Concept:Atomic mass must be measured as a relative value against an internationally agreed-upon baseline isotope that is highly stable and abundant.
Formula:$$ 1 \text{ amu} = \frac{1}{12} \times \text{mass of one } ^{12}\text{C atom} $$
Solution:- In 1961, the International Union of Pure and Applied Chemistry (IUPAC) established Carbon (specifically the Carbon-12 isotope) as the absolute universal standard.
- All other elements on the periodic table have their atomic masses determined relative to exactly 1/12th of this Carbon isotope.
Why other options are incorrect:- H (Hydrogen): Used historically before 1961, but abandoned because it is a flammable gas, making highly precise mass spectrometry difficult.
- P & Cl: Never used as standards in the history of chemistry.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.