You calculate the relative formula mass (Mr) by summing the relative atomic masses (Ar) of all the atoms present in the chemical formula of the substance.
Understanding Relative Formula Mass (Mr)
The relative formula mass, often shortened to Mr, represents the mass of one formula unit of a compound relative to 1/12th the mass of a carbon-12 atom. It's essentially the weighted average mass of all the atoms in a formula unit.
Steps to Calculate Relative Formula Mass:
-
Identify the Chemical Formula: Know the chemical formula of the compound. For example, calcium carbonate is CaCO3.
-
Obtain Relative Atomic Masses: Look up the relative atomic masses (Ar) of each element present in the compound from the periodic table. You'll typically be given these values in exam questions. For example:
- Calcium (Ca): Ar = 40
- Carbon (C): Ar = 12
- Oxygen (O): Ar = 16
-
Multiply and Sum: Multiply the Ar of each element by the number of atoms of that element present in the formula, and then sum up all the values.
Example: Calculating Mr of Calcium Carbonate (CaCO3)
Using the values from above:
Mr (CaCO3) = (1 x Ar of Ca) + (1 x Ar of C) + (3 x Ar of O)
Mr (CaCO3) = (1 x 40) + (1 x 12) + (3 x 16)
Mr (CaCO3) = 40 + 12 + 48
Mr (CaCO3) = 100
Therefore, the relative formula mass of calcium carbonate is 100.
General Formula
The general formula can be represented as:
Mr = (Number of atoms of element 1 x Ar of element 1) + (Number of atoms of element 2 x Ar of element 2) + ... and so on for all elements in the formula.
Key Considerations
- Ensure you are using the correct chemical formula.
- Double-check the number of atoms of each element in the formula.
- Use the correct relative atomic masses.
- Pay close attention to brackets in the formula and multiply accordingly. For example, in Mg(NO3)2, there are two nitrogen atoms and six oxygen atoms.
