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

Question

In a lab, 96g of hydrogen gas and 404g of oxygen gas were collected. Student A says that the starting amount of water cannot be determined, but Student B says that the water must have been 500g. Who is correct?
A. Neither are correct. It was 2000g because the equation shows 2H2O.
B. Neither are correct. It would have been 1112g, because there are 2 H2 and 1 O2
C. Student A, because the water was not measured to start with
D. Student B, because the law of conservation of matter

Answer 1

To determine which student is correct, we can analyze the chemical reaction and the masses involved.

The original equation is:

\[ 2H_2O \rightarrow 2H_2 + O_2 \]

This means that 2 moles of water (H₂O) decompose to produce 2 moles of hydrogen gas (H₂) and 1 mole of oxygen gas (O₂).

From the molar masses:

• The molar mass of H₂ (hydrogen) = 2 g/mol
• The molar mass of O₂ (oxygen) = 32 g/mol
• The molar mass of H₂O (water) = 18 g/mol

Now, Student A says that the starting amount of water cannot be determined, while Student B claims it must have been 500 g.

Calculating the total mass of gases collected:

• Mass of hydrogen gas collected = 96 g
• Mass of oxygen gas collected = 404 g

Total mass of gases = 96 g + 404 g = 500 g

According to the law of conservation of mass, the total mass of the reactants (water in this case) must equal the total mass of the products (hydrogen and oxygen gases).

Since the total mass of the gases collected is 500 g, this means that the starting amount of water must also have been 500 g (because that's the mass that decomposed into these gases).

Thus, Student B is correct, and the correct answer is:

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