What unit of measurement is defined to be the number of atoms in 12g of carbon-12?

• Correct Answer: one ohm
• one quark
• one joule
• one mole

Explanation: The relative scale of atomic masses in amu is also a relative scale of masses in grams. We said before that the mole is officially equal to the number of carbon atoms in exactly 12 g of carbon-12. In other words, one carbon-12 atom has a mass of exactly 12 amu, and one mole of carbon atoms has a mass of exactly 12 grams. This relationship is true for all substances. If one atom of helium has a mass of 4.00 amu, one mole of helium atoms has a mass of 4.00 g. One molecule of water has a mass of 18.0 amu, so one mole of water molecules has a mass of 18.0 grams. Molar mass is defined as the mass of one mole of representative particles of a substance. It is expressed in units of grams per mole (g/mol).

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