To find the formula mass from atomic masses, you multiply the atomic mass of each element in the chemical formula by its respective number of atoms and then add all of these products together.
Here's a breakdown of the process:
Steps to Calculate Formula Mass
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Identify the Chemical Formula: First, know the chemical formula of the compound. For example, water is H₂O, and methane is CH₄.
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Determine Atomic Masses: Find the atomic mass of each element in the formula. These values can be found on the periodic table. For example:
- Hydrogen (H) has an atomic mass of approximately 1 atomic mass unit (amu).
- Oxygen (O) has an atomic mass of approximately 16 amu.
- Carbon (C) has an atomic mass of approximately 12 amu.
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Multiply and Add: Multiply the atomic mass of each element by the number of atoms of that element in the formula. Then, add all these products together to find the formula mass.
Examples:
Methanol (CH₃OH)
As given in the reference:
- Number of Carbon (C) atoms: 1
- Number of Hydrogen (H) atoms: 4 (3 in the CH3 group + 1 in the OH group)
- Number of Oxygen (O) atoms: 1
- Atomic mass of Carbon (C): 12 amu
- Atomic mass of Hydrogen (H): 1 amu
- Atomic mass of Oxygen (O): 16 amu
Formula Mass Calculation:
(1 12) + (4 1) + (1 * 16) = 12 + 4 + 16 = 32 amu
Therefore, the formula mass of methanol is 32 amu.
Water (H₂O)
- Number of Hydrogen (H) atoms: 2
- Number of Oxygen (O) atoms: 1
- Atomic mass of Hydrogen (H): 1 amu
- Atomic mass of Oxygen (O): 16 amu
Formula Mass Calculation:
(2 1) + (1 16) = 2 + 16 = 18 amu
Therefore, the formula mass of water is 18 amu.
Sodium Chloride (NaCl)
- Number of Sodium (Na) atoms: 1
- Number of Chlorine (Cl) atoms: 1
- Atomic mass of Sodium (Na): 23 amu
- Atomic mass of Chlorine (Cl): 35.5 amu
Formula Mass Calculation:
(1 23) + (1 35.5) = 23 + 35.5 = 58.5 amu
Therefore, the formula mass of sodium chloride is 58.5 amu
Key Points
- The formula mass is the sum of the atomic masses of all atoms in a chemical formula.
- It is expressed in atomic mass units (amu).
- It's important to use accurate atomic masses from the periodic table.
