Under certain conditions, water can be separated into oxygen and hydrogen gases according to the equation 2H2O → 2H2 + O2.

Question

In a lab, 112g of hydrogen gas and 888g of oxygen gas were collected. Student A says that the amount of water to start cannot be determined, but Student B says that the water must have been 1000g. Who is correct?

(1 point)
Responses

Neither are correct. It would have been 1112g, because there are 2 H2 to 1 O2.
Neither are correct. It would have been 1112g, because there are 2 H 2 to 1 O 2 .

Neither are correct. It was 2000g because the equation shows 2H2O.
Neither are correct. It was 2000g because the equation shows 2H 2 O.

Student A, because the water was not measured to start with.
Student A, because the water was not measured to start with.

Student B, because of the law of conservation of matter.

Answer 1

To determine who is correct, we will use the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction.

From the balanced chemical equation:

\[ 2H_2O → 2H_2 + O_2 \]

This tells us that 2 moles of water (H₂O) produce 2 moles of hydrogen gas (H₂) and 1 mole of oxygen gas (O₂). The molecular weights are approximately:

• H₂O: 18 g/mol (2 g/mol for H, 16 g/mol for O)
• H₂: 2 g/mol (2 H)
• O₂: 32 g/mol (2 O)

Now, let’s calculate the total mass of the hydrogen and oxygen gases collected:

• Mass of H₂ = 112 g
• Mass of O₂ = 888 g
• Total mass = 112 g + 888 g = 1000 g

According to the law of conservation of mass, the mass of the water used to produce these gases should equal the total mass of the products (H₂ and O₂). Therefore, the starting mass of water must also be 1000 g.

Thus, Student B is correct in saying that the water must have been 1000 g.

So, the correct answer is: Student B, because of the law of conservation of matter.