Hydrogen peroxide, H^2 O^2, decomposes at room temperature to release oxygen gas.

Question

Use the law of definite proportions to explain why doubling the quantity of hydrogen peroxide doubles the quantity of oxygen produced.

Answer 1

2H₂O₂→2H₂O+O₂

2x 2x x

x x 0.5x

Law of definite proportions basically says that a substance always has the same chemical formula.

So, if you double the amount of H₂O₂ decomposing, you double the amount of O₂ that forms.

1 mol of H₂O₂ decomposes to form 1 mol H₂O and 0.5 mol O₂

Doubling it...

2 mol of H₂O₂ decomposes to form 2 mol H₂O and 1 mol O₂

This can be pretty confusing if you're not familiar with stoichiometry and math involving chemical equations.

Answer 2

When learning in chemistry it can be helpful to appreciate quantities in the form of moles. A mole is defined as 6.02214076 × 10²³ of any molecule or atom. (Roughly speaking 1 mol of Hydrogen (H) is 1 gram) Because the number of moles in based on the number of units, one mol of Hydrogen Peroxide will decompose into one mol of Hydrogen gas (H₂) and one mol of Oxygen gas (O₂). So if you double the moles of H₂O₂ you get double the moles of H₂ and O₂ respectively. The law of definite proportions says masses will be the same before and after, and you can use simple math and periodic tables to show that 1 mol (~34g) H₂O₂ = 1 mol (~32g) O₂ and 1 mol (~2g) H₂.