When calculating the relative formula mass (or molar mass) of a compound, we use the relative atomic masses of the elements instead of their actual mass numbers for several reasons. The use of relative atomic masses is a practical approach that provides a more accurate representation of the average mass of the elements involved. Here's why:
Isotopic abundance: Elements can have different isotopes, which are atoms with the same number of protons but different numbers of neutrons. These isotopes have different mass numbers and occur in different proportions in nature. Since a compound can contain different isotopes of an element, using the actual mass numbers of each isotope would be impractical and unnecessarily complex. Instead, the relative atomic mass takes into account the abundance of each isotope and provides an average mass based on their relative proportions.
Fractional masses: Some elements have isotopes with non-integer mass numbers. For example, chlorine has isotopes with mass numbers 35 and 37. If we were to use the actual mass numbers, the result would be fractional masses, which are not convenient to work with. Relative atomic masses provide a more manageable and standardized representation by considering the isotopic abundance and expressing it as a weighted average.
Periodic Table values: Relative atomic masses are based on the values found on the Periodic Table, which is a widely accepted and standardized reference. These values are periodically updated to reflect more accurate measurements and improvements in analytical techniques. By using relative atomic masses, calculations remain consistent and comparable across different sources and scientific disciplines.
It's important to note that the relative atomic mass is a dimensionless quantity, expressed in atomic mass units (amu). It represents the average mass of an atom of an element relative to 1/12th the mass of a carbon-12 atom.
While using relative atomic masses introduces a slight level of approximation due to the averaged nature of the values, it does not significantly impact the accuracy of most calculations. For practical purposes, the relative atomic masses provide a sufficiently accurate representation of the mass of elements in compounds, allowing for consistent and efficient calculations in various fields of chemistry.