Heavy water basics: what is the molecular weight (molar mass) of heavy water (D2O)?

Introduction / Context:
Heavy water (D2O) is used as a moderator and sometimes coolant in certain reactor types. Knowing its composition and molar mass is basic nuclear-engineering literacy.

Given Data / Assumptions:

• D denotes deuterium (hydrogen-2), atomic mass ≈ 2.
• Oxygen in water is the O-16 isotope (dominant), atomic mass ≈ 16.

Concept / Approach:
The molecular weight is computed by summing atomic masses of the constituent atoms: two deuterium atoms (2 × 2 = 4) plus one oxygen (16) equals 20 g/mol for D2O.

Step-by-Step Solution:

Verification / Alternative check:
Isotopic mass tables confirm D ≈ 2.014 and O ≈ 15.999 → sum ≈ 20.027, which rounds to 20 for standard choices.

Why Other Options Are Wrong:

• 18: that is ordinary water (H2O).
• 10, 22, 36: do not match the isotopic composition of D2O.

Common Pitfalls:
Using H instead of D; forgetting isotopic mass differences.

Final Answer:
20