Passage V

The law of definite proportions states that the percent by mass of any given element in a chemical compound is always the same, regardless of how the compound is formed. The law of conservation of mass states that, in a chemical reaction, the total mass of products must equal the total mass of reactants.

To study these chemical laws, students performed an experiment in which zinc (Zn) and iodine (I₂) reacted to form zinc iodide (ZnI₂):

Zn + I₂ → ZnI₂

Experiment

In each of 5 trials, Steps 1 − 9 were performed:

1. A certain mass of Zn metal was placed into a flask.

2. A certain mass of I2 was added to the flask.

3. Twenty-five mL of methanol (a solvent) was added to the flask, and the mouth of the flask was covered with aluminum foil.

4. The contents of the flask were boiled until a color change was observed (which signaled that the I₂ had been completely consumed).

5. The flask was allowed to cool, and the contents of the flask were filtered using filter paper.

6. The filter paper was dried, and the mass of unreacted Zn on the filter paper was determined.

7. The filtrate (the liquid that passed through the filter) was poured into a beaker and heated to evaporate the methanol, leaving a dry solid in the beaker.

8. The mass of the dry solid was measured.

9. The percent by mass of Zn and of I in the dry solid was calculated.

The results are shown in the table below.