Calculating the Mass Percentage of Hydrogen in Water

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Understanding the mass composition of a chemical compound is a foundational skill in chemistry. When it comes to water (H₂O), the calculation is straightforward once you know the atomic masses.

Molar Mass of Water

Using values from the periodic table:

• Hydrogen: 1.008 amu (× 2) = 2.016 amu
• Oxygen: 16.00 amu (× 1) = 16.00 amu
• Total molar mass = 2.016 amu + 16.00 amu = 18.016 amu

Mass Percentage of Hydrogen

The mass fraction of hydrogen is calculated as:

Mass % = (2.016 amu ÷ 18.016 amu) × 100 ≈ 11.19 %

Mass Percentage of Oxygen

Because water contains only hydrogen and oxygen, the remaining percentage is automatically oxygen’s share:

Mass % = (16.00 amu ÷ 18.016 amu) × 100 ≈ 88.81 %

Mass Ratios

The relative mass ratio of oxygen to hydrogen in a water molecule is:

O : H = 16.00 amu ÷ 2.016 amu ≈ 7.94 : 1

Thus, oxygen constitutes roughly eight times the mass of hydrogen in each molecule of water.

All values are based on the latest IUPAC recommended atomic masses, ensuring accuracy for academic and industrial applications.