapped in the inverted graduated cylinder. (This displacement occurred because the H2 did not dissolve in the water.）The volume of water displaced equaled the volume of gas (H2 and water vapor) collected.
In each trial of the experiments, Steps 1-3 were performed:1. The apparatus was assembled, and 25 mL of a 4 moles/L HCI solution was poured into the empty flask.
2. A selected mass of Fe, Ni, or Zn was added to the flask and the stopper was quickly reinserted into the flask.
3. When H2, production ceased, the volume of water that was displaced from the graduated cylinder was recorded.
The apparatus and its contents were kept at a selected temperature throughout Steps 2 and 3. The atmospheric pressure was 758 mm Hg throughout all 3 steps.
Experiment 1
In each trial, a selected mass of Fe, Ni, or Zn was tested at 30°C (see Figure 1).
Experiment 2
In each trial, 0.30 g of Fe, Ni, or Zn was tested at a selected temperature (see Figure 2).
17. Which of the following variables remained constant throughout both experiments?
Your Answer is
Correct Answer is A
Explanation
First exclude option D, because volume of gas collected is the dependent variable;
Options B & C are the independent variables of the two experiments respectively;
Only the atmospheric pressure of option A Strong is constant