Molar Mass

Core Concept

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

  • Account for Subscripts: Always multiply the element's molar mass by its subscript in the formula to ensure every atom is counted.

  • Handle Parentheses Carefully: Multiply the mass of every element inside a set of parentheses by the subscript located outside those parentheses.

  • Watch Your Units: Consistently write $g/mol$ after your calculations to distinguish molar mass from simple atomic mass units ($amu$).

  • Verify Significant Figures: Match the precision of your final molar mass to the number of decimal places provided by your periodic table or the given data.

Test Yourself

Assorted Multiple Choice
A constant current is passed through an electrolytic cell for 45.0 minutes, delivering a total charge of 8,100 Coulombs. How many moles of electrons were transferred during this process? (Faraday's constant = 96,485 C/mol e⁻)

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Molar Mass

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Standard Molar Mass Calculation from Chemical Formula

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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 find a substance's molar mass, you add up the atomic masses of all the atoms in one of its molecules. Here's an example for calculating the molar mass of sulfuric acid (H2​SO4​).

The formula for sulfuric acid is H2​SO4​, which contains two hydrogen (H) atoms, one sulfur (S) atom, and four oxygen (O) atoms.

To find the total molar mass, follow these steps:

  1. Find the atomic masses of each element:

    • Hydrogen (H): 1.01 g/mol

    • Sulfur (S): 32.07 g/mol

    • Oxygen (O): 16.00 g/mol

  2. Multiply each atomic mass by the number of atoms in the formula:

    • Hydrogen: (1.01 g/mol) × 2 = 2.02 g/mol

    • Sulfur: (32.07 g/mol) × 1 = 32.07 g/mol

    • Oxygen: (16.00 g/mol) × 4 = 64.00 g/mol

  3. Add the results together to find the total molar mass:

    • Molar Mass of H2​SO4 ​ =2.02 g/mol + 32.07 g/mol + 64.00 g/mol = 98.09 g/mol

The molar mass of sulfuric acid (H2​SO4​) is 98.09 g/mol.

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.

Brain Hack Tip

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Remember to “mole-tiply”

When converting to moles to grams using molar mass - you will multiply the moles by molar mass to get to grams.

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