Step 1: Understanding the Concept:
The mass of a given substance can be calculated directly from its number of moles if its molar mass is known. The molar mass is defined as the mass of exactly one mole of a substance expressed in grams per mole ($\text{g/mol}$).
Step 2: Key Formula or Approach:
The fundamental relationship connecting mass, moles, and molar mass is:
\[ \text{Mass} = \text{Moles} \times \text{Molar Mass} \]
Step 3: Detailed Explanation:
First, we must determine the molar mass of water ($\text{H}_2\text{O}$).
Using standard atomic masses:
Atomic mass of Hydrogen (H) = $1 \text{ g/mol}$
Atomic mass of Oxygen (O) = $16 \text{ g/mol}$
The molar mass of $\text{H}_2\text{O}$ is calculated as:
\[ \text{Molar mass of H}_2\text{O} = (2 \times 1) + 16 = 2 + 16 = 18 \text{ g/mol} \]
The problem provides the number of moles:
Given number of moles = $0.25 \text{ mol}$
Substitute these known values into the mass formula:
\[ \text{Mass} = 0.25 \text{ mol} \times 18 \text{ g/mol} \]
To simplify the calculation, recognize that $0.25$ is equivalent to the fraction $\frac{1}{4}$:
\[ \text{Mass} = \frac{1}{4} \times 18 \text{ g} = 4.5 \text{ g} \]
Step 4: Final Answer:
The mass of $0.25 \text{ mol}$ water is $4.5 \text{ g}$.