Atomic Foundations of Matter · Lesson 12 of 14
Molecular Mass of Covalent Compounds
“A molecular formula becomes a mass calculation when each atomic mass is multiplied by its atom count and the contributions are added.”
• Define molecular mass. • Read atom counts correctly from molecular formulae. • Calculate molecular mass by adding atomic-mass contributions. • Calculate the molecular masses of water and carbon dioxide. • Apply the same method to nitric acid and methane.
A molecular formula tells us which atoms are present and how many of each occur in one molecule. Once the atomic masses are known, the mass of the whole molecule can be found by adding the contributions from every atom.
Molecular Mass of Covalent Compounds
The total mass of a molecule, found by adding the atomic masses of all atoms present in that molecule.
A Reliable Calculation Method
First read the formula carefully. A written subscript gives the number of atoms of the symbol before it. If no subscript is shown, the count is one. Multiply each atomic mass by its atom count and then add the contributions.
Problem
Calculate the molecular mass of H₂O using H = 1 u and O = 16 u.
- 1.H₂O contains 2 hydrogen atoms and 1 oxygen atom.
- 2.Hydrogen contribution = 2 × 1 u = 2 u.
- 3.Oxygen contribution = 1 × 16 u = 16 u.
- 4.Add the contributions: 2 u + 16 u = 18 u.
- 5.Molecular mass of H₂O = 18 u.
Problem
Calculate the molecular mass of CO₂ using C = 12 u and O = 16 u.
- 1.CO₂ contains 1 carbon atom and 2 oxygen atoms.
- 2.Carbon contribution = 1 × 12 u = 12 u.
- 3.Oxygen contribution = 2 × 16 u = 32 u.
- 4.Total = 12 u + 32 u = 44 u.
- 5.Molecular mass of CO₂ = 44 u.
Problem
Find the molecular mass of HNO₃ using H = 1 u, N = 14 u and O = 16 u.
- 1.HNO₃ contains 1 H, 1 N and 3 O atoms.
- 2.Hydrogen contribution = 1 × 1 = 1 u.
- 3.Nitrogen contribution = 1 × 14 = 14 u.
- 4.Oxygen contribution = 3 × 16 = 48 u.
- 5.Add: 1 + 14 + 48 = 63 u.
- 6.Molecular mass = 63 u.
Problem
Find the molecular mass of CH₄ using C = 12 u and H = 1 u.
- 1.CH₄ contains 1 carbon atom and 4 hydrogen atoms.
- 2.Carbon contribution = 12 u.
- 3.Hydrogen contribution = 4 × 1 = 4 u.
- 4.Total = 12 + 4 = 16 u.
- 5.Molecular mass of methane = 16 u.
Most mistakes come from reading the formula incorrectly. Count atoms first, then calculate. Do not add atomic masses before multiplying by their subscripts.
Quiz
Which description best matches Molecular Mass?
Which term matches this description: The total mass of a molecule, found by adding the atomic masses of all atoms present in that molecule.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Calculate the molecular mass of a molecule containing two hydrogen atoms and one sulfur atom if H = 1 u and S = 32 u.
- Explain what the subscript in CO₂ changes in a mass calculation.
- Why does HNO₃ require three separate atomic-mass contributions?
- Describe the difference between an atomic mass and a molecular mass.
Key Takeaways
• Molecular mass is calculated by adding the atomic masses of all atoms in a molecule. • Subscripts in a chemical formula indicate how many atoms of each element are present. • Each atomic mass is multiplied by the corresponding number of atoms. • The individual mass contributions are then added to obtain the molecular mass.