To determine who is correct, we need to apply the law of conservation of mass which states that mass can neither be created nor destroyed in a chemical reaction.
In the given reaction: \[ 2H_2O \rightarrow 2H_2 + O_2 \]
From the stoichiometry of the reaction:
- 2 moles of water (H₂O) yield 2 moles of hydrogen (H₂) and 1 mole of oxygen (O₂).
- The molar mass of water is approximately 18 g/mol (2 g/mol for H and 16 g/mol for O), so 2 moles of water (36 g) yield 4 g of hydrogen (2 g/mol) and 32 g of oxygen (16 g/mol).
Now let's calculate the total mass of gases collected:
- Total mass of hydrogen gas: 112 g
- Total mass of oxygen gas: 888 g
- Combined mass of gases = 112 g + 888 g = 1000 g.
According to the law of conservation of mass, the mass of the water before the reaction must equal the total mass of the products after the reaction. Thus, if 1000 g of gases were collected (112 g of H₂ and 888 g of O₂), the initial mass of water must also be 1000 g.
Now evaluating the claims:
- Student A says that the amount of water to start cannot be determined, which is incorrect since we can calculate it based on the collected gases.
- Student B claims the water must have been 1000 g, which is correct based on the conservation of mass.
Therefore, the correct answer is: Student B, because of the law of conservation of matter.