The carbon-12 standard defines mass such that one mole of carbon-12 has a mass of how many grams?

• A. 6.02 g
• B. 12.00 g
• C. 24.0 g
• D. 12.000 g

Answer: (D)

The value is exact because the mole is defined so that one mole of carbon-12 has a mass of exactly 12 grams. This ties the atomic mass unit to the gram unit: a carbon-12 atom has a mass of exactly 12 amu, and Avogadro’s number of those atoms per mole makes the total mass 12 g per mole. Writing it as 12.000 g emphasizes that this is an exact value, not an approximate one. Other numbers would either misstate the magnitude (for example, a mass far from 12 g) or mix in non-exact precision.