Which of the following represents the ratio by mass of hydrogen to oxygen atoms (H:O) in a water molecule?

Question

Examoogle · Accepted Answer

The correct answer is Option B: 8. The ratio by mass of hydrogen to oxygen in a water molecule ($H_2O$) is always $1:8$. This is calculated using the atomic masses: hydrogen has an atomic mass of $1	ext{ u}$ and oxygen has $16	ext{ u}$. For two hydrogen atoms and one oxygen atom, the total mass ratio is $(2 	imes 1) : (16) = 2 : 16 = 1 : 8$.

Step-by-Step Solution & Concepts:
The ratio by mass of hydrogen to oxygen in a water molecule ($H_2O$) is always $1:8$. This is calculated using the atomic masses: hydrogen has an atomic mass of $1	ext{ u}$ and oxygen has $16	ext{ u}$. For two hydrogen atoms and one oxygen atom, the total mass ratio is $(2 	imes 1) : (16) = 2 : 16 = 1 : 8$.

Working Principle: According to the Law of Constant Proportions (or Law of Definite Proportions), a chemical substance always contains the same elements combined in the same fixed proportions by mass, regardless of the source or method of preparation. In water ($H_2O$), the atomic mass of $2$ hydrogen atoms is $2 	imes 1 = 2	ext{ u}$ and $1$ oxygen atom is $16	ext{ u}$. Therefore, mass ratio = $\frac{2}{16} = \frac{1}{8}$, which is $1:8$.

Key Formulas: $	ext{Mass of } H = 2 	imes 1	ext{ u} = 2	ext{ u}$; $	ext{Mass of } O = 1 	imes 16	ext{ u} = 16	ext{ u}$; $	ext{Ratio } (H:O) = \frac{2}{16} = 1:8$

Key Points: Water ($H_2O$) consists of Hydrogen and Oxygen in a fixed mass ratio of $1:8$.; By atom count (atomic ratio), the ratio of Hydrogen to Oxygen is $2:1$.; This illustrates Joseph Proust's Law of Constant Proportions.; Atomic mass of $H = 1	ext{ u}$ and $O = 16	ext{ u}$.

Advantages: Fundamental concept for solving stoichiometry problems in competitive chemistry.

Applications: Calculating empirical and molecular formulas in chemical analysis.; Industrial synthesis of water and hydrogen peroxide.

Additional Info: Note the difference between 'ratio by mass' ($1:8$) and 'ratio by number of atoms' ($2:1$ or $2:1$). Questions frequently trap students by mixing these two up.. Option C ($1:8$) is the exact ratio by mass.

Answer: C is correct — The ratio by mass of hydrogen to oxygen in a water molecule ($H_2O$) is $1:8$.

Exam Tip: Always read carefully whether the question asks for 'ratio by mass' ($1:8$) or 'ratio by number of atoms' ($2:1$). Examiners routinely swap these to test conceptual clarity.

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