What is the concentration of 𝐷20 in heavy water (density 1.11 g/ml)?
A. 0.5 M,
B. 5.0 M,
C. 50.0 M,
D. 55.5 M

Understanding the Molarity of D₂O in Heavy Water

Introduction

Heavy water, or D₂O (deuterium oxide), is a form of water where both hydrogen atoms are replaced with deuterium—a heavier isotope of hydrogen. But how concentrated is D₂O in heavy water? In this blog post, we’ll break it down with a straightforward explanation and a little math.

What Is Heavy Water?

Heavy water looks and behaves like regular water (H₂O), but with a key difference: instead of hydrogen, it contains deuterium (D). This changes its physical properties slightly, including density, which is higher than that of regular water.

• Density of heavy water (D₂O): 1.11 g/mL
• Molar mass of D₂O: ~20 g/mol

Step-by-Step Calculation

To find the molarity (mol/L) of D₂O in heavy water, we follow this simple formula:

                      Molarity (M)=Mass of solute (g)   /    Molar mass (g/mol)×Volume (L)

Since we are dealing with pure heavy water, every gram is D₂O.

1. Assume 1 liter of D₂O:

   Since density is 1.11 g/mL:

                         Mass=1.11×1000=1110 g

2. Moles of D₂O:
   

         Moles=1110/20=55.5 mol

3. Therefore, Molarity = 55.5 M

✅ The correct answer is Option D: 55.5 M

Why This Matters

Understanding the molarity of heavy water is essential in nuclear physics, NMR spectroscopy, and biochemical research. Heavy water is used in nuclear reactors as a neutron moderator and in lab studies for isotope labeling.

Conclusion

To summarize, the concentration of D₂O in pure heavy water is 55.5 M, based on its density and molar mass. This mirrors the molarity of H₂O in regular water (55.5 M), thanks to their similar structural characteristics.

Next time you come across heavy water in a research article or science quiz, you’ll know exactly how concentrated it is!