Relative masses of atoms and molecules — common mistakes

In exams, always use the relative atomic mass values given in the question or the periodic table provided. Do not round them prematurely. When calculating $M_r$, pay close attention to brackets, e.g., in Ca(NO₃)₂, the '2' multiplies everything inside the bracket: one Ca, two N, and six O atoms. For hydrated salts, the dot means ADD the mass of the water molecules.

No. Mass number (A) is the total count of protons and neutrons in a single atom's nucleus (an isotope) and is always an integer. Relative atomic mass ($A_r$) is the weighted average mass of all naturally occurring isotopes of an element compared to the carbon-12 standard, and is often a decimal value.

Carbon-12 was chosen as a stable, common, and solid reference. Defining its mass as exactly 12 conveniently makes the relative masses of most other elements and isotopes very close to whole numbers, simplifying calculations. It replaced earlier standards based on hydrogen and oxygen to create a single, unified scale for both physicists and chemists.

No, you will always be provided with a Periodic Table in your exam which contains all the necessary relative atomic masses. However, with practice, you will become familiar with the values for common elements (like C=12.0, H=1.0, O=16.0, N=14.0), which will help you work faster.