One mole of a substance is defined as

  • A. the amount containing as many particles as there are atoms in 12 g of carbon 12
  • B. one gram of the substance
  • C. the mass of one molecule of the substance
  • D. the volume occupied by one gram of a gas

Explanation

The mole is a counting unit fixed by reference to 12 g of carbon 12, which is why the molar mass of any substance in grams contains exactly Avogadro's number of particles. Defining it by mass alone would be meaningless, since one gram of hydrogen and one gram of lead contain hugely different numbers of atoms. The mole is what lets a chemist count particles by weighing.

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