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Summary

Carbon-12 atoms have a mass of 12 grams per mole, which was agreed upon by chemists during the 1960s.
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This means that 16 grams of oxygen atoms have the same number of atoms as 12 grams of carbon-12 atoms.
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Summary
During the 1960s, chemists agreed to use exactly 12 g of carbon-12 as their standard.
They knew that an oxygen atom has a mass of ¹⁶/₁₂ times that of a carbon-12 atom. If they then measured out 16 g of O atoms, they knew that they had the same number of atoms (a mole of each).

How does the mole relate to carbon 12? + Example
socratic.org

molar mass of carbon-12 : Numerical value: 11.999 999 9958 x 10-3 kg mol-1: Standard uncertainty: 0.000 000 0036 x 10-3 kg mol-1: Relative standard uncertainty: 3.0 x 10-10: Concise form…

CODATA Value: molar mass of carbon-12 - NIST
nist.gov