Molar Mass

Related Examples and Practice Problems

Additional Worked Out Examples/ Practice

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Topic Summary & Highlights
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Core Concept

Molar mass is the mass of one mole of a substance. It is usually expressed in grams per mole (g/mol).

Practice Tips

  • Memorize Common Ions: Focus on learning the common polyatomic ions, charges, and patterns.

  • Roman Numerals for Transition Metals: Practice associating transition metals with their possible charges.

  • Cross-Method for Formulas: To determine the correct formula, use the “criss-cross” method to balance charges between cations and anions.

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Core Concept

Molar mass is the mass of one mole of a substance. It is usually expressed in grams per mole (g/mol).

Calculating Molar Mass: 

To calculate the molar mass of a substance, you need to add up the atomic masses of all the atoms in a molecule.

Practice Problem 1: Calculate the Molar Mass of Sulfuric Acid (H2SO4\text{H}_2\text{SO}_4H2​SO4​)

  1. Write the Formula: H2SO4\text{H}_2\text{SO}_4H2​SO4​ (two hydrogen atoms, one sulfur atom, and four oxygen atoms).

  2. Find Atomic Masses:

    • Hydrogen (H): 1.01 g/mol

    • Sulfur (S): 32.07 g/mol

    • Oxygen (O): 16.00 g/mol

  3. Multiply by the Number of Atoms:

    • Hydrogen: 1.01 g/mol×2=2.02 g/mol1.01 \, \text{g/mol} \times 2 = 2.02 \, \text{g/mol}1.01g/mol×2=2.02g/mol

    • Sulfur: 32.07 g/mol×1=32.07 g/mol32.07 \, \text{g/mol} \times 1 = 32.07 \, \text{g/mol}32.07g/mol×1=32.07g/mol

    • Oxygen: 16.00 g/mol×4=64.00 g/mol16.00 \, \text{g/mol} \times 4 = 64.00 \, \text{g/mol}16.00g/mol×4=64.00g/mol

  4. Add Up the Masses:

    Molar Mass of H2SO4=2.02 g/mol+32.07 g/mol+64.00 g/mol=98.09 g/mol\text{Molar Mass of H}_2\text{SO}_4 = 2.02 \, \text{g/mol} + 32.07 \, \text{g/mol} + 64.00 \, \text{g/mol} = 98.09 \, \text{g/mol}Molar Mass of H2​SO4​=2.02g/mol+32.07g/mol+64.00g/mol=98.09g/mol

Answer: The molar mass of sulfuric acid (H2SO4\text{H}_2\text{SO}_4H2​SO4​) is 98.09 g/mol.

FYI - 

Molar Mass and Molecular Weight: Molar mass and molecular weight are often used interchangeably, but they are not exactly the same thing. Molecular weight is the sum of the atomic weights of all the atoms in a molecule, whereas molar mass is the mass of one mole of that molecule.

Molar Mass and Avogadro's Number: Avogadro's number is the number of particles in one mole of a substance (6.02 x 10^23). The molar mass of a substance is directly related to Avogadro's number, as one mole of any substance will always have a mass equal to its molar mass.

Core Concept

FYI - 

Molar Mass and Molecular Weight: Molar mass and molecular weight are often used interchangeably, but they are not exactly the same thing. Molecular weight is the sum of the atomic weights of all the atoms in a molecule, whereas molar mass is the mass of one mole of that molecule.

Molar Mass and Avogadro's Number: Avogadro's number is the number of particles in one mole of a substance (6.02 x 10^23). The molar mass of a substance is directly related to Avogadro's number, as one mole of any substance will always have a mass equal to its molar mass.

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