Question

what do you mean by carbon -12isotope and why was it used as a standard for measuring relative weights​

Answer 1

In 1961 the isotope carbon-12 was selected to replace oxygen as the standard relative to which the atomic weights of all the other elements are measured. Carbon-14, which is radioactive, is the isotope used in radiocarbon dating and radiolabeling.

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Answer 2

carbon 12 isotopes is used to calculate mole
.
Since to measure the weight of diatomic and poly atomic atoms