Determining the Relative Molecular Mass from the Relative Atomic Mass of an Element

To determine the relative molecular mass (molecular weight) of a compound from the relative atomic masses of its constituent elements, you follow these steps:

1. Identify the Elements: Determine which elements are present in the compound and their respective relative atomic masses. You can find these values on the periodic table.

2. Count the Atoms: Determine the number of atoms of each element in the compound. This information is provided by the chemical formula of the compound.

3. Calculate the Sum: Multiply the relative atomic mass of each element by the number of atoms of that element in the compound. Then, sum up these products for all elements present in the compound.

4. Example Calculation: Let's take water (${\text{H}}_2\text{O}$) as an example:

   • Hydrogen ($\text{H}$) has a relative atomic mass of approximately 1.008.
   • Oxygen ($\text{O}$) has a relative atomic mass of approximately 16.00.
   • In water (${\text{H}}_2\text{O}$), there are two hydrogen atoms and one oxygen atom.
   • The calculation for the relative molecular mass (${𝑀}_{𝑟}$) of water is:
     $${𝑀}_{𝑟}=(2\times 1.008)+(1\times 16.00)=2.016+16.00=18.016\text{u}$$

   So, the relative molecular mass of water is approximately 18.016 unified atomic mass units (u).

5. Unit: The relative molecular mass is typically expressed in unified atomic mass units (u) or grams per mole (g/mol).

6. Significance: The relative molecular mass provides the average mass of a molecule of the compound compared to the unified atomic mass unit. It's useful for various calculations in chemistry, including stoichiometry, determining the amount of substance, and understanding molecular properties.