Molecular Weight, Molecular Mass, and TheirCorrect Units.

There are two common (and equivalent) ways to de￾scribe molecular mass; both are used in this text.
 Thef is molecular weight, or relative molecular mass,
denoted Mr. The molecular weight of a substance is de￾fined as the ratio of the mass of a molecule of that sub￾stance to one-twelfth the mass of carbon-12 (12C).
Since Mr is a ratio, it is dimensionless—it has no asso￾ciated units. The second is molecular mass, denoted
m. This is simply the mass of one molecule, or the mo￾lar mass divided by Avogadro’s number. The molecu￾lar mass, m, is expressed in daltons (abbreviated Da).
One dalton is equivalent to one-twelfth the mass of
carbon-12; a kilodalton (kDa) is 1,000 daltons; a mega￾dalton (MDa) is 1 million daltons.
Consider, for example, a molecule with a mass
1,000 times that of water. We can say of this molecule
either Mr  18,000 or m  18,000 daltons. We can also
describe it as an “18 kDa molecule.” However, the ex￾pression Mr  18,000 daltons is incorrect.
Another convenient unit for describing the mass
of a single atom or molecule is the atomic mass unit
(formerly amu, now commonly denoted u). One
atomic mass unit (1 u) is defined as one-twelfth the
mass of an atom of carbon-12. Since the experimen￾tally measured mass of an atom of carbon-12 is
1.9926  10 23 g, 1 u  1.6606  10 24 g. The atomic
mass unit is convenient for describing the mass of a
peak observed by mass spectrometry (see