Calculate the relative molecular mass of water (H2O) using standard atomic masses.

Introduction / Context:
Relative molecular mass is an important concept in chemistry that links the atomic masses of elements to the mass of a molecule. It is used in calculations involving moles, gas laws and solution chemistry. Water is one of the most fundamental compounds, and students are often expected to know or quickly calculate its relative molecular mass. This question asks you to compute the relative molecular mass of H2O given standard atomic masses for hydrogen and oxygen.

Given Data / Assumptions:

• The formula of water is H2O.
• Each molecule of water contains two hydrogen atoms and one oxygen atom.
• The approximate relative atomic mass of hydrogen is 1 u.
• The approximate relative atomic mass of oxygen is 16 u.
• We use simple whole number values for school level calculations.

Concept / Approach:
Relative molecular mass is found by adding the relative atomic masses of all atoms present in one molecule, taking into account how many of each atom there are. For water, the formula H2O indicates two hydrogens and one oxygen. We multiply the atomic mass of hydrogen by two, add the atomic mass of oxygen once and obtain the total. This simple procedure generalises to any molecular or formula mass calculation and is routinely used in stoichiometry.

Step-by-Step Solution:
Step 1: Identify the elements present in water H2O: hydrogen and oxygen. Step 2: Note that there are two hydrogen atoms and one oxygen atom in each water molecule. Step 3: Use the approximate relative atomic masses: hydrogen is 1 u, oxygen is 16 u. Step 4: Compute the contribution from hydrogen: 2 multiplied by 1 u equals 2 u. Step 5: Compute the contribution from oxygen: 1 multiplied by 16 u equals 16 u. Step 6: Add these contributions: 2 u plus 16 u equals 18 u. Step 7: Therefore, the relative molecular mass of water H2O is 18 u.

Verification / Alternative check:
If you use more precise atomic masses, such as 1.008 u for hydrogen and 16.00 u for oxygen, the calculation is slightly refined: 2 times 1.008 equals 2.016, plus 16.00 equals approximately 18.016 u. For most school level work, this is rounded to 18 u. This value is consistent with molar mass calculations where one mole of water has a mass of about 18 grams. Many chemistry tables and problem solutions explicitly state that H2O has a relative molecular mass of 18 u, confirming this result.

Why Other Options Are Wrong:
Option B, 1.8 u, is off by a factor of ten and might arise from misplacing a decimal point, not from correct atomic mass addition. Option C, 2.0 u, corresponds only to the hydrogen atoms and ignores the much heavier oxygen atom. Option D, 20 u, is too high and does not match any reasonable atomic mass combination for H2O. Option E, 28 u, might be confused with nitrogen gas N2, which has a relative molecular mass close to 28 u. None of these values result from correctly adding 2 times 1 u and 16 u. Only option A matches the correct calculation for water.

Common Pitfalls:
Students sometimes forget to multiply the atomic mass of hydrogen by two because there are two hydrogen atoms in the formula. Others may use incorrect atomic masses from memory or confuse water with other molecules. A reliable strategy is always to write out the formula, list each type of atom with its subscript, multiply each atomic mass by the number of atoms and then add the results. Practising this method on common compounds like water, carbon dioxide and methane builds confidence for more complex calculations later.

Final Answer:
The relative molecular mass of water H2O is 18 u, obtained by adding the masses of two hydrogens and one oxygen atom.