Concept:Relative atomic mass must be measured against a highly stable, widely abundant, and solid standard that can accurately calibrate mass spectrometers.
Formula:$$ 1 \text{ a.m.u.} = \frac{1}{12} \times \text{Mass of one } ^{12}\text{C atom} $$
Solution:- Carbon-12 (\( ^{12}\text{C} \)) was adopted internationally in 1961 as the absolute standard.
- By definition, the mass of one atom of Carbon-12 is exactly 12.0000 atomic mass units.
- It is vastly superior to hydrogen because carbon is a solid, easier to handle, and pairs well with mass spectrometry techniques used for accurate mass determinations.
Why other options are incorrect:- H-1.008: Hydrogen was the original historical standard but was abandoned because it is a highly flammable gas and difficult to handle precisely.
- Oxygen-16: Oxygen was a former standard, but chemists and physicists couldn't agree on whether to use the natural mix of isotopes or pure O-16.
- Carbon-13: An isotope of carbon, but it is not the internationally agreed-upon baseline.
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