Calculating the Mass Ratio of Hydrogen and Oxygen in Water

Understanding the composition of water helps us determine the ratio of masses of the elements present. Water has the chemical formula $\text{H}_2\text{O}$. This formula tells us that each molecule of water consists of two hydrogen atoms and one oxygen atom.

Atomic Masses of Hydrogen and Oxygen

To find the mass ratio, we need to know the atomic masses of hydrogen and oxygen:

• Atomic mass of Hydrogen ($\text{H}$) is approximately $1 \text{ u}$ (atomic mass unit).
• Atomic mass of Oxygen ($\text{O}$) is approximately $16 \text{ u}$.

Calculating Total Mass in a Water Molecule

In a water molecule ($\text{H}_2\text{O}$):

• Total mass of Hydrogen = Number of Hydrogen atoms × Atomic mass of Hydrogen
• Total mass of Hydrogen = $2 \times 1 \text{ u} = 2 \text{ u}$
• Total mass of Oxygen = Number of Oxygen atoms × Atomic mass of Oxygen
• Total mass of Oxygen = $1 \times 16 \text{ u} = 16 \text{ u}$

Determining the Mass Ratio

The ratio of the masses of hydrogen to oxygen in water is the ratio of their total masses in a molecule:

Ratio of mass of Hydrogen : Ratio of mass of Oxygen = Total mass of Hydrogen : Total mass of Oxygen

Ratio of mass of Hydrogen : Ratio of mass of Oxygen = $2 \text{ u} : 16 \text{ u}$

The ratio of masses of hydrogen and oxygen in water is $2 : 16$. This ratio can be simplified by dividing both numbers by their greatest common divisor, which is 2. Simplifying $2 : 16$ gives $1 : 8$. However, one of the options provided is $2 : 16$, which is the direct ratio calculated from the atomic masses and the number of atoms in the molecule before simplification.

Analyzing the Options

Let's look at the given options:

Based on the calculation, the ratio of masses of hydrogen and oxygen in water is $2 : 16$.

Revision Table: Key Concepts

Additional Information: Conservation of Mass and Ratios

The ratio of masses of elements in a pure chemical compound is always fixed, regardless of the source or amount of the compound. This is a fundamental concept related to the Law of Definite Proportions (also known as Proust's Law). For water ($\text{H}_2\text{O}$), the mass ratio of hydrogen to oxygen is always $2:16$ or $1:8$. This means that in any sample of pure water, for every 2 grams (or any unit of mass) of hydrogen, there will be 16 grams (or the same unit of mass) of oxygen combined with it.

Understanding mass ratios is crucial in stoichiometry, which is the study of the quantitative relationships between reactants and products in chemical reactions. It helps in predicting the amount of products formed or reactants needed in a reaction.

While $2:16$ is mathematically equivalent to $1:8$, the option $2:16$ directly reflects the contribution of the two hydrogen atoms ($2 \times 1$) and one oxygen atom ($1 \times 16$) based on their atomic masses, before any simplification. Both ratios represent the same proportion.