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What is Molecular Weight and Molar Mass?

Molecular weight is the sum of the atomic masses of all atoms in a molecule, measured in atomic mass units (amu). Molar mass is the same value expressed in grams per mole (g/mol).

Short answer

Molecular weight (or molar mass) is found by adding up the atomic masses of each atom in a formula. For H₂O: (2 × 1.01) + 16.00 = 18.02 amu or g/mol.

Molar Mass Comparison of Common Compounds

x: Compound · y: Molar Mass (g/mol)

Try it: interactive calculator

Molar mass of H₂O

18.02g/mol

= 2*1.01 + 16

Atomic mass of H1.01 amuAtomic mass of O16 amu

Step-by-step worked examples

Calculate the molar mass of CO₂.

C: 1 × 12.01 = 12.01 g/mol
O: 2 × 16.00 = 32.00 g/mol
Molar mass = 12.01 + 32.00 = 44.01 g/mol

Find the molar mass of Ca(OH)₂.

Ca: 1 × 40.08 = 40.08 g/mol
O: 2 × 16.00 = 32.00 g/mol
H: 2 × 1.01 = 2.02 g/mol
Molar mass = 40.08 + 32.00 + 2.02 = 74.10 g/mol

What is the molar mass of NaCl?

Na: 1 × 22.99 = 22.99 g/mol
Cl: 1 × 35.45 = 35.45 g/mol
Molar mass = 22.99 + 35.45 = 58.44 g/mol

Flashcards

Quick quiz

Q1.Molar mass of CO₂?

Correct answer: B. C (12.01) + 2×O (2×16.00) = 12.01 + 32.00 = 44.01 g/mol.

Q2.How many grams is one mole of H₂O?

Correct answer: C. Molar mass of H₂O = 18.02 g/mol, so 1 mole = 18.02 grams.

Q3.Molar mass of Ca(OH)₂?

Correct answer: C. Ca (40.08) + 2×O (32.00) + 2×H (2.02) = 74.10 g/mol.

Q4.What is the unit of molar mass?

Correct answer: B. Molar mass is expressed in grams per mole (g/mol).

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Common mistakes

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Molecular weight and molar mass are different. — Correct: They are the same value — MW in amu, molar mass in g/mol.

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Ignore subscripts when calculating molar mass. — Correct: Each atom count (subscript) must be multiplied by its atomic mass.

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For Ca(OH)₂, ignore the parentheses and subscript. — Correct: Subscript 2 multiplies both O and H: 2×O + 2×H.

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One mole of any substance weighs the same. — Correct: The mass of one mole equals the molar mass, which varies by compound.

FAQ

What is the formula for molar mass?

M = Σ(nᵢ × Aᵢ), where n is the atom count and A is atomic mass.

How do atomic masses affect molar mass?

Higher atomic masses of constituent atoms result in higher molar masses.

Why is molar mass important?

It converts between moles and grams, essential for stoichiometry calculations.

Can molar mass be different for different isotopes?

Yes — different isotopes have different neutron numbers, affecting mass.