What is Molar Mass?
In chemistry, molar mass is the mass of one mole (6.022 × 10^{23} particles) of any pure chemical substance.
It is expressed in grams per mole (g/mol):
- •If water has a molar mass of 18.02 g/mol, then weighing out 18.02 grams of water gives you exactly one mole of water molecules.
The Master Formula
All you need is your Periodic Table and a calculator!
Worked Example 1: Handling Parentheses with Calcium Nitrate
Parentheses appear in chemical formulas when polyatomic groups repeat. The number outside the parentheses multiplies everything inside!
Problem:
Calculate the molar mass of Calcium Nitrate:
Ca(NO_3)_2
Step 1: Count each type of atom
- •Calcium (Ca): 1 atom (no subscript outside)
- •Nitrogen (N): 1 × 2 = 2 atoms
- •Oxygen (O): 3 × 2 = 6 atoms
Step 2: Look up atomic masses on the periodic table
- •Ca = 40.08 g/mol
- •N = 14.01 g/mol
- •O = 16.00 g/mol
Step 3: Multiply and add
The molar mass of Ca(NO_3)_2 is 164.10 g/mol.
Worked Example 2: Hydrate Compounds (What Does That Dot Mean?)
A hydrate is an ionic salt crystal that has water molecules trapped inside its crystal lattice. It is written with a centered dot:
The Centered Dot (·) Means ADDITION, Not Multiplication!
In algebra, a dot means multiply. In chemistry formulas, the dot means the salt and water molecules are physically bound together:
Molar Mass = Mass of CuSO_4 + Mass of 5H_2O
Problem:
Find the molar mass of Copper(II) Sulfate Pentahydrate:
CuSO_4 · 5H_2O
Step 1: Find molar mass of the dry salt (CuSO_4)
- •Cu = 63.55 g/mol
- •S = 32.07 g/mol
- •O = 4 × 16.00 = 64.00 g/mol
- •CuSO_4$ Total = $63.55 + 32.07 + 64.00 = 159.62 g/mol
Step 2: Find molar mass of the 5 water molecules (5H_2O)
- •One H_2O = (2 × 1.01) + 16.00 = 18.02 g/mol
- •5H_2O = 5 × 18.02 = 90.10 g/mol
Step 3: Add both parts together
Common Molar Masses to Memorize for Fast Exams
| Molecule | Formula | Molar Mass |
|---|---|---|
| Water | H_2O | 18.02 g/mol |
| Carbon Dioxide | CO_2 | 44.01 g/mol |
| Sodium Chloride | NaCl | 58.44 g/mol |
| Oxygen Gas | O_2 | 32.00 g/mol |
| Glucose | C_6H_{12}O_6 | 180.16 g/mol |
Check Your Decimals!
Most chemistry professors accept atomic masses rounded to two decimal places (e.g. H = 1.01, C = 12.01, O = 16.00). Rounding too early (using H = 1, C = 12) can cause rounding discrepancies on stoichiometry and AP Chemistry exams!