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.

Practice Problem 2: Determine the Molar Mass of Ammonium Phosphate ((NH4)3PO4)(\text{(NH}_4\text{)}_3\text{PO}_4)((NH4​)3​PO4​)

1. Write the Formula: (NH4)3PO4(\text{NH}_4)_3\text{PO}_4(NH4​)3​PO4​ (three ammonium ions, each with one nitrogen and four hydrogens, and one phosphate group with one phosphorus and four oxygens).

2. Find Atomic Masses:

   • Nitrogen (N): 14.01 g/mol

   • Hydrogen (H): 1.01 g/mol

   • Phosphorus (P): 30.97 g/mol

   • Oxygen (O): 16.00 g/mol

3. Multiply by the Number of Atoms:

   • Ammonium Group (NH4\text{NH}_4NH4​) has 1 nitrogen and 4 hydrogens:

     • Nitrogen: 14.01 g/mol×3=42.03 g/mol14.01 \, \text{g/mol} \times 3 = 42.03 \, \text{g/mol}14.01g/mol×3=42.03g/mol

     • Hydrogen: 1.01 g/mol×12=12.12 g/mol1.01 \, \text{g/mol} \times 12 = 12.12 \, \text{g/mol}1.01g/mol×12=12.12g/mol

   • Phosphate Group (PO4\text{PO}_4PO4​) has 1 phosphorus and 4 oxygens:

     • Phosphorus: 30.97 g/mol×1=30.97 g/mol30.97 \, \text{g/mol} \times 1 = 30.97 \, \text{g/mol}30.97g/mol×1=30.97g/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 (NH4)3PO4=42.03 g/mol+12.12 g/mol+30.97 g/mol+64.00 g/mol=149.12 g/mol\text{Molar Mass of (NH}_4\text{)}_3\text{PO}_4 = 42.03 \, \text{g/mol} + 12.12 \, \text{g/mol} + 30.97 \, \text{g/mol} + 64.00 \, \text{g/mol} = 149.12 \, \text{g/mol}Molar Mass of (NH4​)3​PO4​=42.03g/mol+12.12g/mol+30.97g/mol+64.00g/mol=149.12g/mol

Answer: The molar mass of ammonium phosphate ((NH4)3PO4)(\text{(NH}_4\text{)}_3\text{PO}_4)((NH4​)3​PO4​) is 149.12 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.