The average relative molecular mass (Mr) is the average mass of a molecule relative to one-twelfth of the mass of a carbon-12 atom. It's essentially the sum of the relative atomic masses of all the atoms present in the molecule.
Explanation:
The term "relative" is key because we're comparing the mass of a molecule to a standard: one-twelfth the mass of a carbon-12 atom. Carbon-12 is the isotope of carbon with 6 protons and 6 neutrons. This standard provides a convenient, unitless way to express molecular mass.
To calculate the Mr of a molecule:
- Identify the chemical formula of the molecule. For example, water is H2O.
- Find the relative atomic mass (Ar) of each element in the molecule. These values are usually found on the periodic table. For hydrogen (H), Ar ≈ 1; for oxygen (O), Ar ≈ 16.
- Multiply the Ar of each element by the number of atoms of that element in the molecule. In H2O, there are two hydrogen atoms (2 x 1) and one oxygen atom (1 x 16).
- Add the results together. For H2O, Mr = (2 x 1) + (1 x 16) = 18.
Example:
Let's calculate the average relative molecular mass of carbon dioxide (CO2):
- Chemical formula: CO2
- Ar of carbon (C) ≈ 12
- Ar of oxygen (O) ≈ 16
Therefore, Mr of CO2 = (1 x 12) + (2 x 16) = 12 + 32 = 44.
Why is it an average?
The term "average" comes into play because elements often exist as a mixture of isotopes. Isotopes are atoms of the same element with different numbers of neutrons. The relative atomic mass (Ar) listed on the periodic table is actually a weighted average of the masses of all the naturally occurring isotopes of that element, taking into account their relative abundance. This means that the Mr you calculate is also an average value that reflects the isotopic composition of the elements within the molecule.
