Understanding the Mass Ratio in a Water Molecule

The question asks for the ratio of the mass of hydrogen to that of oxygen in a molecule of water. A water molecule has the chemical formula $\text{H}_2\text{O}$. This means each water molecule contains two hydrogen atoms and one oxygen atom.

To find the mass ratio, we need to consider the atomic masses of hydrogen and oxygen.

• The approximate atomic mass of Hydrogen (H) is $1$ atomic mass unit (amu).
• The approximate atomic mass of Oxygen (O) is $16$ atomic mass units (amu).

Calculating the Total Mass of Each Element

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

• There are two hydrogen atoms. The total mass contributed by hydrogen is $2 \times (\text{Atomic mass of H}) = 2 \times 1 = 2$ amu.
• There is one oxygen atom. The total mass contributed by oxygen is $1 \times (\text{Atomic mass of O}) = 1 \times 16 = 16$ amu.

Determining the Mass Ratio

The ratio of the mass of hydrogen to the mass of oxygen is the total mass of hydrogen divided by the total mass of oxygen.

Mass of Hydrogen : Mass of Oxygen

$2 \text{ amu} : 16 \text{ amu}$

To simplify this ratio, we can divide both sides by the greatest common divisor, which is 2.

$\frac{2}{2} : \frac{16}{2}$

$1 : 8$

So, the ratio of the mass of hydrogen to that of oxygen in a water molecule is $1 : 8$.

Let's look at the options provided:

• $1 : 8$
• $1 : 4$
• $1 : 2$
• $1 : 16$

Our calculated ratio $1 : 8$ matches the first option.

Revision Table: Atomic Masses and Ratios

Mass Ratio (Hydrogen : Oxygen) = Total Mass of Hydrogen : Total Mass of Oxygen = $2 : 16 = 1 : 8$.

Additional Information: Importance of Mass Ratios

Understanding mass ratios in chemical compounds is fundamental in chemistry. It helps in several areas:

• Law of Constant Proportions: This law states that a given chemical compound always contains the same elements in the exact same proportion by mass. The $1:8$ mass ratio of hydrogen to oxygen in water is an example of this law. No matter where water comes from or how it is formed, the mass ratio of hydrogen to oxygen will always be $1:8$.
• Stoichiometry: Mass ratios are crucial for calculating the amounts of reactants and products involved in chemical reactions.
• Chemical Formula Determination: Experimental determination of the mass ratios of elements in a compound can help determine its empirical formula.
• Composition Analysis: Knowing mass ratios allows chemists to analyze the composition of unknown substances.

While we used approximate atomic masses here, more precise calculations would use the average atomic masses from the periodic table (e.g., H $\approx$ 1.008 amu, O $\approx$ 15.999 amu). However, for introductory purposes, using whole numbers is common and gives the simplified integer ratio accurately.