Calculating Molar Mass of Chemical Compounds

Introduction

• Learn to calculate molar mass for a chemical compound by its formula.
• Molar mass can also be referred to by different names but the calculation method stays the same.
• Practice problems ranging from simple to complex.

Basic Steps

1. Identify the number of each type of atom in the chemical formula.
2. Use the periodic table to find the atomic weight of each atom.
3. Multiply the atomic weight by the number of each type of atom.
4. Sum the results to get the molar mass in grams per mole (g/mol).

Example Calculations:

Example 1: SO₂

1. Identify atoms: 1 S, 2 O.
2. Atomic weights: S = 32.7, O = 16.0.
3. Calculation: (1 * 32.7) + (2 * 16.0) = 64.7 g/mol.
4. Result: 1 mole of SO₂ weighs 64.7 grams.

Example 2: C₃H₈O

1. Identify atoms: 3 C, 8 H, 1 O.
2. Atomic weights: C = 12.01, H = 1.01, O = 16.0.
3. Calculation: (3 * 12.01) + (8 * 1.01) + (1 * 16.0) = 60.11 g/mol.
4. Result: 1 mole of C₃H₈O weighs 60.11 grams.*

Complex Calculations with Parentheses

Example 3: Ca(NO₃)₂

1. Identify atoms: 1 Ca, 2 (NO₃).
2. Breakdown: NO₃ means 1 N and 3 O; total: 2 N, 6 O.
3. Atomic weights: Ca = 40.8, N = 14.01, O = 16.0.
4. Calculation: (1 * 40.8) + (2 * 14.01) + (6 * 16.0) = 164.10 g/mol.
5. Result: 1 mole of Ca(NO₃)₂ weighs 164.10 grams.*

Example 4: (NH₄)₃PO₄

1. Identify atoms: 3 (NH₄), 1 P, 4 O.
2. Breakdown: NH₄ means 1 N and 4 H; total: 3 N, 12 H.
3. Atomic weights: N = 14.01, H = 1.01, P = 30.97, O = 16.0.
4. Calculation: (3 * 14.01) + (12 * 1.01) + (1 * 30.97) + (4 * 16.0) = 149.12 g/mol.
5. Result: 1 mole of (NH₄)₃PO₄ weighs 149.12 grams.

Special Case: Hydrates

Example 5: MgSO₄⋅7H₂O

1. Identify atoms: 1 Mg, 1 S, 4 O, 7 H₂O.
2. Breakdown of 7 H₂O: 14 H, 7 O (additional).
3. Combined total oxygen: 4 (MgSO₄) + 7 = 11 O.
4. Atomic weights: Mg = 24.31, S = 32.07, H = 1.01, O = 16.0.
5. Calculation: (1 * 24.31) + (1 * 32.07) + (4 * 16.0) + (14 * 1.01) + (7 * 16.0) = 246.50 g/mol.
6. Result: 1 mole of MgSO₄⋅7H₂O weighs 246.50 grams.*

Conclusion

• No matter the complexity of the chemical formula (parentheses, dots, hydrates), these steps will help you calculate the molar mass accurately.
• Practice with various formulas to become proficient.