Which of the following statements about the molecular weight of a molecule is true?

• A. The molecular weight is always equal to the mass number of the most abundant isotope.
• B. The molecular weight is measured in grams per liter.
• C. The molecular weight of a molecule is always an integer value.
• D. The molecular weight is the sum of the atomic masses of all atoms in a molecule.

Answer: (D)

Verified step by step guidance

1. Understand that molecular weight (or molecular mass) refers to the sum of the atomic masses of all atoms in a molecule, expressed in atomic mass units (amu).
2. Recall that atomic masses are weighted averages of the isotopes of each element, so molecular weight is generally not a simple integer but a decimal value reflecting natural isotope distribution.
3. Recognize that molecular weight is not measured in grams per liter; that unit is used for concentration, not mass.
4. Note that the mass number of the most abundant isotope is a whole number specific to one isotope, but molecular weight accounts for all isotopes and all atoms in the molecule, so it is not simply equal to that mass number.
5. Conclude that the true statement is: molecular weight is the sum of the atomic masses of all atoms in a molecule.