ChemistryChapter 6: Stoichiometry
6.2 Molecular Mass and Formula Mass
### Molecular Mass **Molecular mass** is the sum of the atomic masses
Explanation level
This topic explains how to calculate the molecular mass of a molecular compound and the formula mass of an ionic compound.
Definition
Molecular Mass
Molecular mass is the sum of the atomic masses of all the atoms present in one molecule of a molecular compound.
Simple explanation: Molecular mass is the total mass of all atoms in one molecule.
Formula Mass
Formula mass is the sum of the atomic masses of all the atoms in the formula unit of a substance (used for ionic compounds).
Simple explanation: Formula mass is the total mass of all atoms in one formula unit of an ionic compound.
Main Idea
The term molecular mass is used for molecular compounds. Whereas, the term formula mass is used for ionic compounds.
Ionic compounds consist of arrays of oppositely charged ions rather than separate molecules. So we represent an ionic compound by its formula unit.
A formula unit indicates the simplest ratio between cations and anions in an ionic compound.
Simple explanation: Molecular compounds (like H₂O) have molecules → use molecular mass. Ionic compounds (like NaCl) have formula units → use formula mass.
Core Concept
| Feature | Molecular Mass | Formula Mass |
|---|---|---|
| Used for | Molecular compounds | Ionic compounds |
| Represents | Mass of one molecule | Mass of one formula unit |
| Examples | H₂O, CO₂, C₆H₁₂O₆ | NaCl, MgO, Mg(OH)₂ |
| Unit | amu | amu |
Detailed Explanation
Molecular Mass — In Detail
Example 6.1: Determine the molecular mass of glucose C₆H₁₂O₆ (blood sugar)
Problem Solving Strategy: Multiply atomic masses of carbon, hydrogen, and oxygen by their subscripts and add.
Solution:
Molecular mass of C6H12O6 = 6(12.00) + 12(1.008) + 6(16.00)
= 72.00 + 12.096 + 96.00
= 180.096 amu
Example: Determine the molecular mass of naphthalene C₁₀H₈ (used in mothballs)
Solution:
Molecular mass of C10H8 = 12 × 10 + 1 × 8
= 120 + 8 = 128 amu
Simple explanation: To find molecular mass, just add up the atomic masses of all the atoms in the molecule.
Formula Mass — In Detail
Why formula mass instead of molecular mass?
Ionic compounds consist of arrays of oppositely charged ions rather than separate molecules. So we represent an ionic compound by its formula unit.
A formula unit indicates the simplest ratio between cations and anions in an ionic compound.
For example, common salt consists of Na⁺ and Cl⁻ ions. It has one Na⁺ ion for every Cl⁻ ion. So the formula unit for common salt is NaCl.
The sum of the atomic masses of all the atoms in the formula unit of a substance is called formula mass.
Example 6.2: Determining formula mass
1. Sodium Chloride (NaCl) — table salt, used to flavour food, preserve meat, and in the preparation of a large number of compounds. Determine its formula mass.
Problem Solving Strategy: Add the atomic masses of all the atoms in the formula unit.
Solution:
Formula mass of NaCl = 1 × Atomic mass of Na + 1 × Atomic mass of Cl
= 1 × 23 + 1 × 35.5
= 58.5 amu
2. Milk of magnesia, which contains Mg(OH)₂ — used to treat acidity. Determine its formula mass.
Solution:
Formula mass of Mg(OH)2 = 24 + 16 × 2 + 1 × 2
= 24 + 32 + 2
= 58 amu
Simple explanation: To find formula mass, add up the atomic masses of all the atoms in the formula unit, just like molecular mass — but for ionic compounds.
Intermediate Level
How to Calculate Molecular Mass
- Write the molecular formula.
- Identify the number of atoms of each element.
- Multiply the atomic mass of each element by its subscript.
- Add all the values together.
Example: Glucose C₆H₁₂O₆
= 6(12.00) + 12(1.008) + 6(16.00)
= 72.00 + 12.096 + 96.00
= 180.096 amu
How to Calculate Formula Mass
- Write the formula unit (simplest ratio of ions).
- Identify the number of atoms of each element.
- Multiply the atomic mass of each element by its subscript.
- Add all the values together.
Example: Mg(OH)₂
= 24 + (16 + 1) × 2
= 24 + 17 × 2
= 24 + 34
= 58 amu
Practice Problems (from the book)
1. Potassium chlorate (KClO₃) — used for laboratory preparation of oxygen gas. Calculate its formula mass.
Solution: = 39 + 35.5 + 16 × 3 = 39 + 35.5 + 48 = 122.5 amu
2. Baking soda (NaHCO₃) and carbon dioxide (CO₂) — determine their formula masses.
Solution: NaHCO3 = 23 + 1 + 12 + 16 × 3 = 23 + 1 + 12 + 48 = 84 amu
CO2 = 12 + 16 × 2 = 12 + 32 = 44 amu
3. Fertilizers: Urea (NH₂)₂CO and Ammonium nitrate (NH₄NO₃) — determine their formula masses.
Solution: Urea = 2(14 + 1 × 2) + 12 + 16 = 2(16) + 12 + 16 = 32 + 12 + 16 = 60 amu
NH4NO3 = 14 + 1 × 4 + 14 + 16 × 3 = 14 + 4 + 14 + 48 = 80 amu
Why the Distinction Matters
| Feature | Molecular Mass | Formula Mass |
|---|---|---|
| Applies to | Covalent (molecular) compounds | Ionic compounds |
| Represents | One molecule | One formula unit |
| Examples | H₂O, CO₂, C₆H₁₂O₆ | NaCl, MgO, Mg(OH)₂ |
| Used in calculations | Molar mass of molecular compounds | Molar mass of ionic compounds |
Important Points
- - Molecular mass: sum of atomic masses of all atoms in one molecule of a molecular compound.
- - Formula mass: sum of atomic masses of all atoms in one formula unit of an ionic compound.
- - Molecular compounds: consist of molecules → molecular mass.
- - Ionic compounds: consist of ions in a lattice → formula mass.
- - Formula unit: simplest ratio between cations and anions in an ionic compound.
- - Examples:
- - Glucose (C₆H₁₂O₆): molecular mass = 180.096 amu
- - Naphthalene (C₁₀H₈): molecular mass = 128 amu
- - NaCl: formula mass = 58.5 amu
- - Mg(OH)₂: formula mass = 58 amu
- - KClO₃: formula mass = 122.5 amu
- - NaHCO₃: formula mass = 84 amu
- - CO₂: molecular mass = 44 amu
- - Urea: formula mass = 60 amu
- - NH₄NO₃: formula mass = 80 amu
Remember
- - Molecular mass = for molecules (covalent compounds).
- - Formula mass = for formula units (ionic compounds).
- - Both are calculated by adding atomic masses.
- - Glucose: 180.096 amu.
- - NaCl: 58.5 amu.
- - Mg(OH)₂: 58 amu.
- - Formula unit = simplest ratio of ions in an ionic compound.
Common Mistakes
- 1. Thinking molecular mass and formula mass are the same thing. They are calculated the same way, but used for different types of compounds.
- 2. Using molecular mass for ionic compounds. Use formula mass for ionic compounds.
- 3. Using formula mass for molecular compounds. Use molecular mass for molecular compounds.
- 4. Forgetting to multiply by subscripts. Each atom's mass must be multiplied by its subscript.
- 5. Forgetting to distribute subscripts outside parentheses. In Mg(OH)₂, the (OH)₂ means 2 O atoms and 2 H atoms.
- 6. Thinking formula mass has different units. Both are in amu.
Quick Revision
- Molecular mass: sum of the atomic masses of all the atoms in one molecule of a molecular compound.
- Formula mass: sum of the atomic masses of all the atoms in the formula unit of an ionic compound.
- Molecular mass is used for molecular compounds; formula mass is used for ionic compounds.
- Formula unit indicates the simplest ratio between cations and anions in an ionic compound.
- Examples:
- Glucose (C₆H₁₂O₆): molecular mass = 180.096 amu
- Naphthalene (C₁₀H₈): molecular mass = 128 amu
- NaCl: formula mass = 58.5 amu
- Mg(OH)₂: formula mass = **58 amu**
- KClO₃: formula mass = 122.5 amu
- NaHCO₃: formula mass = 84 amu
- CO₂: molecular mass = **44 amu**
- Urea: formula mass = 60 amu
- NH₄NO₃: formula mass = **80 amu**
60-Second Revision
- Molecular mass = for molecules.
- Formula mass = for formula units (ionic compounds).
- Both = add atomic masses.
- Glucose: 180.096 amu; NaCl: 58.5 amu.
- Warning: use formula mass for ionic compounds, not molecular mass.
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Review the key facts, formulas and checks before you move on.6.2 Molecular Mass and Formula Mass
Sticky Notes · Quick Revision
Key Point 1
- Molecular mass = for molecules (covalent compounds).
Key Point 2
- Formula mass = for formula units (ionic compounds).
Key Point 3
- Both = sum of atomic masses.
Key Point 4
- Glucose: 180.096 amu.
Key Point 5
- NaCl: 58.5 amu.
Key Point 6
- Mg(OH)₂: 58 amu.
Key Point 7
- KClO₃: 122.5 amu.
Key Point 8
- Formula unit = simplest ratio of ions.
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