Slide presentation
Molecular Mass and Formula Mass
General Chemistry • Chemical Compounds
Topic launch
A formula tells you the mass of one molecule or formula unit.
Molecular mass and formula mass are found by adding the atomic masses of every atom shown in a chemical formula. Subscripts are the counting instructions.
Learning target
Use atomic masses from the periodic table, interpret subscripts, calculate compound mass, and distinguish molecular mass from formula mass.
Why it matters
Compound mass connects formulas to measurable samples.
A chemical formula is particle-level information. Molecular mass, formula mass, and molar mass translate that information into quantitative chemistry.
Stoichiometry
Balanced equations require mole ratios, and molar mass is the bridge between grams and moles.
Formulas and composition
Percent composition, empirical formulas, and molecular formulas all depend on formula mass.
Real compounds
Medicine doses, fertilizer analysis, and reaction yields all require converting formula information into mass.
Core concept
Subscripts multiply atomic masses.
Every element symbol has an implied count of 1 unless a subscript says otherwise. The total compound mass is the sum of each element’s mass contribution.
Formula reading rules
- No subscript means 1 atom of that element.
- A subscript multiplies only the element or group immediately before it.
- Parentheses multiply everything inside the group.
Vocabulary and variables
Use the correct name for the kind of compound.
The calculation is the same addition process, but the vocabulary depends on whether the formula represents a molecule or an ionic formula unit.
| Term | Meaning | Typical unit | Example |
|---|---|---|---|
| Molecular mass | Mass of one molecule, found by summing atomic masses. | amu | H2O, CO2, C6H12O6 |
| Formula mass | Mass of one formula unit, commonly used for ionic compounds. | amu | NaCl, MgO, CaCl2 |
| Atomic mass | Average mass of an element’s atoms from the periodic table. | amu | C is about 12.01 amu |
| Molar mass | Mass of one mole of particles or formula units. | g/mol | H2O is about 18.02 g/mol |
Molecular compound
Discrete molecules are present, so molecular mass is a natural term.
Ionic compound
A repeating crystal lattice is present, so formula mass refers to one formula unit.
Numerical connection
The amu value for one particle has the same numerical value as g/mol for one mole.
Main relationship
Compound mass is the sum of element contributions.
For each element, multiply the number of atoms by the atomic mass, then add all element contributions.
The same relationship gives molecular mass for molecular compounds and formula mass for ionic compounds.
For CO2
Use 1 carbon and 2 oxygen atoms: \(1(12.01) + 2(16.00)\).
For NaCl
Use 1 sodium and 1 chlorine: \(1(22.99) + 1(35.45)\).
Interactive formula builder
Build a formula and watch the mass update.
Adjust atom counts for common elements. The formula and total mass update as each element’s contribution changes.
Dynamic relationship
Each atom added increases the total mass by its atomic mass.
The graph compares how total mass changes as more atoms of a single element are added. Heavy elements increase the mass faster.
Hydrogen contribution
Each H atom adds about 1.008 amu, so the line rises slowly.
Oxygen contribution
Each O atom adds about 16.00 amu, so the line rises much faster.
Worked example
Calculate the molecular mass of glucose, C6H12O6.
Use approximate atomic masses: C = 12.01 amu, H = 1.008 amu, and O = 16.00 amu.
Count atoms from subscripts. C6H12O6 contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.
Multiply each count by atomic mass. Carbon: \(6(12.01)\), hydrogen: \(12(1.008)\), oxygen: \(6(16.00)\).
Add contributions. \(72.06 + 12.096 + 96.00 = 180.156\ \text{amu}\).
Round reasonably. The molecular mass of glucose is about 180.16 amu.
The molar mass has the same numerical value: about 180.16 g/mol.
Common mistake
Do not ignore parentheses or subscripts.
A subscript outside parentheses multiplies every atom inside the parentheses. This is one of the most common formula-mass errors.
Incorrect reasoning
“Ca(OH)2 has 1 calcium, 1 oxygen, and 1 hydrogen because O and H each have no subscript.”
Correct reasoning
The outside subscript 2 multiplies the whole OH group, so Ca(OH)2 has 1 calcium, 2 oxygen atoms, and 2 hydrogen atoms.
| Formula | Correct atom count | Mass setup |
|---|---|---|
| Ca(OH)2 | Ca = 1, O = 2, H = 2 | \(1(40.08) + 2(16.00) + 2(1.008)\) |
| Al2(SO4)3 | Al = 2, S = 3, O = 12 | \(2(26.98) + 3(32.06) + 12(16.00)\) |
Practice check
Calculate the formula mass of MgCl2.
Use approximate atomic masses: Mg = 24.31 amu and Cl = 35.45 amu. Pay attention to the subscript on chlorine.
Guide questions
- How many magnesium atoms are in one formula unit?
- How many chlorine atoms are in one formula unit?
- Which atomic mass must be multiplied by 2?
The result is a formula mass in amu and has the same numerical value as molar mass in g/mol.
Show answer
MgCl2 has 1 Mg atom and 2 Cl atoms. The formula mass is \(1(24.31) + 2(35.45) = 24.31 + 70.90 = 95.21\ \text{amu}\). The molar mass is 95.21 g/mol.
Continue learning
Apply formula mass to compound calculations.
Use molecular mass and formula mass to connect formulas, atom counts, molar mass, and gram-to-mole conversions.
Molecular Mass and Formula Mass Calculator
Calculate compound mass from a chemical formula and periodic table atomic masses.
Molecular Mass and Formula Mass Questions
Practice atom counting, subscripts, formula units, and mass calculations.
Summary
Formula mass is atom counting plus atomic masses.
Read subscripts carefully
A missing subscript means 1. A subscript after parentheses multiplies the entire group.
Use periodic table masses
Use atomic masses, not atomic numbers, when calculating compound mass.
Connect to moles
The amu value for one molecule or formula unit has the same numerical value as molar mass in g/mol.
This is the central relationship for both molecular mass and formula mass.