MCQ Detailed View

The molecular mass of a water (H₂O) molecule is 18 atomic mass units (amu). This value is calculated by summing the atomic masses of its constituent atoms: two hydrogen atoms and one oxygen atom.

Here's how it breaks down:

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  Each hydrogen atom has an approximate atomic mass of 1 amu.

  
  


• 
  

  One oxygen atom has an atomic mass of approximately 16 amu.

  
  

Therefore:
Molecular mass of H₂O = (2 × 1 amu) + (1 × 16 amu)
= 2 amu + 16 amu
= 18 amu

Atomic mass unit (amu) is a standard unit used to express the mass of atoms and molecules. It is based on one-twelfth the mass of a carbon-12 atom.

Water is a polar covalent molecule with a bent shape due to the lone pairs on the oxygen atom. The structure plays a crucial role in many physical and chemical properties, but the molecular mass depends purely on the atoms that form it.

Let’s review the incorrect options:

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  16 amu: This is the atomic mass of oxygen only, without the two hydrogen atoms.

  
  


• 
  

  17 amu: Incorrect total; undercounts the hydrogen component.

  
  


• 
  

  20 amu: Overestimation; does not match the actual atomic composition.

  
  

Understanding molecular mass is a key concept in chemistry, used in calculations involving moles, chemical equations, and reactions.

Thus, the correct answer is C. 18 amu — the total mass of a water molecule based on its atomic components.