Molar Mass, Molar Volume, and Mass-Volume Calculations

Understanding Molar Mass

The molar mass of a substance is the mass of one mole of that substance. It is expressed in grams per mole (g/mol). To find the molar mass, sum the atomic masses of all atoms in a molecule.

For example:

• The molar mass of water (H₂O) is calculated as follows:
  • Hydrogen (H): 2 atoms × 1 g/mol = 2 g/mol
  • Oxygen (O): 1 atom × 16 g/mol = 16 g/mol
  • Total molar mass of H₂O = 2 g/mol + 16 g/mol = 18 g/mol

Molar Volume of Gases

At standard temperature and pressure (STP), one mole of any gas occupies a volume of 22.4 liters. This is known as the molar volume of a gas.

For example:

• 1 mole of carbon dioxide (CO₂) at STP occupies 22.4 L.
• 2 moles of nitrogen gas (N₂) at STP occupy 44.8 L.

Mass-Volume Calculations

To calculate the mass or volume of a substance, use the relationships between moles, molar mass, and molar volume.

Steps for Mass-Volume Calculations

1. Find the number of moles: Use the formula $\text{moles} = \frac{\text{mass}}{\text{molar mass}}$.
2. Calculate the volume: For gases at STP, use $\text{volume} = \text{moles} \times 22.4 \text{ L/mol}$.
3. Calculate the mass: Use $\text{mass} = \text{moles} \times \text{molar mass}$.