ChemGemTM

How to Calculate Molar Mass (Step-by-Step, with Practice)

Molar mass = the mass of one mole, in g/mol. Add the atomic mass of every atom in the formula.

3 steps, 1 worked example, 4 practice problems. About 10 minutes. Free.

ChemGemUpdated Section 4.2

The route · 3 steps

  1. Count each element’s atoms. Parentheses multiply.
  2. Look up each atomic mass on the periodic table.
  3. Multiply and add: molar mass = Σ (atoms × atomic mass), in g/mol.
Try one now See a worked example
Start here

Periodic table

Atomic masses come from here. Tap any element.

Tap an element.

123456789101112131415161718Row11 shell22 shells33 shells44 shells55 shells66 shells77 shells57–7189–103

Swipe sideways for the full table.

Spot it

How to recognize a molar mass question

Asked for “molar mass,” “formula mass,” or “g/mol” of a formula.

Clue words marked:

What is the molar mass of H₂O?

Not this type?

Step-by-step

Worked example: molar mass of water

The idea

Add up every atom
O16.00 H1.008 H1.008 1.008 + 1.008 + 16.00 = 18.02 g/mol
H₂O has 2 H and 1 O. Every atom adds its atomic mass.
Molar mass
M = Σ (number of atoms × atomic mass)
Units: g/mol. Use the table’s decimal masses.
Parentheses multiply everything inside
Ca(OH)2 1 Ca 1 × 2 = 2 O 1 × 2 = 2 H the outside 2 multiplies both O and H
Count atoms first. Then look up masses.
Counting atoms
FormulaAtoms counted
NaCl1 Na, 1 Cl
CO₂1 C, 2 O
Ca(OH)₂1 Ca, 2 O, 2 H
Al₂(SO₄)₃2 Al, 3 S, 12 O
A subscript belongs to the symbol (or parentheses) right before it.

Atomic mass (table decimal)

Use it for molar mass.

O: 16.00

Atomic number (top number)

Protons. Not a mass.

O: 8

On the periodic table: H 1.008, C 12.01, O 16.00, Na 22.99, Cl 35.45, Ca 40.08, Al 26.98, S 32.06.

Worked example · 3 steps

What is the molar mass of water, H₂O?

  1. 1
    Count atoms
    ElementAtoms
    H2
    O1
  2. 2
    Look up masses

    1H1.008On the periodic table: hydrogen (H) 1.008, oxygen (O) 16.00.

  3. 3
    Multiply and add
    ElementAtomsAtomic massSubtotal
    H21.0082 × 1.008 = 2.016
    O116.001 × 16.00 = 16.00
    Total18.02 g/mol
  4. ✓
    Answer

    H₂O = 18.02 g/mol.

Your turn

Molar mass practice problems

Try first, then check. Hints if you need them.

Problem 1 of 4 · warm-up

What is the molar mass of CO₂ (in g/mol)?

Hint 1 · the clue

1 C and 2 O.

Hint 2 · the next move

C and O masses from the periodic table; double the O.

Your path:

  1. Count: 1 C, 2 O
  2. Look up masses
  3. Multiply and add
Answer

Answer: 44.01 g/mol

Full worked solution

What you know: CO₂.

  1. 1 C × 12.01 = 12.01; 2 O × 16.00 = 32.00.
  2. 12.01 + 32.00 = 44.01 g/mol.
ElementAtomsAtomic massSubtotal
C112.011 × 12.01 = 12.01
O216.002 × 16.00 = 32.00
Total44.01 g/mol

6C12.01On the periodic table: carbon (C) 12.01, oxygen (O) 16.00.

Answer: 44.01 g/mol

Problem 2 of 4

What is the molar mass of NaCl (in g/mol)?

Hint 1 · the clue

One of each.

Hint 2 · the next move

Add sodium and chlorine from the periodic table.

Your path:

  1. Count: 1 Na, 1 Cl
  2. Look up masses
  3. Add
Answer

Answer: 58.44 g/mol

Full worked solution

What you know: NaCl.

  1. 22.99 + 35.45 = 58.44 g/mol.
ElementAtomsAtomic massSubtotal
Na122.991 × 22.99 = 22.99
Cl135.451 × 35.45 = 35.45
Total58.44 g/mol

11Na22.99On the periodic table: sodium (Na) 22.99, chlorine (Cl) 35.45.

Answer: 58.44 g/mol

Problem 3 of 4 · watch for the trap

What is the molar mass of Ca(OH)₂ (in g/mol)?

Hint 1 · the clue

The 2 outside the parentheses doubles O and H.

Hint 2 · the next move

1 Ca, 2 O, 2 H. Masses from the periodic table.

Your path:

  1. Count: 1 Ca, 2 O, 2 H
  2. Look up masses
  3. Multiply and add
Answer

Answer: 74.10 g/mol

Full worked solution

What you know: Ca(OH)₂.

  1. Count: 1 Ca, 2 O, 2 H.
  2. 40.08 + 32.00 + 2.016 = 74.096 → 74.10 g/mol.
ElementAtomsAtomic massSubtotal
Ca140.081 × 40.08 = 40.08
O216.002 × 16.00 = 32.00
H21.0082 × 1.008 = 2.016
Total74.10 g/mol

Watch out: 58.10 means you doubled only H; 57.09 means you doubled neither.

20Ca40.08On the periodic table: calcium (Ca) 40.08, oxygen (O) 16.00, hydrogen (H) 1.008.

Answer: 74.10 g/mol

Problem 4 of 4 · exam-style

What is the molar mass of aluminum sulfate, Al₂(SO₄)₃ (in g/mol)?

Hint 1 · the clue

The 3 outside the parentheses multiplies S and O.

Hint 2 · the next move

2 Al, 3 S, 12 O. Masses from the periodic table.

Your path:

  1. Count: 2 Al, 3 S, 12 O
  2. Look up masses
  3. Multiply and add
Answer

Answer: 342.14 g/mol

Full worked solution

What you know: Al₂(SO₄)₃.

  1. Count: 2 Al, 3 S, 4 × 3 = 12 O.
  2. 53.96 + 96.18 + 192.00 = 342.14 g/mol.
ElementAtomsAtomic massSubtotal
Al226.982 × 26.98 = 53.96
S332.063 × 32.06 = 96.18
O1216.0012 × 16.00 = 192.00
Total342.14 g/mol

13Al26.98On the periodic table: aluminum (Al) 26.98, sulfur (S) 32.06, oxygen (O) 16.00.

Answer: 342.14 g/mol

Watch out

Common molar mass mistakes