Answered

Under certain conditions, water can be separated into oxygen and hydrogen gases according to the equation $2 H _2 O \rightarrow 2 H _2+ O _2$.



In a lab, 112 g of hydrogen gas and 888 g 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 1000 g. Who is correct?


ANeither are correct. It would have been 1112 g, because there are $2 H _2$ to $1 O _2$.
BNeither are correct. It was 2000 g because the equation shows $2 H _2 O$.
CStudent A, because the water was not measured to start with.
DStudent B, because of the law of conservation of matter.

Verified Answer :

  • The law of conservation of mass states that the total mass of reactants equals the total mass of products.
  • Given the products are \(112\text{ g}\) of \(H_2\) and \(888\text{ g}\) of \(O_2\), the total mass of products is \(112\text{ g} + 888\text{ g} = 1000\text{ g}\).
  • By the conservation of mass, the initial mass of water must be equal to the total product mass of \(1000\text{ g}\).
  • Consequently, Student B is correct. Final choice: \(\boxed{\text{Student B, because of the law of conservation of matter.}}\)

Explanation

  1. Analysis of the Law of Conservation of Mass

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants must equal the total mass of the products. The given chemical reaction is:
\[2 H_2O \rightarrow 2 H_2 + O_2\]
Here, the reactant is water (\(H_2O\)) and the products are hydrogen gas (\(H_2\)) and oxygen gas (\(O_2\)). To determine the initial mass of water, we must sum the masses of the products collected, which are \(112\text{ g}\) of hydrogen and \(888\text{ g}\) of oxygen.

  1. Calculating the Initial Mass of Water

To find the total mass of the products, we add the mass of hydrogen and the mass of oxygen:
\[\text{Total mass of products} = \text{mass of } H_2 + \text{mass of } O_2\]
\[\text{Total mass of products} = 112\text{ g} + 888\text{ g} = 1000\text{ g}\]
According to the law of conservation of mass, the total mass of the products must equal the initial mass of the reactant (water). Thus, the initial mass of water must be \(1000\text{ g}\).

  1. Evaluating the Students' Claims

Comparing our result with the claims made by the students:

  • Student A claims that the amount of water cannot be determined. This is incorrect, as the law of conservation of mass allows us to calculate the initial amount based on the products.
  • Student B claims that the water must have been \(1000\text{ g}\). This is consistent with our calculation based on the law of conservation of mass.

Therefore, Student B is correct. Among the provided options, the correct statement is: "Student B, because of the law of conservation of matter."

Examples

The law of conservation of mass is fundamental to chemical stoichiometry. For example, if you bake a cake using 500g of flour, 200g of sugar, 100g of eggs, and 200g of milk, you expect the total mass of the ingredients to be 1000g. Even after the chemical reactions that occur during baking (which release some water vapor as steam), the total mass of the cake and the released steam will still sum up to the initial mass of the ingredients. Understanding this principle helps scientists and engineers predict the yields of chemical reactions and account for all components in any closed system.

Related Questions