Relative molecular mass is the sum of the relative atomic masses of all the atoms in a molecule. In simpler terms, it's a way to express the mass of a molecule relative to the mass of a carbon-12 atom.
Understanding Relative Molecular Mass
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Definition: The relative molecular mass (often abbreviated as Mr) is a dimensionless quantity. It represents the ratio of the average mass per molecule of a substance to 1/12th the mass of an atom of carbon-12.
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Calculation: To calculate the relative molecular mass of a compound, you simply add up the relative atomic masses (Ar) of each element present in the molecule, multiplied by the number of atoms of that element.
- Formula: Mr = (Number of atoms of element 1 × Ar of element 1) + (Number of atoms of element 2 × Ar of element 2) + ...
Example
Let's calculate the relative molecular mass of water (H₂O):
- Identify the elements present: Hydrogen (H) and Oxygen (O).
- Find the relative atomic masses: Ar(H) ≈ 1, Ar(O) ≈ 16. (These values are typically found on the periodic table).
- Apply the formula: Mr(H₂O) = (2 × 1) + (1 × 16) = 2 + 16 = 18.
Therefore, the relative molecular mass of water is 18.
Significance of Relative Molecular Mass
- Chemical Calculations: It's crucial for various chemical calculations, such as determining the number of moles in a given mass of a substance and calculating the concentration of solutions.
- Stoichiometry: Relative molecular mass is fundamental to stoichiometric calculations, allowing chemists to predict the amounts of reactants and products involved in chemical reactions.
- Identification of Substances: While not a definitive identification method on its own, it can be used in conjunction with other data to help identify unknown compounds.
Relative Molecular Mass vs. Molecular Weight
While often used interchangeably, "relative molecular mass" is technically more accurate when dealing with molecules, while "relative formula mass" is more appropriate for ionic compounds (which don't form discrete molecules). However, both terms are calculated in the same way - by summing the relative atomic masses of the elements in the formula unit.
