Which unit of mass is designated as one-sixteenth the mass of oxygen-16?

The unit of mass designated as one-sixteenth the mass of oxygen-16 is the Dalton. The Dalton, also known as the unified atomic mass unit (u), is defined specifically in relation to atomic and molecular masses and is based on the standard atomic weight of carbon-12.

Since oxygen-16 is a key reference point in atomic mass calculations, the Dalton provides a scale where the mass of other atomic and molecular structures can be expressed in relation to a specific isotope. Thus, one Dalton is defined as 1/12 the mass of carbon-12, making it a convenient unit when discussing particles such as atoms and molecules.

The other options are units of mass but do not carry the specific definition associated with atomic scale measurements:

• The gram is a larger unit of mass (1,000 milligrams) and not concerned with atomic scale.

• The mole is a unit that quantifies the amount of substance, based on the number of elementary entities (atoms, molecules) and does not directly relate to mass.

• The milligram is a subunit of grams and is also inappropriate for atomic and molecular mass contexts.

In summary, the Dalton is specifically constructed for discussions surrounding atomic mass, making it the correct answer.