To solve for molar mass, you need to find the atomic masses of each element in the compound from the periodic table and then add them together, taking into account the number of atoms of each element present in the chemical formula.
Here's a step-by-step guide:
1. Identify the Chemical Formula
First, determine the chemical formula of the compound you are working with. For example, water is H₂O, meaning it has two hydrogen atoms and one oxygen atom.
2. Find Atomic Masses from the Periodic Table
Locate each element present in the compound on the periodic table. The atomic mass (also known as atomic weight) is usually found below the element symbol. This value is typically expressed in atomic mass units (amu), but for molar mass, we use grams per mole (g/mol).
For example:
- Hydrogen (H): Approximately 1.01 g/mol
- Oxygen (O): Approximately 16.00 g/mol
3. Multiply Atomic Masses by Subscripts
If an element appears more than once in the chemical formula (indicated by a subscript), multiply its atomic mass by that subscript.
- In H₂O, there are two hydrogen atoms, so multiply the atomic mass of hydrogen by 2: 2 * 1.01 g/mol = 2.02 g/mol.
- There is one oxygen atom, so its atomic mass remains 16.00 g/mol.
4. Add the Results
Sum the values obtained in the previous step to find the molar mass of the compound.
For H₂O:
Molar mass = (2 Atomic mass of H) + (1 Atomic mass of O)
Molar mass = (2 1.01 g/mol) + (1 16.00 g/mol)
Molar mass = 2.02 g/mol + 16.00 g/mol
Molar mass = 18.02 g/mol
Therefore, the molar mass of water (H₂O) is approximately 18.02 g/mol.
Example: Calculating Molar Mass of Glucose (C₆H₁₂O₆)
- Chemical Formula: C₆H₁₂O₆
- Atomic Masses (from the periodic table):
- Carbon (C): 12.01 g/mol
- Hydrogen (H): 1.01 g/mol
- Oxygen (O): 16.00 g/mol
- Multiply by Subscripts:
- Carbon: 6 * 12.01 g/mol = 72.06 g/mol
- Hydrogen: 12 * 1.01 g/mol = 12.12 g/mol
- Oxygen: 6 * 16.00 g/mol = 96.00 g/mol
- Add the Results:
- Molar mass of C₆H₁₂O₆ = 72.06 g/mol + 12.12 g/mol + 96.00 g/mol = 180.18 g/mol
Thus, the molar mass of glucose is approximately 180.18 g/mol.
